This dissertation has been microfilmed exactly as received 66—1751 ABRAMSON, Fred Paul, 1941- KINETICS OF THE RADIOLYSIS OF CHLOROFORM: COMBINED EFFECTS OF DOSE RATE AND TEMPERATURE.
The Ohio State University, Ph.D., 1965 Chemistry, physical
University Microfilms, Inc., Ann Arbor, Michigan Copyright by Fred Paul Abramson 1966 KINETICS OF THE RADIOLYSIS OF CHLOROFORM:
COMBINED EFFECTS OF DOSE RATE AND TEMPERATURE
DISSERTATION
Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University
Fred Paul Abramson> A. B*
******
The Ohio State University 1965
Approved by
Adviser Department of Chemistry ACKNOWLEDGMENTS
I wish primarily to thank Dr. Richard F. Firestone for his guidance and instruction throughout the past years. It is hoped that this work» as well as future work» will partially repay him for the time and effort which he has put forth on my behalf.
The other members of the Radiation Chemistry section are also owed a great debt of gratitude for it has been their assistance and enthusiasm which has made my graduate program more complete and re warding.
I also gratefully acknowledge the support of the Department of
Healthy Education and Welfare for its award of a National Defense
Education Act Title IV Fellowship from 1962 to the present time.
This work was also supported by the United States Atomic Energy Com mission (Contract AT(ll-l)-ll6 ) which sponsored my Research Assistant- ship during the summer months.
A special acknowledgment is made to Dr. Clinton Foulk and the
Numerical Computation Laboratories of The Ohio State University for their support and assistance with the digital computer programs and the award of computer time.
ii VITA
January 15» 19**1...... «... Born - Columbus * Ohio
1961-1962 ...... N.S.F. Undergraduate Research Participant* Western Reserve University* Cleveland* Ohio
1962 ...... A.B. cum laude* Western Reserve University* Cleveland* Ohio
1962-1965 ...... N.D.E.A. Research Fellow* The Ohio State University, Columbus* Ohio
Summers* 1962-196**...... U.S.A.E.C. Research Assistant The Ohio State University Columbus* Ohio
PUBLICATIONS
Abramson* F. P.* Buckhold, B. K . * and Firestone* R. F., "The Effects of Temperature and Various Solutes on the Radiolysis of CCl^," J. Am. Chem. Soe., 8J*, 2285 (1962).
FIELDS OF STUDY
Major Fields Radiation Chemistiy
Studies in Radiation Chemistry. Professor R. F. Firestone
iii CONTENTS Page
ACKNOWLEDGMENTS ...... ii
VITA ...... iii
TABLES ...... v
ILLUSTRATIONS ...... vi
CHAPTER
I. INTRODUCTION...... 1
II. EXPERIMENTAL PROCEDURE...... 6
Analytical techniques Sample preparation Irradiation procedure Dosimetry Reagents Data processing
III. EXPERIMENTAL RESULTS ...... 35
Preliminary results Dosage dependence Combined dosage and dose rate Combined dose rate and temperature Hydrogen yield Chlorine yield
IV. DISCUSSION...... 51
Hydrogen production Chlorine production Dosage experiments Mechanism of the radiolysis
BIBLIOGRAPHY...... 98
iv TABLES.
Table Page
1* 100-ev. Product Yields at Various Dose Rates at 2^ • ••••••••••••»»♦•* 3
2, 100-ev, Yields in Liquid CHCl-j...... • . . 4
3» Gas Chromatographic Retention D a t a ...... 8
4, Sample Size vs Relative Response ...... 15
5, G Values at Various Temperatures and Dose Rates ...... 44
6 , Chlorine Absorbance and Dose ...... 49
7, G Values ...... ^
8, Radiochemical Yields ...... ^2
9* Relative Rate C o n s t a n t s ...... r,g
10# 100-ev, Yields in Solid Chloroform ......
v ILLUSTRATIONS
Figure Page
1. Computer drawn chromatograms...... 10
2. Sample series chromatogram...... 12
3« Chromatograph linearity . . 16
4. Vacuum system ...... 23
5« Dosage study irradiation vessel ...... 26
6. Vessel holder ...... 28
7* Concentration vs. d o s e ...... 37
8*. Concentration vs. dose at twodose r a t e s ...... 38
9* G vs. T for CH2CI2 and C2H2C1^ ...... 40
10. G vs. T for CgCl^ and CgHCl^ ...... 41
11. G vs. T for CCl^ and C2CI4...... 42
12. G vs. T ...... 43
1 3 . Chlorine production ...... 50
14. Log G(CH2C12 ) vs. 1 / T ...... 62
15. Analog computer output ...... 66
16. Digital computer program...... 69
17. Computer results ...... 70
18. CEzC12 yield v s . B ...... 75
19. Log k5 vs. 1 / T ...... 80
20. X : Y vs. B ...... 82
vi CHAPTER I
INTRODUCTION
Radiation chemistry represents that branch of science whose purpose is the investigation of chemical changes induced by par
ticles which are capable of ionising the substance of interest.
Although the use of radioactive substances to study chemical changes
extends throughout this century* only in about 1945 did radiation
chemistry start its rapid growth which today finds the number of
radiation chemists exceeding the number of photochemists in this 1 country. It has grown so that a number of books have been published 2 ^ in the field. The reader who is unfamiliar with the basic aspects
of radiation chemistry “may be referred to the introductory chapters
of these texts. The only point of explanation will be of the G value.
This is a yield which has units of molecules formed or destroyed per
100 ev of energy deposited into the system. It is exactly analogous
1. Office of Professional and Government Relations* The Ameri can Chemical Society* Chemical & Engineering News* 42, Jan. 20* 1964* p. 56 . 2. A. J. Swallow* "Radiation Chemistry of Organic Compounds," Pergamon Press* Oxford* England* I960.
3* S. C. Lind* "Radiation Chemistry of Gases»" ACS Monograph No. 131, Reinhold Publishing Corp., New York, N. Y., 1962.
4. A. Chapiro* "Radiation Chemistry of Polymeric Systems," Interscience Publishers* London* 1962.
I to the M/N ion pair yields of the early radiation chemist or to S» the quantum yield in a photochemical system.
A distinction between tracks and spurs might be made here in order to make the discussion easier. A track is the path which a charged high energy particle takes through the system. Along this path the particle will leave small clusters of radicals and ions. It is such a cluster that one calls a spur. The distribution of spurs along a track is a physical phenomenon which one cannot alter but by increasing the temperature the rate of spur expansion may be altered.
Further* by increasing the dose rate the distance from one track to another may be reduced. Thus* by combining dose rate and temperature as parameters in a radiolysis* the extent of spur overlap might be expected to vary.
Such reasoning led to this thesis. After investigating several workers' results* it was concluded that a combined dose rate and tem perature study should yield some fruitful information. Gardner and
Harper^ studied the effects of dose and dose rate on liquid chloroform.
Their results are presented in Table 1. They further showed that the yields of products have no dependence on dosage up to doses of 180
Mrad.
Other evidence of dose rate dependence was given by Meaburn.^
Using dose rates of 6.6 and 0.11 x 1 0 ^ ev/gm.-hr. he found that
5* J. B. Gardner and B. G. Harper, Paper No. 53» 8th Annual Meeting of the Radiation Research Society, San Francisco* Calif.* May 9-11* I960.
6 . G. M* M e a b u m * Ph.B. dissertation, University of Leeds* Great Britain* 1959* 3
TABLE 1
100-ev. Product Yields at Various Dose Rates at 25°
Dose rate (megarads/hr.) 18,000 360 0.25 0.06 ch2ci2 0.03 0.13 2.9 3-6
CCl^ 0.03 0.08 0.23 0.4
C2C14 0.04 0.05 0.13 0.14
C2H2C14 0.5 0.5 0.6 0.7 c2hci5 0.8 0.8 1.0 0.9 c2ci6 0.5 0.7 2.5 2.8
CgCl^ » 1»1»2-C2H^C1^» sym-C^H^Cl,.* C2 C14 and CC1if a11 showed inverse dose rate dependences of some sorti while CgHCl,. and C^H^Gl^ have some positive dependence. CgHgCl^* Hg and HC1 showed no dose rate dependence• 7 Werner* who also presents a fine historical review of the 8 radiation chemistry of chloroform* and Werner and Firestone studied
the effects of temperature. Their data are presented in Table 2.
Werner and Firestone also used Br2 as a scavenger and they briefly discuss the problems involved in its use. Furthermore* it will be
shown that bromide increases the decomposition of chloroform by at least 3*4 G units. For these reasons, the pure system, rather than
the scavenged one* was investigated.
7. H. R. Werner* Ph.D. dissertation* Western Reserve University* 1963.
8. H. R. Werner and R. F. Firestone, J. Phys. Ghem.* 69* 840 (1965). TABLE 2
100-ev, Yields in Liquid CHCl^
Temperature C°c) -62 *30 0 20 25 63
Dosage rate (ev/g-hr x lO”1^) 1.74 1.73 1.72 1.65 a 2.01
Dosage (ev/g x 1 0 ) 4.13 4.16 4.14 0.99 a 4.79
C0C12 0.11 0.09 0.17 — 0.15 0.11 c h 2c i 2 0.23 0.66 1*5 2.2 2.4 3*4
GCl^ 0.59 0.68 0.76 0.79 0.80 0.9 c2ci4 0.01 0.02 0.05 0.08 0.08 0.2 W h 0.97 0.92 0.82 0.60 O .58 0.50 C2HC15 1.6 1.8 1.8 1*3 0.91 0.60 c2c16 0.58 1.0 1.9 2.2 2.4 2.8 c h c i 2c c i 2c h c i 2 0.15 0.13 0.10 0.05 0.04 —
CHC12CC12CC13 0.17 0.20 0.20 0.08 0.1 MM
C^Clg 0.03 0.06 0.04 — — MM
—CHCl^ 8.3 10 13 12 12 12
HC1- 4.0 4.6 5.4 5.3 5*3 5.0
- Selected best values at 25° at dosagesgup to 2.6 x 10 ev/g. and dosage rates in the range 0,28 x 10 ^ ev/g.-hr. to 2.2 x lO1? ev/g.-hr. v — Computed from material balance for all products observed Q Ottolenghi and Stein' Investigated the dosage dependence of
pure chloroform and found that several products did exhibit dose dependence. In order to resolve their results and those of Gardner
and Harper-* the study of dose dependence was also undertaken. If
there were a dose dependence it would make the study of dose rate
and temperature considerably more complex.
When this research was begun» there was no work published where the combined effects of dose rate and temperature were used
as radiochemical tools. During the course of this investigation
two groups have successfully undertaken such studies. Hblroyd and
Fessenden^® use two temperatures and a number of dose rates in the
investigation of liquid ethylene. They find that those products
that show temperature dependence also show dose rate dependence.
Their conclusions are that both effects result from the competition
between radical-solvent and radical-radical reactions. Gale and
Wagner^ investigated short chain polymerizations where again the
competition between radical-solvent and radical-radical reactions
is present. Using dose rates which differed by 42 and several tem
peratures they found that both the G value and the temperature coef
ficient for their 1:1 adduct differed with dose rate.
9* M. Ottolenghi and G. Stein» Radiation Research* 14* 281 (1961).
10. R. A. Holroyd and R. W. Fessenden* J. Phys. Cham.* 67* 2743 (1963).
11. L. H. Gale and C. D. Wagner* J. Am. Chem. Soc.» 8 6 * 4531 (1964). CHAPTER II
EXPERIMENTAL PROCEDURE
Analytical techniques
Gas chromatography
An F & M Model 609 gas chromatograph which was equipped with linear temperature programing and a flame ionization detector was used for quantitative work# For this work a number of different columns were made in order to achieve the best resolution for the radiolysis products. The columns were of varied length and were constructed from 1/4 inch soft copper tubing according to Johns
One departure from this technique was necessary for the preparation of silicone gum rubber columns because this material is not suffici ently soluble in methylene chloride or any other of the recommended solvents. In this case it was necessary to dissolve the substrate in hot benzene. After the packing had dried the column was filled by use of a mechanical shaker. The shaker consisted of a flat plate of metal which had a hole near one end which was large enough to accomodate the tubing diameter and which had three set screws to hold it to the column. This plate was periodically struck by a
1. T. Johns* "Beckman Gas Chromatography Applications Manual*" Beckman Instruments, Inc., Bulletin 75^-A, Revised and Reprinted* June I960* p. 32*
6 metal cam attached to a small electric motor equipped with a variable transformer so that the striking rate could be varied. The column* which was made as straight as possible* was supported by a large ring stand and appropriate clamps. After packing, the column was bent into the proper shape and fitted with Swagelok connections.
The column was conditioned by attaching only one end to the chromatograph and programming at .33 degrees per minute to the maximum permissible temperature for that column for at least 12 hours. After this primary conditioning, column were readied for analysis by running them for several hours at least 50 degrees above the maximum temperature which the analysis would require.
Two types of columns were used in this work. For the dosage studies* a series column was employed. This was 1-1/2 feet of Carbo- wax 4-000 followed by 2-1/2 feet of silicone gum rubber both 25# by weight on Johns-Manville Chromosorb P» non-acid washed* 4-5-60 mesh.
It was found that appreciably better separation was obtained when the initial column temperature was above 50°• Therefore this series column was operated isothermally at 75° until 5 minutes after injec tion. It was then programmed at 6.4- degrees/minute to a final temperature of 150°. All the major radiolysis products were at least partially separated. Tetra-, penta-, and hexachloroethane formed a poorly resolved trio of peaks. Retention data are shown in Table 3«
This inferior separation was tolerated because the CCl^ peak was in front of the CHCl^ peak and was, therefore* easily integrable. This was the only column* or combination of columns, which gave an in tegrable peak for every radiolysis product down to less than 0 .1# TABLE 3
Gas Chromatographic Retention Data
Retention time (min.) Column- Series SGR McNairs
CH2 C12 3.1 2.9 3.6 trans-C2 H2Cl2 mm 3.1 - cis-C2H2Cl2 - 3.8 -
CHCI3 5.0 4.5 5.5
CCI4 4.2 6 . 6 2.3
C2 HCl3 - 7.9 -
C2 H3Cl3 - 1 0 . 6 -
G2 C3j(, 9 . 5 1 1 . 1 -
G ^ C l y - 1 3 . 2 -
C2H 2 CI4 2 0 . 4 1 3 . 6
C 2 H C I 5 1 9 . 7 1 5 . 3 -
C 2C 1 6 2 1 . 3 1 7 . 7
C 3H 3 CI5 - 1 8 . 6 - assym-^HCly - 2 7 . 3 •*»
* See text for the operating conditions for each column* - Not determined. conversion. This eliminated the special column for CCl^ which
Werner2 *-^ used. Pure Carbowax 4000 or 20M columns placed CgCl^, on the tail of the main peak and did not resolve Cgl^Cl^ and CgCl^.
Apiezon L and Dow-Corning Silicone Oil DC-200 would not properly resolve CCl^ and both bled substrate at higher temperature. The series column was ultimately abandoned because it had aged such that adequate resolution was no longer obtainable. Attempts to prepare new series columns failed to produce one which would give the same resolution as the original.
In order to achieve good accuracy in integrating the trio of two carbon peaks which the series columns gave* W. J. Pobst and the writer undertook an investigation of the integration of unresolved chromatograph peaks. Using a Heathkit ES-series Analog Computer with Donner Quarter Square Multipliers we observed that the function sin^x gave curves which looked very much like chromatograms. This computer is equipped with two relay systems so that two of these peaks could be generated and begun independently of each other so that the separation of the two peaks could be varied to represent varying de- 4 grees of resolution. This is shown in Figure 1. Pecsok shows the method of approximate triangles to be superior to the method of dividing the peaks at the lowest point between them. Our work shows that this is not the case* but that* if the two peaks are fairly well
2* Werner, op. cit.
3* Werner and Firestone* op. cit.
4. R. L. Pecsok* "Principles and Practice of Gas Chromato graphy," R. L. Pecsok, ed., John Wiley and Sons, Inc.* New York* N. Y., 1959* p. 146. Figure '• Computer 11 resolved and symmetrical} the latter method is superior. For a peak such as the last one in Figure 1 this technique gave results which agreed with the separate areas to A real chromatogram and the method of integration are shown in Figure 2 .
After it was decided to abandon the series type of column an attempt was made to incorporate the higher efficiency of lightly loaded 1/8 inch packed columns with the larger sample capacity of
1/4 inch columns. Chromosorb P, 80-100 mesh*, was chosen for this column on the basis of two outside pieces of work***^ and because this material hhd been used successfully in 1/8 inch columns in 5 this laboratory. Frederick et al . reported that the optimum mesh size for Chromosorb P in their 5 foot by 1/4 inch column is 80-100. 6 They also say that the optimum loading is Sawyer and Barr evaluated support materials in an 1/8 inch column and found that
Chromosorb P gives the greatest number of theoretical plates per unit of length. To minimize whatever inconsistencies in loading might occur 5$ loading was utilized rather than the recommended 3$.
The choice of liquid phase was again silicone rubber. Urone* Smith* n and Katnik' studied retention volumes of a number of chlorine con taining compounds such as are expected from radiolysis mixtures. They found that paraffin wax gave the best separation and that the DC 550 and 710 silicone oils were somewhat unsatisfactory with regard to the
5* D. H. Frederick, B. T. Miranda, and W. D. Cooke, Anal. Chem., 34, 1521 (1962).
6 . D. S. Sawyer and J. K. Barr, ibid., 1518.
7* P. Urone,. J. E. Smith, and R. J. Katnik, ibid., 476. CHCI
CH2CI2
I I__ 0 10 - 15 Time ( minutes) Figure 2. Sample series chromatogram, low boiling compounds although they did not present data for these latter columns. The 5 $ SGR (silicone gum rubber) column was indeed somewhat unsatisfactory with regard to CHgClg for it was not properly resolved. The 5 $ column gave essentially no baseline drift at CgCl^ which came at 22 minutes and 100° when programmed at 3 *3°/min* from room temperature. The column had very good separation otherwise* but it tended to cause the rather polar compounds, C HgC^ and GgHgCl^, to tail. This was probably due to the light loading so that a 10$
SGR on 80-100 mesh Chromosorb P column was prepared. The 10$ column exhibited better low end resolution. The heavier loading also ap peared to minimize the tailing of the polar compounds than the more lightly loaded one. The column temperature was held at k0° for ^*5 minutes followed by a linear increase of 9°/®in. to 150°, where the temperature was maintained until the last peak had eluted. Retention data for this column are given in Table 3» This column was used for all of the quantitative high and low dose rate determinations. The column has been used for over a year and little, if any, column deterioration has been noted. The baseline drift is gradual enough to allow the instrument to run at attenuation ^ and range 1 (here after noted as k x 1 ) at 150°.
Since no non-polar column tried would adequately resolve CCl^, a polar column was used for CCl^ analyses. With such a column CCl^ precedes CHCl^ rather than follows it, thus allowing accurate in tegration of this product. The column chosen was packed with ty0-60 mesh Chromosorb E» which was impregnated with 25$ by weight of
McNair’s Phase, 1,2*3-Tris(2-cyanoethoxy)propane. It was operated isothermally at 40°. It was found that the retention time for any two carbon product was more than three times that between the sample injection and the return to the baseline following the CHCl^ peak.
This meant that three samples could be analyzed for CClj^ without any interference from the two carbon products. After these three analyses> the exit end of the column was disconnected and the two and three carbon compounds allowed to bleed off at 90-100°for an hour or so. Retention data for this column are given in Table 3*
The chromatograph was always run using the same operating con ditions. The temperature of the injection port was set at 115-120° although high temperature conditioning caused this to be somewhat higher at times. The detector was kept at 105°• The flow rates of hydrogen*, compressed air* and nitrogen carrier gas (N2 at 30 psi) kept at 6.0 on the rotameters which are on the instrument. This corresponds roughly to flows of 40* 300* and 20 ml./min. respectively
The carrier flow of 6.0 was found to be optimum for this column.
W. J. Pobst evaluated the other two best settings for this instrument
Chart speed was 1 inch/min. Samples were always injected through the silicone rubber septum into a glass injection sleeve by the use of a Hamilton Model #701-NW/G syringe of 10 microliter capacity.
Several of these were used in the course of this work and they all gave indistinguishable results. One solution was injected three suc cessive times to check on the instrument's reproducibility as well as the injection techniques* The three areas were 180* 178, and 181 planimeter units.
The linearity of the instrument's response was also investigated Two syringes were used; one with a capacity of 0.5 microliters
(Hamilton Model #7000*5 N) and the standard 10 microliter one.
The data for CHCl^ were obtained on the 2556 McNair’s Phase column at 70°• The linearity data are presented in Table 4 and in Figure
3* The instrument’s attenuation devices were accurate to within the manufacturer’s tolerances.
TABLE 4
Sample Size vs. Relative Response
Sample Relative size 4l» response —
10.0 10.0 7.5 9.30 5.0 8.19 3.0 6.92
2.0 5.93 1.0 4.15 •5 2.56 •3 1.63
.2 1.15 .1 .5^5 .05 .227
— The response of 10.0 represents 2.35 square chart units at 256 x 1 0 0 .
It may be seen by consulting Figure 3 that the instrument’s response is linear below 0.3 lambda. Therefore> the standardization procedure was to add to 1.00 cc. of CHCl-j 10.0 lambda of solute.
The 1.00 cc. was measured with a 1 cc. capacity syringe with 0.01 cc. divisions. The 10.0 raicroliters were measured with a 50 M-l. syringe
(Hamilton Model #705) which could be read to less than 0*5 Ml* In none of this work were any volumetric instruments calibrated beyond 16
a> v> c o a. a>
a> > o a> 0 2 3 4 5 6 7 8 9 10 Sample size (jxl ) Figure 3. Chromatograph linearity that which was on the instrument. Samples (5*0 pi.) of this mixture constituting the equivalent of an 0.05 Hi* sample of solute were in jected. This is well within the linear range. For calibration of CgCl^ a one-dram vial was filled.with CHCl^ and weighed on a Mettler micro-balance. A few grains of CgCl^ were then added to the vial and the difference in weight noted. Two replicate samples of CgClg were prepared and three samples of all liquid solutes were prepared to lessen volumetric errors. The solutions 'were run at least twice on the chromatograph to lessen any chromatographic errors. The agree ment between the samples averaged *3$. This entire technique was repeated somewhat later and gave the same results within the experi mental error. Later the sensitivity factor for CgCl^ was judged to be incorrect for it gave unacceptably large values for that product. Then the ex perimental techniques were changed. Solute (25 Hi.) was added, by the use of the same syringe as before, to 25 cc. of CHGl^ in a volumetric flask. For the hexachloroethane, the flask was weighed empty, then a roughly weighed sample of the solid was added and the weight again measured. These samples were all successively diluted by factors of two from their original concentration of 1/10$ by volume to l/l6o$. Peak areas were found to be linear with respect to concentration in this region and all agreed with the older values except C2Cl£. A calibration factor differing from the older work by almost 200$ was found. It was also found that there was an effect on sensitivity associated with the temperature program of the instru ment. This phenomenon was studied only for 02^6* Four samples of 18 CgCl^ were run* and the programmed peaks were generally larger than the isothermal ones by about 5#* There was no trend with concentra tion* The instrument was regularly checked for sensitivity changes by the use of a secondary standard. In the early work 1.00# CCI4 was used and later 0 .0100# of was used because of its exceptional purity. CCljj' had to be abandoned as a standard when the series column was no longer used* since there would not be good enough resolution to unambiguously integrate that peak* whereas C2.CI1} was completely resolved and its integration was routine on the SGR column. The in strument’s sensitivity was generally checked before each series of runs and any time that the instrument had been repaired. The instru ment was remarkably stable. Periodic checks on the sensitivity showed that only small changes* about 5#» had taken place from time to time and it was assumed that whatever change took place in the ob served sensitivity for the secondary standard also took place for all the other compounds. Only two times was there a large change in the instrument’s sensitivity. Once* the electrometer tubes were replaced and the collector ring was moved so that it was more concentric about the flame tip. This raised the sensitivity about 20#. The second change took place without any apparent reason and raised the sensi tivity 33#* Both of these changes, as well as the general stability of the instrument were also observed by other members of this labora tory. The first part of a routine analysis was to insure column stability. If the column had just been installed, it would be baked 19 as has already been described. If not, the previous run would not be stopped until the last possible peak had been eluted. This pre vented any trace high boiling component from staying in the column and causing any abnormally high baseline drift in the next run. The syringe was always rinsed with acetone and blown dry with dried high purity nitrogen prior to each injection. The 10 lambda syringe had a dead volume of air which was hard to eliminate. One procedure was very successful in eliminating this air. The syringe was filled twice with the solution of interest and its contents rejected. Then the needle was held in the liquid again and the plunger was withdrawn to about 5 ^1* slowly and then very rapidly depressed again, still, keeping the needle in the solution. By repeating the slow withdrawal and rapid depression two or three times, the entire syringe could be filled with liquid. Before the syringe was filled, however, the gas flows on the chromatograph were carefully adjusted, and the instruments range and attenuator switches turned to appropriate settings. Hie analysis was begun by flipping on the switch of the recorder where for the sake of easy measurements, the chart paper had been turned to an even inch line and a mark made signifying where the run had started. Then the above procedure was used to fill the syringe to somewhat more than the 5 pi. which was always the volume injected. The plunger was then gently adjusted just to 5*0 Pi. but not less. After adjustment to the correct volume, the needle was wiped off with a tissue, the needle was grasped in the fingers and quickly inserted through the septum and injected. The handling of a syringe by the fingers is 20 not a recommended practice for it causes the liquid to expand and change the volume retained in the syringe. However* the needles on the 10 lambda, syringes are so thin that it is not generally pos sible to insert them in the septum without bending the needle or possibly aborting the entire run by a faulty injection. Therefore the needle was firmly grasped and guided into the injection port by the fingers. It was hoped that by doing this quickly* little volume expansion takes place. Once the injection had taken place* the chart was turned on and the appropriate range and attenuator settings were made for each peak. At temperatures less than about 50° the cap supplied for the chromatograph oven gave such good insulation that heat from the injector block caused the oven to rise to about 50° without any other heat supplied. Therefore an uninsulated sheet metal cylinder without a top was used for the isothermal parts of a program below 50°. This Cylinder fitted inside the regular cap* so that when the temperature was programmed* the cap could be put on and the programmer switch thrown. The lower limit of detectability on this instrument is about 1 x 10"^ moles of product per mole of chloroform. Spectrophotometry A Bausch and Lomb Model 505 recording spectrophotometer was em ployed for analysis of FeSO^ dosimeter solutions. One cm. silica cells were used and were cleaned with HCl-EtOH solutions. The instru ment was not equipped with a thermostatted cell compartment so that after the absorbtion curve was made* a thermometer was placed into the solution in the cell and the temperature of the liquid measured* 21 For studies of chlorine production a Beckman DU spectrophoto meter with a 10 cm. cell compartment was used with 2.5 cm. pyrex cells which held about ^ ml. This instrument was employed rather than the 505 since very low (less than 0 .10) absorbances were anticipated and the DU has a scale which is easier to read at such absorbances than the chart of the recording instrument. The extinc- 2 tion coefficients were taken from Werner. Mass spectrometry For the determination of hydrogen yields, a Consolidated Electro dynamics Corporation Model 21-620 Mass Spectrometer modified with a DC amplifier and Cat. No. 21-0?2 isotope ratio accessory was used. For this determination the hydrogen from the CHCl-j was diluted with a known amount of deuterium. The isotope ratio accessory has two separate circuits. The mass selector of one of these circuits was set to give the maximum peak height for the isotopic f o m present in the smallest quantity. The other circuit is identical except that it contains a graduated voltage divider. The mass selector in this circuit was then set to a voltage which maximizes the peak for the isotope present in the largest quantity. If the height of the two peaks is then visually balanced by adjusting the voltage divider in the circuit of the larger peak, an observed signal ratio can be read from the graduations on the voltage divider. However, it was found that the signal ratio changed as a function of time, because hydrogen diffuses through the molecular leak faster than deuterium and the remaining sample enriches in Dg. Thus the signal ratios at a given time were plotted on a semi-log scale against the time and an extra polation to zero time was obtained. This extrapolated value is the 22 actual value one would get if the instrument could give exact readings as soon as the sample was introduced. The run was accompanied by.one made on a known mixture of H2 and Dg- which had been prepared volumetrically. This gave a calibra tion for the instrument. The observed ratiofbr the unknown sample was multiplied by a factor equal to the ratio of the actual ratio of the calibration mixture as measured volumetrically to the observed ratio as measured on the mass spectrometer. This gave the corrected value for the ratio of the unknown. Thus* by knowing the volume of Dg that had been added* one could obtain the volume of H^. produced from the radiolysis of chloroform. Sample preparation Vacuum system A conventional all glass vacuum system was employed in this work. The pumping was provided by a Todd single stage mercury diffusion pump backed by a Welch Duo-Seal mechanical vacuum pump. Liquid nitro gen traps were used so that the line was kept mercury free. The system is shown in Figure For single samples 5*0 ml. of CHCl^ was placed into a 50 ml. flask containing BaO and placed on the vacuum system by use of a 10/30 standard taper joint. Dow Corning high vacuum silicone grease was used where required in the vacuum system. The 5*0 ml. were dis pensed from a 10 cc. medicinal syringe which was readable to less than to.l cc. The flask wb then cooled with liquid nitrogen and several freeze-thaw cycles were used to degas the liquid. The cessation of bubbling upon thawing was used as an indication that the sample i Teflon i Vent ! stopcock A To mercury 5 ml. and mechanical volumetric Liquid vacuum pumps -flask storage bulb Sample Sample 4 mm. preparation intro duction tubing manifold flask Figure 4. Vacuum syste m 2k has been degassed. The freeze part of the cycle was maintained until little or no spark was produced by use of a Tesla coil. After no more ebullition was seen*, the liquid was transferred as a vapor to the irradiation vessel by immersing the vessel in liquid Ng* Samples were pulled off of the vacuum line by constricted 6-8 mm. tubing or alternatively k mm. tubing* which can be conveniently pulled off without the use of a constriction. For preparing more than one vessel at a time two different techniques were used. The first required the use of the optional Fischer and Porter #795609 Teflon stopcock and storage flask in Figure k* Fifty ml. of liquid were cold transferred to this flask and carefully degassed. Then* when a sample was to be prepared* 5*0 ml. of liquid were cold transferred to the 5 ml* volumetric flask which was also in the vacuum line. This technique was abandoned when it was found that the Teflon stopcock did not maintain a good enough vacuum around the glass-Teflon seat. After that, when more than one sample was prepared, a sufficient amount of liquid was placed in the 10/30 flask and 5.0 ml. aliquots were transferred to the volumetric flask. The stopcock which goes from the 10/30 flask directly to the vacuum was added later to allow the sample flask to be removed with out disturbing the rest of the vacuum system. The vessels and the rest of the vacuum system were routinely flamed with a Meeker burner. For the hydrogen determinations a small oven rather than the flame was used to bake out the vessels. Irradiation vessels g The vessels were of annular design^ and had a volume of about 7 cc. 9* R* F. Firestone and J. E. Willard* Rev. Sci. Instr.* 2k % 90k (1953)* When single samples were made for the early dosage studies the vessel had a side arm of 7 mm. O.D. tubing attached to it. This side arm served as a manifold to which six "pulloffs" of 4 mm. tubing were attached. This vessel is shown in Figure 5» A small part of the liquid was poured into the pulloff which was farthest from the vessel and both the vessel and the pulloff were frozen in liquid nitrogen before the pulloff was removed. In this fashion aliquots of an irradiated sample were removed periodically without exposing the bulk of the liquid to air. Most of the high and low dose rate experiments were carried out using multiple sample preparations from the 10/30 flask. No zero time samples were taken, and it was assumed that the CCl^ (the only major impurity) would be distilled without fractionation along with the CHCl^ so that meaningful zero time readings could be taken from the stock solution. Only at -57° were zero time samples properly taken, either by using a pulloff of 4 mm. tubing, or by a single preparation in which the problem of fractionation is avoided. For the determination of hydrogen,, the irradiation vessels were equipped with breakseals. For halogen measurements the vessels had 2.500 ± .055 cm. by 15 mm. Aminco pyrex optical cells as well as a reservoir so that the cells could be read eu^ty in order to permit corrections for radiation produced coloration in the cell windows. New irradiation vessels were boiled in concentrated nitric acid for several hours followed by two boilings in distilled water. If the vessels had been used for dosimetry,. they were again boiled twice in water. After the washings the vessels were dried in an oven F ' 9 u r e 5 » Dosage r r a d i a t j 0n vessel fo O n 27 at 135°* This oven was also used for storage of the vessels between fillings. After a chloroform irradiation and subsequent analysis the liquid was poured or shaken from the vessel which was then placed in the drying oven. No other routine treatment for cleaning was used* for whatever cleaning agent was used would leave the vessel with more* and more harmful* contamination than it started with. The procedure may have allowed some high boiling compounds to remain in the vessels* but it is felt that the flaming under vacuum which was done on every vessel kept them as clean as was possible. There was no experimental evidence in this work which was interpreted as indicating problems with contamination of the vessels. Irradiation procedure The source used in all of the radiations was of modified Firestone- 9 Willard design. It was manufactured by Ohio-Nuclear and originally contained 205 - 5 curies of Co-60. This source material was doubly encapsulated in stainless steel so th't the metallic cobalt was 9/32 inch from the bottom of the capsule and was itself about 9*5 ram. high. Since a number of constant temperature baths were planned for use* it was advisable to keep the source rod itself from coming into contact with the bath materials. Therefore the vessel holder was based around a thin walled piece of 13/l6 inch steel tubing such that the annular vessels would fit around it while the source rod would fit inside it. This required that this tube be exactly vertical and centered in the pit so that the source rod would enter the tube and not bind in it* The vessel holder is shown in Figure 6. Temperatures were measured with a -60 to +110°C pyrometer (As sembly Products Inc.) which utilized an iron-constantan thermocouple. Adjustable slides for centering the vessel holder in irradiation Source entrance Legs fit outside of temperature bath FI gure 6. Vessel holder Temperatures above room temperature were; maintained with a Tecam fluidized sand bath along with a Variac and a temperature control device and thermistor (Yellow Springs Instrument Co.# Model 63-RA). Control of the air for the sand bath was maintained by a Norgren precision pressure regulator. Temperature control to ^2° was ob tained. Two problems prevented the precise temperature control of which the system should have been capable. The thermistor and its insulation were sensitive to the radiation. This caused drifting of the temperature which was most prevalent when the thermistor probe was closest to the radiation source. When the probe was moved away from the source to minimize radiation damage# poor temperature con trol also resulted# for the temperature in the sand bath varied with position. In some of the latter runs, equally good control was ob tained by merely using a Variac with no other device. For temperatures below 0° an automated slush bath such as has been previously des cribed^ was used. The major modifications were the use of liquid nitrogen as the coolant rather than a cooled gas and the use of a stirrer# which was not needed in the prior work. The two liquids used for this work were ethyl benzoate (Matheson*. Coleman and Bell) for -32° and n-octane (Phillips Petroleum Co.) for -57°• They were contained in an insulated metal bucket of 7-1/2 inch I.D. and about 6 liter capacity. An air driven stirrer was used for the presence of organic vapors might affect the insulation of electric ones. 10. R. Vallee and S. Harrell, ibid., 56? (1962). 30 Fifty liters of liquid nitrogen served to maintain a bath tem perature of -3.2° overnight so that the irradiations could be carried out continuously for the 20+ hours needed for the low dose rate runs# At -57° a 50 liter supply did not last that long so that at *57° shorter low dose rate times had to be used. Temperature control with the stirred system was excellent. Control at -57° was tl°. For a zero degrees bath the bucket was filled with crushed ice and then with pre-cooled distilled water. Such a bath would maintain 0° t 1° for 12 hours without stirring. Dosimetry On June 4, 1963 dosimetry experiments were performed. The dosimetry solution was prepared according to Lind j1-*- while the ex tinction coefficients for Fe+^ were those of Lind,^ Swallow*^ and L e e ^ at the appropriate wavelengths. Lind recommends 275 mii» Swallow uses the conventional 304 mp. and Scharf and Lee use the larger Fe+^ peak at 224 mp.. Irradiations were done on both 5 cc. and 3 cc. samples. Vessels were filled and emptied with 10 cc. syringes and each vessel was irradiated at least once. First 30 second irradia tions were made. All of the five vessels gave agreement to within 2$ of each other. The 30 second dose rates were 0.98* 0.96* and 0.94 x 10^® ev/gm.-inin. at 304 * 275* and 224 respectively. One minute dosages were then given. The average dose rate was .94 t *01 x 10^® ev/gm.-min. as determined from the Fe+^ absorbance at 304 mp.. 11. Lind» op. cit.» p. 59* 12. Swallow* op. cit.t p. 43* 13. K. Scharf and R. M. Lee* Radiation Research* 16* 115 (1962). 31 The 275 rail and 224 raji readings were .92 and .85 x 1 0 ^ ev/gm.-min. respectively. Since the extinction coefficient at 304 mji has been reported from so many different laboratories, data taken at 304 mji have been used. The value obtained by irradiations for one minute will also be employed in this work. A lower dose rate position was also calibrated. It was created by placing a piece of tubing around the handle of the source rod so that it could not descend to its limit, thus keeping the Co-60 farther from the vessel. Irradiations were made for 5 minutes, and the dose rate in the low position was 1.14 £ .03 x 101? ev/gm.-min. The effect of liquid level was studied by placing 3 cc. rather than 5 cc. in the vessel. This gave high and low dose rates of 9*8 and .97 x 1 0 ^ ev/gm.-min., respectively. It is seen that the liquid level is more critical at low than high dose rate,, and the effect is significant at the lower dose rate. These dose rates were corrected for nuclear decay using a half-life of 14 5.27 years for Co-60. The dose rate in CHCl^ was calculated from that in water by the use of the appropriate electron densities of the two liquids. This is the only correction necessary for Co-60 fl-ray in GHCl^ for the contributions from pair production and photoelectric effect are less than 0.2$^. At 25° the ratio of electron densities in electrons/ml. is 1.26 ?^» Thus the two dose rates in CHCl^ were 4.85 and 0.588 x 101* ev/gm.-hr. or 0.778 and 0.09^4 Mrad/hr. 14. W. Sullivan, "Trilinear Chart of the Nuclides,M U. S. Govern ment Printing Office, Washington, D. C.» January 1957» revised through December 1961. 3 2 Reagents Mallinckrodt AR chloroform was used throughout these experi ments. For the early work the chloroform was thoroughly washed with demineralized doubly distilled water to remove the ethanol stabilizer before, distilling under nitrogen on a 22 mm. by 1 meter column filled with glass helices. The middle third of the distillate was used and had only traces of CCl^ and an unidentified impurity. This latter impurity was neither formed nor removed by radiolysis and definitely was none of the compounds in Table 3* There were also much smaller amounts of CHgClg and cis-C2H2Cl2 * An experiment was made to determine the effectiveness of water in removing the ethanol from chloroform. Two percent of ethanol by volume was added to a sample of well washed CHCl^. This sample was analyzed on the VPC (McNair's column, 50°). Then the sample was well shaken with an equal volume of demineralized double distilled water and the chloroform was separated and dried with Drierite. The ethanol peak was 1/8.8 of its original size. Thus, a rough distribution co efficient was obtained. The standard washing procedure, which was to wash the CHCl^ five times with several times its volume of water*, removed the ethanol to less than the limits of chromatographic detect ability. A second lot of chloroform was used for the dose rate work. It was purer than the previous lot, and the only purification was washing with water to remove ethanol and then drying over Drierite. Chloro form thus treated contained 4.8 x 10“5 mole fraction CCl^, 4 x 10”^ mole fraction of GH2CI2 and a peak of the unknown impurity the same size as that of methylene chloride* No other impurity was detected. Because the unstabilized CHCl^ had little shelf life purification was performed immediately before sample preparation. All other gas chromatographic standards were obtained from com mercial sources. Their purity was determined by gas chromatography. Only C2HC1^ was less than 99$ pure and the appropriate correction was applied to it. The deuterium used for the hydrogen determination was Matheson research grade and was further purified by R. H. Lawrence by passage through a heated palladium thimble. The substrates used in the preparation of the chromatographic columns were obtained from F & M Scientific with the exception of McNair*s Phase [ 1»2,3-1 r is(2-cyanoethoxy)propane] which was Eastman white label. Data processing Individual chromatographic peaks were integrated with a plani- meter. Each peak was integrated until consistent results (tl plani- meter unit) were obtained. This area was multiplied by the attenuation and divided by 6o» the conversion from square chart units (not square inches as was thought) to planinimeter units. Then it was multiplied by the compound*s sensitivity which had the units* (moles of product)/ (mole of CHCl.j)-(sq. chart unit). For the dosage studies the G values were obtained from the slopes of the lines while on the dose rate oc\ studies for which the dose was constant at 10 ev/gm. the factor 5.06 x 10? converted the moles of product/mole of chloroform to G values. In the latter case* the chromatography was done on two aliquots to lessen analytical error. If agreement between the two areas was not within 2#» a third aliquot was analyzed to obtain con sistent results. The radiolysis experiments were carried out at least twice. No values were tolerated which differed by more than 5$ from the average of the two runs. This was not followed for C2C1^ for it would require accuracy to thousandths of G units. Replicate CgCl^ yields generally agreed within tO.Ol G. At 27° the agreement for the two values of CCl^ was somewhat less than 5$. For the dosage studies rapid evaporation from the small ampoules took place so that only the first chromatograph was assumed to be reliable* Hie main purpose of these experiments was qualitative* and they were not individually reproduced* CHAPTER III EXPERIMENTAL RESULTS Preliminary results In some very early work designed for orientation to the problem* ampoules of 7 ram* tubing containing about 0.1 cc. were irradiated. These results confirmed qualitatively the results of W e r n e r ^ in that methylene chloride* carbon tetrachloride* tetra-* penta-* and hexachloroethane are the major radiolysis products, while tetrachloro- ethylene along with trichloroethane* trichloroethylene* penta-* and hexachloropropane are minor products.. The flame ionization detector does not respond to COClg.so that phosgene was never detected in any experiments. Phosgene has a strong and characteristic odor which was never detected. Werner's G values for C0C1 were low (less than 0.2) and COCI2 may well have been present in the writer's samples also. Dosage dependence When the annular vessels became available* vessels with six pulloffs were prepared for the study of effects of absorbed dose. At 25° and the high dose rate and at both dose rates at 62° no dose dependence was seen for any product from 2.4 x lO1^ ev/gm. to 2q 2.9 x 10 ev/gm. Some curvature was seen in one run for C2CI5 and 1. Werner, op. cit. 2. Werner and Firestone, op. cit. in two runs for CgCl^. However* in the latter experiments combining dose rate and dosage* the lack of curvature of any lines leads to the belief that the curvature is merely a fault of experimental procedure* The results of the dose study at 25° and a dose rate of k,85 x 1 0 ^ ev/gm»-hr. is shown in Figure 7» The G values calculated from the early chromatographic standardization values are given also on the graph. These G values were redetermined in the combined dose rate and temperature experiments. All. of the plots appear to be linear with dose. As was mentioned previously* two different lots of CHCl^ were used. With the first* which had small traces of cis-dichloro— ethylene* trichloroethane was observed with a small yield which grew to some constant concentration with dose. The cis-dichloroethylene was not observed to decrease since it had been observed on a different column from the series columns used in the dose studies. The latter column would not resolve the dichloroethylene but the former* which was squalane* would. The sizes of the trichloroethylene peak before irradiation and the trichloroethane peak after irradiation were of similar size also. With the second lot of CHCl^ no dichloroethylene was present as an impurity and no trichloroethane was observed as a radiolysis product* Combined dosage and dose rate experiments Before the experimental procedure utilizing the six pulloffs was abandoned, several experiments were done using them. The first three samples were taken at various doses at one dose rate and the last three taken at greater doses at the other dose rate* The results of two runs at 25° are shown in Figure 8. CCl^ is not included because 0 3 6 9 12 15 18 21 24 27 30 33 36 Dosage ( EV/9 x I0"19) Figure 7. Concentration vs Dose 38 * 15 o ro O X H i gh Low o dose dose rote rote TJ O ,( 2 ) (A 5 (2) (2) (2) 0 0 3 6 9 12 15 18 21 24 27 30 34 36 Dosage ( EV/g x I0""19) F igure 8; Concentration vs Dose at two dose rates the SGR column was used for these experiments, and because the use of the small ampoules would not allow two samples to be taken accurately* Such runs were made at 25°» 48°, and 60°. The results were rather scattered but they did qualitatively show that the G value for methylene chloride was inversely proportional to dose rate as was G(hexachloro- ethane)* G(pentachloroethane) was directly proportional to the dose rate however G(tetrachloroethane) was independent of dose rate above 25°* No dose rate variation was found for C^Cl^. In the future, the inverse dependence on dose rate will be called negative and the direct dependence of G value on dose rate will be called positive. The effects of temperature showed that the dose rate coefficient» the difference between the two values divided by their average, gen erally decreased with increasing temperature. Combined dose rate and temperature A total of 28 irradiations were made to study the effects of dose rate and temperature. These were all single runs at constant dose. 20 The dose was arbitrarily set at 10. ev/g. to further insure: the con sistency of the results. This dose represented a conversion of 0.2- 0.4$. Only the experiments at -57° and low dose rate were done at 20 lower doses, in this case the samples were given 0.209 and 0.223 x 10 ev/g. doses. The results of these experiments are given in Figures 9> 10t lli and 12 and in Table 5» The CCl^ yields at 62° were taken from the earlier experiments. Included in the table are the three calcu lated values G(-CHCl^), GCCHClg*)! and G(CCLj*)» The first is obtained by summing the products of each compound*s G for production times it*s carbon number. The second is obtained by summing G C & ^ C ^ ) * G (Molecules / 100 EV) 2.4 4.2 3.6 0.6 3.0 1.8 -60 iue . v T o CH for T vs G 9. Figure oi pit - ih oe rote dose High - points Solid pn ons o ds rote dose Low - points Open 2) (2 0 3 - (2) (2) mprtr (°C) perature em T (2) 0 V2) (2) (2) 30 2 CI 2 n C ond 60 2 H 2 CI 4 *K) 6 (Molecules /100 EV) 3.0 4.2 0.6 2.4 3.6 -60 iue 0 G s fr C for T vs G 10. Figure 2) (2 oi pit ih oe rote dose High - points Solid pn ons o ds rote dose Low - points Open 30 e eaue (°C) perature Tem 0 (2) (2) (2) + 30 2 CI 6 n C' and (2) (2) + 60 2 HCI (2) 5 41 4 2 Solid points-H igh dose rate Open points-Low dose rote - 0.9 > UJ CCI o 0.8 o (2) » °-7 a> Z 0.6 0) o 2 0.5 o OAOf- 0.75 C,CI 0 .5 0 (2) (2) 0 .2 5 0.00 - 6 0 - 4 5 - 3 0 -1 5 0 +15 + 3 0 + 4 5 + 6 0 +75 Temperature (°C) Figure II. 6 vs T for CCI4 and C2CI4. t G (Molecules / 100 EV) 0.3 0.3 3.0 0.9 2.4 2.7 0.6 3.3 3.6 3.9 2.1 1.8 1.5 1.2 60 -5 3 -5 +5 3 +4 +6 +75 + 60 + 45 +30 +15 0 -15 -30 -45 0 -6 0 iue 2 G s T vs 12. G Figure . : ~ ~T ~ ~T T : —.T - T - Bpth dose rates rates dose Bpth mprtr (°C) perature em T — — High Low— dose dose rate CCI rate 1 3 4 44- TABLE 5 G Values at Various Temperatures and Dose Rates Temp. ’ Dose rateS Date ch2ci2 CCl^ C^Cl^ C2H2C] (°c) 63 Low 6/23/64 3.85 1 .10* .100 .68 65 Low 7/08/64 4.08 .086 .62 High 6/24/64 3.36 63 •.94* / • .102 .69 66 High 7/09/64 3.44 .086 .64 48 Low 7/15/64 3.46 .89 .097 .67 48 Low 7/22/64 3.62 .87 .083 .59 48 High 7/16/64 2.90 .97 .091 .69 48 High 7/22/64 2.89/ .87 .083 •59 48 High 7/16/64 2.90 .97 *091 .69 48 High 7/22/64 2.89 .07 .086 •66 27.5 Low 7/28/64 2.70 .77 .074 .57 26.5 Low 8/01/64 2.59 .61 .090 .52 25 Low 8/24/64 2.8 5 .93 .068 .53 26 Low 8/25/64 2.84 .77 .064 .49 2 7.5 High 7/28/64 2.32 .71 .076 .70 see footnote b 2.19 .71 .076 .68 26*5 High 7/29/64 2.25 .57 .096 .64 0 Low 8/11/64 2.08 .65 .074 •66 0 Low 8/18/64 2.26 .77 .060 .67 0 High 8/07/64 1.34 .73 .065 .86 0 High 8/13/64 1-37 .72 .069 .90 O 0 -33 Low 12/08/64 1.09 .66 . .85 -33 Low 12/11/64 1.09 .75 .048 .81 -33 High 12/08/64 .70 .81 .058 1.18 -33 High 12/09/64 .58 .87 .048 1.12 -33 High 12/22/64 •66 .77 .04 7 1.20 -33 High 12/22/64 •57 .60 .055 1.18 -57 Low 3/30/65 *51 .51 .89 -57 Low 3/30/65 •^9 .51 — .76 -57 High 3/24/65 .28 •66 .029 1.13 -57 High 3/24/65 .26 .61 .034 1.16 45 TABLE 5 (Contd.) T0IQ.pl C°c) Dose rate^ Date CgHCl^ c2c16 0013* c h c i 2* -CHCI3 63 Low 6/23/64 .61 3.58 8.87 5.82 14.88 65 Low 7/08/64 .62 4.04 9.80 5.94 15.91 63 High 6/24/64 .97 3.23 8.37 5.71 14.27 66 High 7/09/64 .98 3.46 8.84 5.70 14.71 48 Low 7/15/64 .75 3.37 8.38 5.55 14.12 48 Low 7/22/64 .76 3.65 8.93 5.56 14.65 48 High 7/16/64 I .23 2.90 8.00 5.50 13.69 48 High 7/22/64 1.18 3.10 8.25 5.39 13.81 2? .5 Low 7/28/64 .90 2.82 7.31 4.74 12.19 26.5 Low 8/01/64 .84 2.64 6.73 4.47 11.38 25 Low 8/24/64 •91 2.84 7.52 4.82 12.47 26 Low 8/25/64 .86 2.87 7.37 4.67 12.17 27.5 High 7/28/64 1.52 2.50 7.23 5.24 12.62 see footnote b 1.48 2.43 7.05 5.03 12.23 26.5 High 7/29/64 1.45 2.27 6.56 4.98 11.73 0 Low 8/11/64 1.41 2.26 6.58 4.81 11.54 0 Low 8/18/64 1.45 2.29 6.80 5.05 11.97 0 High 8/07/64 1.81 I .56 5.66 4.87 10.66 0 High 8/13/64 1.91 1.62 5.87 5.08 11.09 -33 Low 12/08/64 1.81 1.34 5.15 4.60 9.85 -33 Low 12/11/64 I .83 1.30 5.21 4.54 10.03 -33 High 12/08/64 2.23 1.17 5.38 5.29 10.79 -33 High 12/09/64 2.06 1.02 5.29 4.88 9.9^ -33 High 12/22/64 2.21 1.08 5.14 5.27 10.50 -33 High 12/22/64 2.15 1.05 4.85 5.08 10.04 -57 Low 3/30/65 1.61 •77 3.66 4.00 7.66 -57 L&w 3/30/65 1.49 .65 3.30 3.50 6.80 -57 High 3/24/65 1.98 .74 4.12 4.52 8.70 -57 High 3/24/65 1.98 .73 4.05 4.56 8.68 46 TABLE 5 (Contd.) a High dose rate is 3.88-4.85 x lO1^ ev/gm.-hr. depending on the date and the low dose rate is 4.70-5*88 x 1018 ev/gm.-hr. — This value is obtained by not considering one of the three chromatograms which was run for the above sample. The one chromatogram was of questionable reliability. * This value taken frcxn experiments of dose dependence. - Not determined. 47 GCCgHCl^) and 2G(C2H2C1^)> and the last by summing G(CC1^)» G(C2HC1^)» and 2G(C2C1^)» This procedure* of course* implies that the mechanism is known for the production of the observed products. The radical- radical and radical-solvent reactions which are: used for these calcu lations are discussed in Chapter IV. In these experiments some other minor products were observed qualitatively. Trichloroethylene was always a product of irradiation. Quantitative analysis of the peak was difficult* for it*, like CCl^, came on the tail of the solvent peak. It did not.seem to be very sensitive to the temperature of irradiation and its G value would be approximately 0.1-0.3* No effects of dose rate were observed within this uncertainty. There were traces of pentachloropropane and a later 3-carbon peak which has not been actually identified but whose identity is assumed to be hexachloropropane on the basis of its retention time. This latter compound was the major three carbon product. Its G value is estimated between 0.01 and 0.1. The C^HgCl^ showed a steady inverse temperature; coefficient from +48° to -57°» It is positively dose rate dependent, its peak areas at high and low dose rates being 104 and 73 planimeter units on x 10 at -32°* respectively. At that temperature no higher molecular weight products were observed. At -57° at least three more products were observed with areas less than 20$ that of the hexachloropropane. Two of these small peaks were close together; per haps they are isomers of heptachloropropane as they have similar retention times to the assym-C-jHClr, which was tested. The third peak may have been octachloropropane as Werner^*^ reported. The pentachloropropane peak was very small in all cases and no effect **8 of temperature or dose rate was obtainable from these data. Both the penta- and hexachloropropane were formed at all temperatures o below 62 • Hydrogen yield The G value of hydrogen previously determined by Wemer^*^ was redetermined using the more sophisticated and precise method previ ously discussed. Three irradiations were made. Two were made on 5 ml. samples of CHCI3 in carefully (and equally) baked annular 2i vessels. They were given doses of 1.93 and 3*96 x 10 ev/gm. and then opened on a vacuum line. The lower dose sample gave 105*2 microliters of noncondensable gas corresponding to G(noncondensable gas) = 0.029* From the mass spectrometer* using the ratio of sensi tivities of hydrogen to deuterium of 4.30 one obtains a volume of Hg equal to 106*8 microliters. This corresponds to G(H2) = 0.030* In the higher dose sample 285*2 lambdas were obtained from the volumetric measurement giving G(noncondensable gas) = 0 .039* while the mass spectrometric measurement gave 257*0 lambdas of Hg for a G(Hg) = 0.035* A third baked but empty vessel which was irradiated for the same amount of hydrogen observed compared to that in the samples which had liquid in them. The blank also had veiy small amounts of nitrogen* oxygen* and water in amounts scarcely discernible from the instrument's back ground. Since there appears to be no appreciable hydrogen yield which is not dependent on the presence of chloroform* G(H2) = 0.032 - .003* 49 Chlorine yield The observed concentrations of chlorine at 330* 335» and 340 up and various doses are given in Figure 13• Data were obtained at 325 up also» but those were excluded because of their variation from the data obtained at the other wavelengths. Cell coloration was checked after the irradiations were made and then after several hours in the annealing oven to bleach the color. These corrections to the absorbances have been subtracted from the observed absorbances prior to the calculation of the chlorine concentration. The corrected ab sorbances are given in Table 6 . The initial slope value for the production of Cl2 gives G(Cl2)0 - 0 .2-0 .3 . TABLE 6 Chlorine Absorbance and Dose Absorbance *2 at Dosage £ 330 335 340 0.947 .016 .013 .012 1.894 .028 .024 .024 0 0 3.790 . .025 .02? a in ev/gm. x lO”^ £ all absorbances ±10$ $ 6) 0 00 01ro Chlorine concentration (nrioles/ mole I CHCI3 x O ro <3 o o b o iq - - Figure 13* Chlorine production CHAPTER IV DISCUSSION Without mentioning the elementary steps which may actually take place the three overall reactions which initially decompose chloro form are: CHC13 - c h c i 2* + Cl* (1) CHC13 - CC13* + H* (2) CHC13 - cci2: + HC1 (3) Werner and Firestone"* present the various reactions in chloroform that involve these species. They report: Cl* + CHC13 HC1 + CGly w CHC12* + CHC1-J ch2ci2 + C d • (5) -4 2 CHCI2* C2H2C14 (6 ) CHC12* + CCly —* c2hci5 (7) 2 C C l y G2Cl6 (8) H* + CHCl^ h2 + CCl^* (9) H* + CHCl^ HC1 + CHC12* (10) CC12 : + CHCI3 -4 c2hci5* (11) c2hci5* — » C2HC15 (12) CgHCl^* — t CgCli,, + HC1 (13) 2 Cl* - C12 m Cl* + CCly “4 CCl^ (15) 1. Werner and Firestone* op. cit. 52 Cl* + chci2* - CHCl^ (16) Cl* + CHC13 - Cl2 + CHC12* (17) CC13* + Cl2 - CC\ + Cl* (18) In their paper they also discuss these reactions and their relative importance as well as corroborating evidence for them. In this work these reactions will again be discussed as the foundations of a physical model. It may be stated here that the results of these experiments have generally supported, the mechanism that Werner and Firestone presented. Werner found considerably lower yields of C2Cl£ at 63° than were found in this work. From his comparison of G(CC1^*) calculated and observed* a shortage of CCly observed is apparent. Using the present data for C2C1^ at 63° one finds that G(C2Clg) is 0 .3-1.0 greater than Werner and Firestone obtained. The spread of values is due to the different dose rates used in this work. This increase in GCCgCl^) also will give much better agreement to Werner and Firestone*s observed and calculated G(-CHCl^) at 63°. 2 When Werner added bromine to his chloroform he noted that G(-CHCl^) did not go down; it was 13^1 rather than the 1 2 H which they obtained for the pure system at the same temperature. He also noted that G(CH2C12) was decreased by 2.4 by adding bromine. Ac cording to reaction 5 * the formation of methylene chloride is accompanied by the loss of another molecule of chloroform. Therefore* if one has decreased the amount of methylene chloride formed* one 2. Werner* op. cit. has also decreased the amount of chloroform used up. Since he found that G(-CHCl-j)gr was not less than G(-CHCl-j)pUre> the bromine must 2 be decomposing chloroform. This does not take into account the amount of reaction b which is also scavenged by the bromine. This scavenging would make the attack of bromine on chloroform even greater. Hydrogen production The observed GCHg) of 0.032 is in good agreement with the 0.03 reported by Werner and Firestone.^" The G values here show that the hydrogen yields are affected slightly by dose* if at all. Aside from reaction 9» a possible source of H£ could be H* + HCl - H£ + Cl* (19) 3 This reaction has been proposed in the radiolysis of liquid HCl and has a G value of 2.2 at -79°. If this reaction were responsible for hydrogen production* G(H£) would be expected to show a strong dose dependence since HCl is not a primary species* but rather a product whose concentration grows with dose. This dose independence for GCHg) further supports reaction 9 as the major source of hydrogen. Chlorine production 2 This work also supports that of Werner. He found that GCClg^ = 0 .3 * which is very good agreement with the present value of 0 .2^i.05 . Wemer also found that the chlorine concentration reached some reason ably steady value at 1.5 x 10”^ moles/liter while Figure 13 shows that the steady state would be about 1.2 x 10"^ moles/liter. This is quite 3* R. C. Rumfeldt and D. A. Armstrong* J. Phys. Chern.* 68 * 761 (196*0 • similar to the self scavenging level which is attained in the radioly- h, sis of liquid CC1/|. We must attribute the majority of the chlorine destruction to reaction 18 and the analogous reaction for the CHClg radical chci2« + Cl2 - CHCl^ + Cl* (20) It is hard to see how reaction 14 can contribute to the chlorine production at 25° • The values which Werner and Firestone'*’ calculate for G(HC1) are essentially constant above 0° indicating that all of the Cl atoms are abstracting. Reaction 17 would be a more reasonable source of Cl2 since it involves the same reactants as reaction 4 with which it is competing. Reaction 17 is considerably endothermic. The Cl-Cl bond energy is reported to be 57 Kcal/mde^ and the CHClg-Cl bond energy has been calculated to be 78 Kcal/moleThis gives a minimum E-^ of 21 Kcal/mole. This value is indeed high for a process which is taking place with any degree of efficiency at 25°. The only alternative to this type of process is one involving ions. This will be discussed later. Dose experiments The present observation of a total lack of dose dependence is in agreement with that of Gardner and Harper.^ While it is always 4. F. P. Abramson* B. M. Buckhold, and R. F. Firestone* J. Am. Chem. Soc., 84, 2285 (1962). 5. T. L. Cottrell* "The Strengths of Chemical Bonds," Butter- worths Scientific Publications, London,, 1954* 6 . G. Glockler, J. Phys. Chem., 63, 828 (1959). 7* Gardner and Harper, op. cit. dangerous to criticize other workers results * those of Ottolenghi O and Stein are far enough away from those of other workers to war rant some comment. They found the following G values as a function of dose: TABLE 7 G Values Dose* 10^9 ev/ral. Product if .2 5 5.9 11 HCl 5 5.6 6.3 CH3CI2 - 0.9 o.if5 C2H3C13(1 ,1 ,2 ) - 0.5? 1.2 C2H2Cl2f(sym) - 1.1 1.1 c2hci5 - 1.2 1.3 c2ci6 - 3.1 3.4 CgCli* - 0.15 0.15 The observation of trichloroethane as a major radiolysis product is supported only by Meabum.^ As was mentioned in Chapter III* when samples which contained no traces of cis-dichloroethylene were ir radiated* no trichloroethane was observed. An impurity with a boiling point only 1 .1° below the solvent is often difficult to ob serve chromatographically and is generally difficult to remove by distillation. A reasonable reaction to write for the formation of CgHoGlo is HCl + (cis)C2H2Cl2 - (lil,2)C2H3Cl3 (21) 8 . Ottolenghi and Stein, op. cit. 9 « Meabum, op. cit. If the concentration of the dichloroethylene were fairly large* one would see the yield of trichloroethane increase with increasing dose since the HCl concentration is increasing with dose. No attempt will be made to explain all of the dose effects which are seen* but rather to suggest that impurities caused the observed dose dependence of Ottolenghi and Stein. The present observation of a lack of dose dependence presumably allows all reactions which involve any product (except chlorine) as a reactant to be eliminated from consideration. Such a reaction is CHC12* + CH2C12 - C12HC-CH2C1 + Cl* (22) which Ottolenghi and Stein present to explain their yields of tri chloroethane. Even tetrachlorethylene shows no dose dependence. This probably is due to that compound's unreactivity compared to other olefins. A 0.01 M solution of C2C1^ in CCl^ at 99° only decreased G(C12) from if. .75 "to .25 . If C2C1^ were reactive* one would expect essentially no chlorine to form. In unpublished work*, repeated photochlorination -2 did not remove trace (10” mole per cent) amounts of C2C1^ from CCl^. Thus* one may anticipate that no deviation from linearity would be seen for the low (ca. 10“-^ mole per cent) concentrations of olefin present in these chloroform solutions. Mechanism of the radiolysis One of the basic principles of liquid phase radiation chemistry which distinguished it from photochemistry is the non-uniform spatial distribution of the initial events and the differences which result in the kinetics due to this lack of homogeneity. In this discussion two different models will, be used and compared. The traditional model which employs spurs and tracks and requires the use of dif fusion kinetics and the homogeneous model which assumes a Uniform spatial distribution of reactive species will both be discussed. The attempted application of homogeneous kinetics to a solute free radiochemical system is a relatively novel one and has been fairly successful in this case. The two reasons for this moderate success are that the chloroform is both a reactive solvent and one which cap tures electrons. A reactive solvent here connotes one which may react with one or more of the intermediates that are produced from the solvent*s decomposition. In this case we have a number of reac tions which are of this type; reactions 4, 5 * 9 » 10 * 11 » and possibly 17* This type of behavior along with a sufficiently high dose rate to allow some track overlap will give a dose rate dependence. One can see that at a very low dose rate radicals or ions from one track do not live long enough to interact with radicals or ions from an other track. Therefore the kinetics will be independent of the dose rate because each radical or ion will react in its own track. If this takes place, the probability of a radical reacting with the sol vent as opposed to reacting with another radical is determined by the various rate constants which apply to that radical as well as 10 by diffusion kinetics. It may be seen that the broader the initial 10. A. Kuppermann> "The Chemical and Biological Action of Radia tions," Vol. V, M. Haissinsky (ed.) Academic Press Inc., London, England, 1961, Chapter III. distribution of intermediates, the lower the dose rate at which some traek-track overlap occurs. A very broad distribution of intermediates approaches a homogeneous distribution in the limit. It is on this basis that the homogeneous model is used. Before the models are discussed further, some discussion of the results may be made. Qualitatively, the results are very similar to those of Gardner and Harper'* and Meaburn^ except for Gardner and Harper's dose rate independence of CgHCl^ and Meabum's negative de pendence for C2C1^. They both see a negative dependence for CCI4 which was not seen in the present work. This compound is quite difficult to analyze by the single, non-polar column technique which Gardner and Harper used. The typical result of such an analysis is to find that G(CCl^) is considerably smaller than its.real value. In this work, G values of 0-0.4 were obtained for CCI4. when the one column technique was employed. This may explain some of the differences be tween other workers CCl^ yields and the present ones. Gardner and Harper used a 1 Mev resonant transformer as the source for their two highest dose rates. The beam then passed into several centimeters of chloroform. Using a range-energy relationship it,is observed that a 2 11 1 Mev electron has a range of 400 mg/cm in Al. This roughly corres ponds to a range of 3 Dim* in chloroform. This means that the bulk of the liquid receives no dose at all at any given time. Because the entire beam is absorbed by the liquid, the G values are determined only by the total integrated dose and are, consequently, correct. 11. G. Friedlander and J. ¥. Kennedy, "Nuclear and Radiochemis try," John Wiley and Sons, Inc.,, New York, N. Y.» i960, p. 202. 59 However the position on the dose rate axis cannot be accurately de termined without some consideration of the variation of the dose rate through the solution. One may be certain that the experiments done with electron irradiation have considerably higher local track densi ties near the surface than do the Co-60 irradiated samples. Thus the overall behavior of the system with respect to dose rate may be observed from the results of Gardner and Harper but the dose rate scale is undetermined because of the variation in track density from top to bottom. A good example of the value of a study of the combined effects of dose rate and temperature is shown in Figure 11. From the points one may see that G^Cl^ is temperature dependent but essentially dose rate independent. This observation strongly supports the sequence of reactions 11-13 for CgCl^ production. Two other pathways for its formation involve C^HCl^ formation through a reaction like reaction 7 followed by the subsequent decomposition or stabilization of the CgHCl^* or the bimolecular association of two GClgJ species. Since the former should have a temperature dependence which is similar to that for C2HC1 j. and since both would have positive dose rate depend ences, -neither could be correct. The positive temperature coefficient is just what one would expect both for the insertion and for the sub- ♦ 12 sequent decomposition of C^HCl^ • Recent evidence is interpreted to show that the carbene generated by the themal decomposition of CHCl^ has a singlet ground state and reacts in that state. Thus, one may presumably anticipate the insertion reaction 11 rather than 12. L. D. Wescott and P. S. Skell, J. Am. Chem. Soc., 87, 1721 (1965). 6o one which involves the abstraction which would be due to a diradical. The plots of G vs. T which are presented in Chapter III show that several of the compounds (CH2C12, C2HC1^» and CgCl^) have an essentially constant difference between the high and low dose rate plots at temperatures above -33°• The anticipated result based on the spur overlap model was that the dose rate dependence would vanish at higher temperatures because of the higher reactivity of the dichloro- metbyl radical at the higher temperatures. The plot for Cg^Cl^ ex hibits this particular type of behavior. It may be noted here that the low temperature end of the plot of CgH^Cl^ has a peak in it although the data in this region suggest the straight lines which are also drawn. If a compound shows a dose rate dependence without showing a temperature dependence this indicates that the two competing processes have the same temperature coefficient. For this to be true in the case of C^H^Cl^ reactions 5 and 6 would have to have the same temperature dependences. This is not impossible since reaction 6 may be diffusion controlled and thus have an activation energy of at least 3*3 Kcal/mole, the activation energy for self dif- 13 fusion in CCl^. It is assumed that a similar activation energy might Ik be expected for the CHCl^ system. Recent experiments-1- have shown that the temperature coefficient for diffusion of iodine atoms in CCl^ is higher than that for I2 and similarly the activation energy for radical diffusion in CHC12 might be higher than 3*3 Kcal/mole if 13• H. Watts, B. J. Alder, and J. H. Hildebrand, J. Chem. Phys.,, £.> 659 (1955). 14. S. Levison and R. M. Noyes, J. Am. Chem. Soc., 86, 4525 (1964). one generalizes that atoms or radicals will have higher temperature coefficients than molecules with saturated valences. In order to have E,. - l/2E£ equal zero* E^ would have to be 8 Kcal/mole because E^ will be subsequently shown to be 4 Kcal/mole* This value for E^ is rather large and therefore a straight line for C„H0C1., is not 2 c M" probable• A maximum in the G vs. T plot might also be considered reasonable for C2H2C1^ by analogy to the C2HG1^ plots which definitely show such behavior. It would be expected that tetrachloroethane would peak at a lower temperature than would pentachloroethane since CCl^* produc tion is a much sharper function of temperature than that for CHC12*. This will cause the G(pentachloroethane) to have a stronger tempera ture dependence than G(tetrachloroethane) as well as the maximum at a higher temperature. Figure 14 shows a plot of log GCCH^Clg) vs. l/T. The unusual results indicated that the kinetic considerations which allow such a plot are not properly fulfilled in this case. This observation of non-linear and dose rate dependent activation energy plots required further investigation into the kinetics of this system. One problem with the track overlap model is that the dose rate dependence for CgCl^ is very difficult to predict. On the basis of track-track overlap, one would predict a positive dose rate de pendence. On the other hand,, the production of CCl^ radicals is, in part, inversely dose rate dependent as a result of reaction 5 and thus CgCl^ would show an inverse dependence. The actual system is a combination of both and it is difficult to predict which result would be the stronger. 62 0.6 0.4 0.2 Low dose rote High dose rote 0.0 CP - 0.2 - 0 . 4 - 0.6 2 .8 0 3.20 3 .6 0 4 .0 0 4.40 4 .80 1000/ T (°K ) Figure 14. Log G(CH2CI2) vs l/T The higher reactivity of the CHCI2 radical at elevated tempera tures should cause the difference between the high and low dose rate plots to diminish according to the track-track model so that an al ternate mechanism must be proposed for the observation of a non diminishing difference between the high and low dose rate values above -33° as well as the shape of the curves in Figure 1^. In order to achieve the most contrast a purely homogeneous model was next formulated. In complete form* the rate equations for the radicals are R(CHC12’} = R(l) + R(10) + R(l?) - [R(5) + 2R(6) + R(7) + R(l6)] R(CC13*) = R(2) + R(4) + R(5) + R(9) - [R(7) + 2R(8) + R(15) + R(18)] There is no rate law presented for the H atom because its high reac tivity insures no competition for reactions 9 and 10 and therefore its steady state is easily determined. The Cl atom will also be as sumed to have no competitive step above 0° other than reaction This assumption is based on the reported"*" independence of G(HC1) upon temperature from 0° to 63° which means that reaction 4 is com plete and* as a result, chlorine atoms may have no other reactions. The low value for GCl^i (G & 0.4"*") allows its omission from all rate equations. The H atom reactivity means that R(2) will act as a constant source of radicals but the G value for reaction 2 is only 0.25^. Compared to the G value for reaction 4 of 5*^ this is indeed a negligible quantity. Therefore R(2), R(9)» and R(10) may be omitted 64 from these rate expressions. The completeness of reaction 4 allows reaction 15 to be omitted and the endothermicity of reaction 17 re quires that R(17) be very small with respect to R(4). R(l8) is a possible source of CCl^ because G(C12) = 0.2-0.3. The total yield of CCl^ is about 0.8. This is not clearly a negligible quantity compared to some of the G values which are being retained. For the present discussion R(l8) is also being omitted and the validity of the amission will be checked subsequently. Because reaction 1 produces Cl atoms, and because reaction 4 uses them to produce CCl^ radicals with complete efficiency* those two reactions may be condensed into P single reaction which maintains the stoichiometry but is mechanistically incorrect. Thus reactions 1 and 4 become 2CHC13 CHC12* + CC1 • + HCl (23) if the temperature is 0° or above. By invoking all of these assumptions, two differential equations are obtained: d(CHCl2*)/dt = R(23) - k5(CHCl2-)(CHCl3) - 2k6(CHCl2’)2 - k7(CHCl2*)(CCl3*) d(CCl3*)/dt = R(23) + k5(CHCl2*)(CHCl3) - 2kg(CCl3*)2 - k7(CHCl2*)(CCl3*) R(23) is not given any explicit form here. It represents the rate of production of initial radical species and is equal to the dose rate times G(23) divided by 100No if the rates are expressed in moles/liter-second. The above equations may be transformed into an other set which will be more useful. These equations are: dX/dt = A - BX - 2DX2 - CXI dY/dt = A + BX - 2EY2 - CXY where A = B(23) = dose rate (in ev/l.-sec.) x G(23)/l00No , B is a pseudo-first-order rate constant which equals k^(CHCl^), C, D» and E are the rate constants for reactions 7» 6 , and 8 respectively and X «? CHC12* and Y = CCl^*. These two equations represent a set of non linear differential equations which cannot be solved directly. The rate of production of any product is expressed in the normal way, e.g., d(CH2Cl2)/dt = k5 (CHCl2 *)(CHCl3) = BX. The G value is the rate of production times 100 and divided by the dose rate. Thus BX/A = G(CH2C12 )/G(23) and similarly CXY/A = G(C2HC15 )/G(23), DX2/A = G(C2H2C14 )/G(23) and EY2/A = G(C2C16 )/G(23). In order to prove that these equations do establish a steady state for the system they were placed on the Heathkit Analog Computer. The result of a typical run is given in Figure 15* The ordinate is given in volts which, in this case,, is the computer's analog for con centration and the abcissa is the time, in seconds, for the computer to draw the curves: it is not the reaction time, but rather some quantity which is proportional to it. It may be seen that both of the radicals do reach a steady state, although at different times. The CCI3 radical cannot reach a steady state before the CHCI2 radical because reaction 5 produces GGl^ Concentration (volts ) IOOI 40 20 50 90 30 60 80 70 iue 5 Aao cmue otu. g; output. computer Analog 15. Figure Time (seconds) CpHCI CCU* 67 radicals from the CHClg species. It was also observed that the effect of raising the temperature (the temperature was said to rise when B was increased because B was assumed to be the only one of the rate constants which was temperature dependent) was to raise the steady state concentration of CCly and to lower that for CHClg** The im plicit relationships of the previous equations were observed when B was changed by a factor of 4 and BX only increased by 2.4* This type of relationship made it difficult to obtain desired results for one's predictions were seldom carried out quantitatively. The analog computer was eventually abandoned because the instru ment did not have a great enough dynamic range to completely expose the system and it did not have enough accuracy when its normal range was exceeded. After establishing that the radicals did reach a steady state* the differential equations became implicit, non-linear algebraic equations by setting dX/dt = dY/dt - 0. Such equations may be solved by some method of successive approximations. A digital computer was chosen to do this task. The implicit algebraic equations were solved on an IBM 709/7094 computer using a two dimensional Newton-Raphson iteration methodThe quantity A was varied over 8 orders of magni tude and B was varied from 0.05 to 3*2 by factors of 2 although only four cases of B are presented in one figure. C» D, and E were initially given the relative values of 0.0104» 0.0100* and 0.0074 respectively. These numbers result from those relative rate constants which Semeluk 15* F. B. Hildebrand* "Introduction to Numerical Analysis," McGraw-Hill Book Co., Inc., New York, N. Y., 195&* p. 450. 68 and Unger reported in the photolysis of CHCl^ vapor. The magnitude of B relative to these nutmers was approximately obtained by use of the analog computer* where the adjustment of such parameters is a simple process. 17 The computer program is written in 3catran* ' a language for the IBM 709/709^ which is used at The Ohio State University. The program is reproduced in Figure 16. X represents CHC^** X represents CCl^* and the other constants have the meanings which have been discussed. Scatran uses certain signs to represent computer operations. An asterisk* *, represents the multiplication operation while the slash* /> indicates division. The .L. means less than and .P. indi cates a power operation. The dash at the end of each statement is to signify the end of an order. FX» FY, GX» and GY are all derivatives; i.e.* d(F)/dX = FX, etc.. Statement 21 shows all of the outputs which are written. Thus the computer gives solutions to the equations of statements 10 and 11 and presents the X and Y which satisfies that choice of A* B, C» D» and E along with the yields of the four products. The best fit to the experimental data was obtained when C and D were given half of their original value. These data were put into a data plotter and the resulting curves shown in Figure 17. The sig moidal shape of a plot of G vs. log dose rate has already been shown 1 ft experimentally by Schuler and Kuntz. When the relative rate constants of Semeluk and Unger were used* the yields of pentachloroethane and 16. G. P. Semeluk and I. Unger, Nature * 198* 853 (1963)* 17* "Scatran Reference Manual," Numerical Computation Labora tories* The Ohio State University, 1964. 18. R. H. Schuler and R. R. Kuntz, J. Phys. Chern.* 67, 10C& (1963). ------.FOULK, C. JOB BAA 100 C4/20/&5 _ 9 3 9 C U PAGE 3 ______ SOURCE LANGUAGE STATEMENTS 1 DIMENSION (A(2S),R(7)>- 2 LITERALS (A|li}03C>t3CwO)f20'J3*|ir0^pi3'‘0 u C 2 I . G1 1i.CQ3it- 002,.00 I,.COOS,.0002,.CCCl,B ,3.2,I.6,.8,.A , .2,.1,.05,U,.:1,C,.0104,E,.00 74)- 3 START 00 THROUGH (L11,K=G,l,K.L.5- _ . . . _:_____ 4 D*D/2.- 5 C*C/2.- - - -• 6 00 THROUGH (L1),J*C,1,J.L.7-...... ______ 7 DO THROUGH I L11,I*C,1,1.L .25- 8 x » A i n - “ '...... .9 v«A IT OELfAX*(-F*GY*G*FY)/(FXiGY-GX*rYr- .______18 DELTAY*(F*GX-G*FX)/IFX»GY-&X»FY1- ...... ______19 X*X*DELTAX------20 Y*Y*0ELTAY- .. ------21 PROVIDED I .ABS, I DELTAX/X 1. L . .OGC I . AND, , ABS. I DEL.TAY/Y1 .L..00011, TRANSFER 1 L 11- ______22 TRANSFER IL01— ------ 23 LI WRITE OUTPUT ,F1, ID, BU ) ,A1 1 1 ,X,Y,BU) »X/A7i 1 , C * X * Y / A U 1 ,D»X.P.2/A( I) ,E*Y.P.2/A< 1 1 1- ______24 F FI _ 11P9E 13.61 - ______25 END PROGRAM (START)- . .------ Figure 16. Digitol computer progrom. ___ __...ir. .ILHZIZIZ Yields o 1 lODO 4.00 0.00 6.00 2.00 8.00 4.00 iue 7 Cmue results 17. Computer Figure CHoC 4 - B - 4 = 0.40 B3 - = 0.20 = 05 .0 0 =B 3.00 2.00 1.00 Log 3 \ A + 1.00 + 2.00 + 3.00 +4.00 71 tetrachloroethane were too great. There is no reason to believe that these rate constants are incorrect or that they are necessarily correct in the liquid system* rather that they do not fit into the approxi mate model which was used. In Figure 17 there are four families of curves* each corres ponding to a given product. As B is changed by a factor of two* these four curves, which represent a given choice of B*. do not change their shape but only translate along the dose rate axis by 0.60 units. This 0.60 represents a change in the dose rate of a factor of 4 since the dose rate scale is logarithmic. Thus, for a given yield* the ratio 1/2 of B/A is constant. The ordinate scale represents the radiochemical yield. This yield is equal to G(product) divided by G(23). If the experimental G values are divided ty G(23) these values will fit on the ordinate. Reaction 23 is the sum or reactions 1 and k and its rate equals that of reaction 1 or 4. If the G value for either of these reactions were known* the scaling of G values to the relative yield values would be complete. Werner and Firestone^ report that the G value for CHCI^Br in the B^-CHCl^ system is This represents the scavengable yield of CHCI2 * in the system and thus approximates the homogeneous yield of GHClg*. This G of will be taken as G(23) and represents a scaling factor, that quantity which transforms real units into computer units. Table 8 gives the relative yields along with the calculated yields which will be discussed later. The experimental G values are taken from the plots of G vs. T which were presented in Chapter III. CH2CI2 was taken as the reference compound since the desired result TABLE 8 a Radiochemical Yields — ch2ci2 c2h2C14 c2hci5 C2C16 Temp* Dose Rate obs calc obs calc obs calc obs calc 65° High .63 .63 .12 .06 .18 .21 .63 .71 Low .74 .74 .12 .05 .11 .17 .74 *78 O -3*CO High •54 .12 .12 .22 .23 .54 .65 Low .65 .65 .12 .08 .15 .20 .64 .72 26° High .42 .42 .12 .16 .28 .26 * 3 .58 Low *5^ .12 .12 .20 .23 *54 .65 0° High . 2 6 .26 .16 .23 •34 .29 .30 .48 Low •39 •39 .12 *18 .26 .27 .41 .56 - obs yields = G(product)/G(23) calc yields= rate(product)/A of this operation*, along with an investigation of the homogeneous model* was to obtain kj and E^. The procedure was to pick any one group of curves which correspond to a particular temperature (those with the same number in Figure 17) and to obtain the calculated yields of the other compounds from the observed yield of methylene chloride• If one looks at the group of curves which are at the far left of Figure 17* one may observe that a yield of Ct^Clg of 0.50 corres ponds to a yield of CgHCl^ of 0.25. This is found by following the CHgClg along the dose rate axis until the CH^Clg curve crosses the 0.50 line of the yield axis. Once there* one remains at that dose rate* in this case -0.58* and finds the intersection between the curve for CgHCljj and that dose rate. Thus a yield of O.25 is obtained. Simi larly the yield of 0.63 and the Cgf^Cl^ yield of 0.13 are found by noting the intersections of those curves with -0 .58* the dose rate which corresponds to the choice of CHgC^ at the temperature (value of B) selected. By using the next group of curves which corresponds to another temperature* a methylene chloride yield of 0.50 is found to correspond to a different dose rate*, 0.02, but still represents the same yields of the other products which are found by observing the intersections of the lines which belong to the second group of curves with 0.02 on the dose rate axis. The same relative yields would be found at a different dose rate for the third or fourth group of curves. Thus, to obtain the calculated yields of any product relative to that for CH2CI2 all that needs to be done is to find the yield of CHgClg on any of the curves which represent that compound and then find the yields of the other products from the other curves in that group. A fit to the experimental data means that Figure 17* and thus that set of C» D* and E* gave the closest correspondence between the observed and calculated yields. Larger or smaller rela tive ratios of C» D» and E gave inferior results. The yields of CHgClg* C^I^Cl^* CgHCljj and CgCl^ obtained in this manner are also presented in Table 8. The agreement between the calculated and observed values is reasonable. Since methylene chloride was picked as the standard* it agrees perfectly. As the temperature decreases* the major defect in the system is that the calculated yields of C2Cl£ do not decrease fast enough. The largest error is for C2Cl£ with an observed yield of 0.30 and a calculated value of 0.JJ-8. This error would have been halved if CHgClg were not taken as the standard. At temperatures below 0° a lack of agreement would be expected for C2Cl£ because reaction *4- is no longer complete but at the upper temperatures such an argument is not valid. The yields of C^H^Cl^ are not in good qualitative agreement. In the calculated results* the yield of C2H2C1^ increases as the temperature decreases while* in fact*, the yield is constant. The observed tetrachloroethane yield is attributable to a great extent to a spur reaction and the correct values which ought to be included in a homogeneous model might be very close to zero. With some changes in the rate constant for its formation* the calculated yields of C2H2C1^ could probably be reduced but that would result in a very unrealistic set of rate constants. The rate constants may be evaluated from Figure 18. yield ( BX/A) x 10 On Figure 18 are a number of curves , equally spaced from one another and of identical shape. These curves were generated from the same data which were used in Figure 17» only they have been replotted. Each of the curves represents the effect of temperature at constant dose rate. The shape of the curves is similar to that in Figure 17. It may be noticed that those curves which represent dose rates that differ by 100 are separated by one full log unit on the B axis. This is further evidence of the earlier statement that, for a con- l/2 stant yield, B/A is constant. The rate constants are determined by observing the intersection of the yields with the curve of interest and reading the B which corresponds to that choice from the abcissa. Because each of the curves in Figure 18 is similar, any of them will serve to give the same value of when the problem is scaled. It is easily shown that the same results from any choice of A and B. If the high dose rate 26° yield of methylene chloride is taken with the lines for A = 1.00, A = 10.0 and A = 100 then three rate constants are obtained, 0.076*. 0.24, and 0.79 respectively# Two absolute 19 numbers are known; the dose rate and kg. The dose rate is 4.85 x 10 *7 10 20 ev/g.-hr. and kg is 5*0 x 10 l/mole-sec. * The computer value for E is 0.0074 c ' V 1 where c is a computer concentration unit and a is computer time. Because both E and kg represent the same rate constant, they can be equated and a scaling factor obtained. Ely -1 -1 7 setting 0.007^ c O = 5.0 x 10 l/mole-sec. the conversion between computer and real second order rate constants may be made by using the factor 1 = 6.76 x 10^ l/mole-sec. 19* H. W. Melville, J. C. Robb, and R. C. Tutton*. Discussions Faraday Soc., 12, 154 (1951). 20. vMelville et al., ibid., 14, I50 (1953). The dose rate may be expressed as the rate of formation of radicals by converting from ev to molecules by multiplying by G(23)/100* from molecules to moles by division by Avogadro's number and from grams of chloroform and hours to moles and seconds by the standard conversions. The dose rate then becomes 18.1 x 10"^ moles/l-sec. The computer dose rate* A* is expressed as a rate also and has the implicit units of c/a. By equating the laboratory dose rate to the ones which have been chosen from the computer, three different conversion factors are obtained. The three logarithmic dose rates, 0 .00, 1 .00, and 2 *00, correspond to computer rates of 1.00 c/a, 10.0 c/a, and 100 c/a respectively. When each of these is set equal to the laboratory rate of 18.1 x 10”^ moles/l-sec. the following conversion factors are obtained: for A = 1.00 1 c/a = -8 18.1 x 10”' moles/l-sec., for A = 10.0 1 c/a = 18.1 x 10” raoles/l-sec., and for A = 100 1 c/a = 18.1 x 10“^ moles/l-sec. By dividing these factors by the rate constant scaling factor, a set of numbers are 2 2 obtained which have the units, 1 c = (some number)(moles/l) . By taking the square root of this type of factor, a scaling factor for concentration may be obtained. The factors are 1 c = 1.6^ x 10“® moles/l, 1 c = 5*18 x 10“^ moles/l and 1 c = 1.64- x 10“^ moles/l for A equal to 1.00, 10.0, and 100 respectively. The computer con centration for CHCl^ is required because the only rate constant conversion factor is for second order reactions while B is pseudo- first order. The above numbers are inverted and multiplied by 12.6, the molarity of CHCl-j at 26°. The resulting computer concen trations are 7.70 x 10® c, 2 ^ 3 x 10^ c, and 7 .70 x 10^ c respectively. If the three B values are divided by the chloroform concentration 78 which is appropriate, the three second order rate constants which result are 1.00 x lO”'1’0, 0.99 x 10"l0» and 1.02 x 10"*° in units of c”^cT^. When the conversion lc'^o”^ = 6.76 x 10^ l/mole-sec. is applied to the above result, is found to be 0.67 l/mole-sec. regardless of the dose rate which is selected. Therefore the posi tion on the dose rate axis may be arbitrarily chosen without altering the rate constants which will be calculated. From Figure 18, the value of B at each of the temperatures and dose rates may be determined. If the high and low dose rate experi ments are considered separately, the same curve (choice of A) may be used for both since the resulting will assume its appropriate o value when the B's are scaled. The B's for the line A = 10 are given in Table 9 as are the k^'s which are obtained by setting the chosen A of 1.00 equal to the high dose rate (expressed in moles of radicals produced per liter-second) and scaling the high dose rate B's as previously described and then letting the A of 1.00 equal the low dose rate and scaling the other B's. TABLE 9 Relative and Absolute Rate Constants (Oq ) Relative rate constants (B»cT^)Absolute rate constants(k^,^'msecT) Temp. High dose rate Low dose rate High dose rate Low dose rate 63 .16 .25 1.40 .77 48 .11 .17 .98 .52 26 .076 .11 .67 .33 0 .040 .067 .35 *20 79 Figure 19 is a plot of log k against l/T with the points arbitrarily- assigned accuracies of tlO$. The result is E^(HDR) = 3»9^ Kcal/mole and E^(LDR) = 3»9g» If the model which was proposed for the chloroform system were entirely correct, no difference would have been found for E^ and, moreover, no differences would have been noted in the rate constants which were found at two dose rates but the same temperature. As long as a homogeneous model is proposed, a rate constant cannot be a func tion of the dose rate. To evaluate how well the system has behaved, the dose rates were found which corresponded to the same rate con stants. The low dose rate yields for methylene chloride are the circu lar points in Figure 17 and have been arbitrarily placed at log A = 0.00. The high dose rate points are placed at log A = 0.91 since the differ ence in the two experimental dose rates was 8.2 and 0.91 is the log of 8 .2 . Since any one of the curves for CHgClg looks like any other of that family, a translation along the abcissa will suffice to create a new curve. The lowest point at 0.00 lies 0.25 units to the right of the B = 0.05 line and corresponds to a certain rate constant— evaluated by a linear interpolation between the two lines which enclose it; in this case B = 0.068— so that any point which lies 0.25 units to the right of the B =• 0.05 line will also represent B - 0.068. If one proceeds 0.25 units to the right of the 0.05 line until the yield of 0.26 (the high dose rate yield) is reached, this will correspond to the dose rate change required to reduce the yield of CHgClg the correct amount at the same temperature. These points are marked as x's in Figure 17* These logarithmic differences are .47» *35* *37» - L o 9|0 0 4 . 0 - - 0.00 0.20 0.20 1.00 90 . .03. 0 .7 3 3.10 . 0 .9 2 gur 9 Lg k Log 19. re u ig F o ds rate dose Low / x 0 ) 103 x ( l/T ih oe rate dose High 30 • 50 .5 3 • 0 .3 3 5 s /T l/ vs 81 and *40 at 0°* 26°» 48°» and 63° respectively. This represents a change in dose rate of approximately 2.5 rather than the 8.2 which was experi mentally observed. This comparison is the clearest test of the model. Although the dose rate dependence is over 200$ off* this seems to make only a slight difference in the observed activation energies which differ little at the two dose rates. On this basis the activation energy appears to be accurate to ^0*5 Kcal/mole* Because the computer has given solutions for X and Y, the steady state concentrations of the radicals may be evaluated. The high dose rate 26° concentrations will be calculated. From Figure 18» the A and B which correspond to this yield are 1.00 and 0.077 respectively. Once all five of the variables of the algebraic equations have been specified* a solution for X and Y may be obtained. Because the chosen value for B is not one which the computer used*, an interpolation must be made. In Figure 20 the values of X/Y have been plotted as a function of B with A = 1.00. If A did not have some value which the computer had used exactly* a double interpolation would have to be made. One of X/Y against A at various B*s and then a second interpolation from the points which were obtained in the first for X/Y against B at this particular A. From Figure 20* X/Y is 0.60 at A = 1.00 and B = 0.077. The rate of production of CCl-j radicals is approximately equal to the dose rate: (in ev/liter-sec.) times GCCCI^) divided by 100NQ . At high dose rate and room temperature G(CCl^) = 7 . The rate of its production then becomes R(p) = 2.35 x 10“^ mole/liter-sec. The rate of reaction of CCl-j* is R(r) =-k^C'CHClgOCCCl^*) + 2kg(CCl-j*)2. 19*20 8 . 2kg has been determined to be 1.0 x 10 liter/mole-sec. and ky may be shown to have the value 3*5 x 10^ liter/mole-sec. from the 82 0.8 0.7 0.6 0.5 >- 0.4 x A « 1.00 0.3 0.2 0.1 0 1.0 2.0 3.0 4.0 B Figure 20. X -f- Y 3*5, B 83 computer ratio of C/E = 0.0052/0.0074 which equals k^/kg. The ratio X/Y is equal to CHCl^/CCl^* so that CHClg* = (X/Y) x CCl^*. The *7 2 rate of reaction is R(r) = 0.60 x 3.5 x 107 liter/mole-sec. (CCl^*) 7 2 17 + 10 x 10 liters/mole-sec. (CClg*) . R(r) then becomes 12.1 x 107 2 liters/mole-sec. (CCl^*) . Equating R(p) and R(r) one obtains that CCly = 1.4 x 10“7 moles/liter. Ey recalling that X/Y - 0.60 the CHCI2 radical concentration may be set at 0.84 x 10“7 moles/liter. Now that all the unknowns have been assigned values» the assump tion that was made earlier in this discussion about the omission of reaction 18 may be checked to find if it is a significant omission. The rate law for C C l y with reaction 18 included is: d(CClg*)/dt = 0 = k^'(Cl*) + k5 »(CHCl2*) -k?(CHCl2 ‘)(CCl3«) - 2k8 (CCl3*) -kl8 (cci3 ‘)(ci2) If this expression is set up as a quadratic in CCl^* it takes the 2 standard form (CCl^*) + b(CCl3#) + c = 0. Here b is [kl8 (Cl2) + k7 (CHCl2 *)]/2k8 and c is -Dq/CCl*) + k^(GHC12 *)]/2kg. k^ = 3*5 x 107 liters/mole-sec.» 2kg = 10 x 107 liters/mole-sec.» k«j* = 7.8 sec.”1 and (CHC12*) = 0.84 x 10-7 moles/liter. k^»(Cl*) gives the rate of initial CCl^* production and is equal to R(23) = -6 1.81 x 10” moles/liter-sec. Figure 13 shows that the steady state Clg concentration is 1*2 x 10**5 moles/liter. The b term is composed of two parts» one of them is the k^ term and the other is the k^g term. The ratio of R(l8) to R(7) is equal to k^gCCl^/k^CHClg*) after cancelling the common (CCl^*) term. This ratio is also equal to the ratio of the G values for the two processes. In this case m G(l8 ) will be G(C12) and is approximately 0.3. The ratio of the G values is G(l8)/G(7) = 0.3/l«5 so that the omission of reaction 18 alters the b term by about 20$. For the moment, the b term then is 1.2k,-,(CHCl *)/2kg. Inserting the proper values b becomes -8 3.5 x 10 moles/liter. The quantity c is 2.46 x 10"^ moles2/liter2. The CC1-* con- o l/2 centration is found from CCl^* = -b/2 + (b -4c) /2. This becomes -1.8 x 10“^ + (12 x 10"^ + 9*8 x 10"^)^^/2. The dominant term in the entire expression is obviously the 4c term in the root. The b2 term in the root is only 1$ of the 4c term and the -b term in front is about 12$ of the entire root. Therefore the contri bution to the CGl^* concentration from reaction 18 can only be 2-3$ and reaction 18 clearly may be omitted from the reaction sequence which was used without any serious effects upon the kinetics. The kj. values in Table 9 are very small but similarly small rate constants have been observed for hydrogen abstractions by 21 organic peroxides in solution. The analogy is far from direct since the molecules involved in the peroxide reactions are much larger than the one carbon radical and solvent which is present in the reaction under consideration but the comparison simply shows that such rate constants are not unheard of. Using the van der Waals b as four times the volume of a chloroform molecule and inserting the resulting molecular diameter in an expression for collision theory rate constant one obtains k^ = I.56 x 10® liter/mole-sec. 21. C. Walling* "Free Radicals in Solution*" John Wiley and Sons, Inc.* New York, N. Y., 1957* P* 422. using Etj = 4.0 Kcal/mole and 299°K. This result combined with the experimentally observed rate constant produces a steric factor> p, a which equals 4 x 10 . This is an extraordinarily small value for such simple reactants. While chloroform is probably freely rotating at temperatures much below those of interest» the presence of dipole-dipole inter- 22 actions may cause some of the molecules to rotate in unison as well as translate in swarms. What this means is a radical may rotate toward a favorable orientation for reaction only to find that the solvent molecule which it was attacking has rotated in a similar manner. This will give rise to an anomalous steric factor which will be smaller than geometric considerations would predict. Such argu ments are weak at best since a radical might be held in a favorable orientation and give rise to a relatively large p factor. In this dipolar medium molecules may hot move in a completely independent fashion. They may move in groups and such behavior will cause the collision frequency to be smaller than what one calculates assuming freely moving spheres. The motion of a molecule towards another may result not in a collision but> only in a displacement of the second molecule. On the simple basis of geometry a small steric factor may be expected. The chloroform molecule may have an electron cloud which looks rather mushroom shaped. The three chlorines with their high 22.- J. Frenkel, "Kinetic Theory of Liquids," Dover Publica tions, Inc., New York, N. Y., 1953 > p» 303. electron affinity and large physical shape should present most of the surface of the molecule along with withdrawing some of the electron density from the carbon-hydrogen bond. The CCI3 group is indeed a strongly electrophilic group. The CCl^ group has a Taft o* value of +2 .65* ^ which shows the increased electron attrac ting power of CCl^ over C E y The o* values are obtained from the logarithm of the ratio of the rates of the acidic and basic hydrolyses of the acetate esters which have the particular group alpha to the carbonyl group. This a* - 2.65 represents a change in rate of 2 k ,5 x 10 which is larger than that for most other simple groups. Thus one might expect a significant reduction of electron density about the C-H bond as well as the diminuation of bond energy which is observed."1. Thus any attacking species might have a difficult time locating a hydrogen atom in this cloud of chlorine. When the attacking species is a bulky one whose odd electron may also be partially hidden and somewhat reduced in density due to the chlorine atoms* one may get a very small steric factor indeed. The reaction of a methyl radical with an isobutane molecule is similar to the reaction being considered inasmuch as the hydrogen which is being abstracted is buried between three large groups although the induc tive effects of the chlorine atoms as well as the bulky abstracting radical are absent. The p for this reaction^ is 2 x 10~^. With 23* R. W. Taft*. Jr.* in “Steric Effects in Organic Chemistry*1* M. S . Newman* ed.» John Wiley and Sons, Inc.* New York, N. Y.» 1956* pp. 660-665. 24. Walling, op. cit.*, p. 5 0 . 25. A. A. Frost and R. G. Pearson* “Kinetics and Mechanism,” John Wiley and Sons, Inc.* New York, N. Y.» 1961* p. 105* the bulky attacking radical which also has a.high electron attrac tive tendency (a* = +1*94)^ the p factor may well be approaching the small value which is found. Thus a very small rate constant has been found the magnitude of which can probably be explained through a combination of the reasons stated above. Having discussed both of the physical models individually one may now compare them directly. From the lack of agreement between the calculated and observed dose rate dependence it may be seen that the homogeneous model does not completely describe the system. The direction of the disagreement is easily understood. The homo geneous model is the most dose rate dependent model which one may propose. A smaller dose rate dependence indicates that the actual system is not quite homogeneous but consists of super-spurs which are very broad but still capable of diffusing towards a neighboring track. The homogeneous model does explain what was mentioned earlier* that the high and low dose rate curves for CHgClg* 02^ 1^* and C2CI6 are parallel. It may be noted that in the region of Figure 17 where the experimental points for CH2CI2 were drawn* the lines for CHgClg are rather parallel also* thus giving rise to a similar reduc tion of G with a change in dose rate at any of the temperatures of interest. This system has been shown to be easily scavengable^ and on that basis it is thought to be relatively homogeneous since a track system is 100 times more difficult to scavenge.^ The homogeneous model adequately explains the log G vs. l/T plot of Figure 14. In order to achieve such behavior, some of the 88 experimental points on Figure 17 must lie above 0 .6 * for above that valuer the vertical differences between two adjacent curves for GHgClg begin to diminish. That is* the temperature coefficient of reaction 5 approaches zero as the yield approaches unity. Ely looking at Figure 17* one may see that the points at low dose rate are higher than those at the high dose rate and that some of the low dose rate points lie farther into the region of diminishing temperature coefficients by virtue of their greater magnitude. This type of behavior would cause the low dose rate experiments to exhibit a higher value but a lower temperature coefficient than those at the higher dose rate. This is essentially what is shown in Figure 14. The agreement between the prediction and the result substantiates the choice of 5*4 as G(23). If a much higher value were adopted* the experimental yields would be very low with respect to the curves in Figure 17• The high dose rate points rather than the low dose rate points would lie in the region of diminishing temperature coeffici ent because as the yield of CHgClg goes to zero the temperature coefficient also goes to zero. A lower value for G(23) would place the experimental points higher on the curves of Figure 17 than they are now and would cause the predicted temperature coefficient to more nearly approach zero. The experimental results of Figure 14 show that nowhere is the temperature coefficient zero so that the choice of 5*^ for G(23) cannot be in great error. The value of G(23) must be larger than the greatest G(CH2Cl2) which is 4.0 and thus that is the lower possible value for G(23). Now a semi-homogeneous model has been proposed. What are the reasons for this behavior? Other systems do not show a dose rate dependence at such low dose rates indicating that these other systems do not have as broad an initial distribution. In hexane» 26 Widmer and Gaumann saw no dose rate dependence for saturated products in the dosage rate region of 1 Mrad/hr. indicating that no appreciable track-track overlap was taking place. Sutton and Rotblat^ studied the ratio of G(Fe+^) to G(Ce+^) in ferrous am monium sulfate and eerie ammonium sulfate solutions and found that the ratio was independent of dose rate below 3.6 x 10^ Mrad/hr,. above which they propose track-track overlap to explain the onset of dose rate dependence. Since the chloroform system is already dose rate dependent at dose rates below 1 Mard/hr. the size of the spur in chloroform must be considerably broader than in water. On this basis an argument may be made that the k^ which is calculated in this work is a lower limit. The presence of localities of high radical concentration* rather than the case where the radicals are uniformly distributed, will favor radical-radical reactions at the expense of radical-solvent reactions. This means that if the irradiated liquid chloroform had a uniform distribution of radicals, rather than the diffuse spur distribution which it actually has, the yield for CH2CI2 would be greater than that which actually was observed. This would require that a larger B would be obtained from Figure 18 and thus a larger k^ would be obtained. Therefore the application of this heterogeneous system to the homogeneous model results in a rate constant which is a lower limit. 26. H. Widmer and T. Gaumann, Helv. Chim. Acta, 46, 944 (1963)* 27. H. C. Sutton and J. Rotblat, Nature, 180, 1332 (1957). If one assumes that the diffusing species in Sutton and Rotblat's work is the H02 radical and that it has a smaller rate constant for reaction with the solute than the H* or OH* do» then Kuppermann*s10 initial conditions of a 10 A spur with 12 radicals/ spur gives good agreement with the onset of their dose rate depend ence* Kuppermann presents the following formula for I» the dose rate where overlap should occur: I = N0w/|i.t.P(t) where NQ is the initial number of radicals in a spur, u> is the energy in ev required for production of a radical, u is the density of the medium, t is the time required for the radicals to have oQ decayed to 10$ of their original value and r(t) is the radius of a spur at time t. If one has a longer time for reaction then one has a correspondingly low dose rate resulting. The reason for a longer time for reaction in a spur in CHCl^ partially stems from the slower rate constant for radical-radical reactions than were postulated for water. Ely using a value which is more nearly correct one would find the onset of dose rate dependence is at a lower dose rate so that the discrepancy between the calculated and observed results would be somewhat less. Along with the longer time one in creases in the radius for the longer the track expands, the larger it becomes. Even with the decreased rate of radical-radical reac tion it seems necessary to postulate that a broader initial distribu tion occurs in chloroform than in water to make up for the difference 28. This 10$ is the arbitrary amount of track-track overlap which Kuppermann designates as sufficient for observing a dose rate dependence* in dose rate which are probably three orders of magnitude. Thus the chloroform is shown to have a broad initial dis tribution by yet another argument. What this must mean is that there are some primary reactions which cause the distribution of radicals to be larger than in the case of water or hexane. The phenomenon of ionization probably has a similar distribution in any medium*, that is the distribution of primary events in chloroform would not be expected to differ appreciably from water. The alter native is that the ions which are formed by ionization are not neutralized rapidly but that they diffuse from the center of the spur and are neutralized at some distance from their point of creation. The only way that this can occur is if the negative species in solu tion is much less mobile than is an electron. In hexane the neutrali zation reaction which forms reactive intermediates involves electrons while chloroform is a molecule which might not be expected to show this behavior. Several workers^ have observed electron attach ment in chloroform and still others have noted that electron capture in CC1^» a very similar molecule* is dissocative and requires no 31-33 activation. One might, by analogy, expect chloroform to behave similarly. The analogous reaction is CHCl^ + e“ - GHC12 * + Cl" (2!±) This reaction would create a broad distribution of dichloromethyl Z 9 I T . 0. Lee* J. Phys. Chem., 67* 360 (1963). 30. E. P. Bertin and W. H. Hamill*. J. Am. Chem. Soc.* 86* 1301 (1964). 31• M. Reese* V. H. Diebler, and F. L. Mohler* J. Res. Nat. Bur. Stds.«. 57* 113 (1956). 32. W. M. Hickham and D. Berg* J. Chem. Phys.*. 22* 517 (1958). 33* R. E. Fox and R. K. Curran* ibid.* 1595 (1961). 92 radicals compared to the initial distribution of ionization events. The chloride ion must not have a mobility which allows rapid reac tion with the positive species for rapid reaction will cause the concentration of radicals in the spur to be large and thus create an unscavengable yield much larger than is seen. The nature of 3^»3 5 the positive species is not clear. The mass spectrum of CHCl^ is complex and shows the parent ion to be 2-4$ of the main species- + 36 CHCI2 • Considerations of quasi-equilibrium theory lead to the prediction that the parent CHC1^+ ion will be much more important in a system where the collision time is shorter. In a liquid» one has a very short collision time and hence a large probability of stabilizing a CHC1^+ before it would have the opportunity to uni- molecularly decompose. The last reactions to be considered are also the most difficult to discuss since there is no evidence to substantiate them. The last ionic reactions must involve CHCl^+ » CHClg** and Cl"" in some sort of neutralization scheme. There are at least four types off neutralization possible; association* dissociation* electron trans- fer,. and proton transfer. In this system these become: CHC13+ + Cl” - CHCl-j + Cl* (25) - CHCl£ + Cl2 (26) - CC1* + HC1 (27) 34. "Mass Spectral Data," American Petroleum Institute Pro ject 44, Nat. Bur. Stds., Washington, D. C., No. 604* 691-2. 35. D. L. Hobrock and R. W. Kiser, J. Phys. Chem., 68, 575 (1964). 36 . D. P. Stevenson, Radiation Research, 10, 6l0 (1959)* 93 CHCl/ + Cl“ - CHClj (28) ** C11C12* + Cl* (29) -* CC12 1 + 1101 (30) It is not presently possible to discuss these reaotlona In detail since neutralisation schemes ar© not known. However* some comment about them is possible, lie actions which involve the parent ion are somewhat more plausible since the formation of the CHCljg* species will also form Cl atoms in the spur. These Cl atoms will react with the solvent and produce CCl^ radicals. Werner and Firestone* have already accounted for the CCl^* production in the spurs via reaction sequence 2, 9» and 10 so whatever the contribution of reactions involving the CHClj>+ species* an unexplained yield of CCl^* should result. Using an activation energy of 3*3 Kcal/mole for reaction 4*^*^ one finds that the exponential term at Z$°C is 1/250. Using the classical interpretation of the exponential term as the probability of a collision having sufficient energy to react, one concludes that* with a atoric factor of unity* one out of 250 collisions ia successful. If one assumes that the diffusion coeffl- 14 cient for Cl atoms in CHCl^ la similar to that for I* in CCl^* in serts this in the equation^ where x is the mean-square displacement, T is 250 times the col- -13 llslon frequency (assumed to be 10 sec.)* one obtains that the 37# J. H. Knox, Trans. Faraday 3oc.* £0* 275 (1962). 38. P. Q. Ashmore and M. 3. Spenoer, ibid.» 6 0 * I.608 (1964). 39* ‘'Physical Chemistry," D. F. ISggera, Jr., John Wiley and dons* Inc.* New York, N. Y.» 1964* p. 397# minimum distance a Cl atom would travel before reacting is approxi mately io Angstroms. Compared to the initial distribution of these eventsi this is not a very large distance but the presence of a small steric factor may allow for more diffusion. Each of the two positive ions is necessary to explain various aspects of the chloroform radiolysis. As was mentioned earlier* no radical reaction seems plausible for the production of chlorine. Reaction 26 seems like a reasonable one. The analogous reaction involving a CHC12+ is not included for it would be accompanied by the production of a CHC1: diradical. There is no substantial evi dence for the formation of that species although it may precede the C?HC10 which is observed. 3 i Werner and Firestone calculated the amount of CCl^* produced in the spur and found excellent agreement with the observed yield, assuming that the formation of C2HC1jj is entirely due to reaction 7 and not reaction 12. Both in their paper and in their work, the , reaction sequence 11-13 has been supported. In order to preserve the calculations of Werner and Firestone, it must be postulated that reaction 3 does not occur in the spur but occurs in a more homogeneous area so that the CClgJ species may be scavenged. This requires that the formation of the CCl£! be ionic rather than pro ceeding from an excited chloroform molecule because excitation is probably associated with the spur. Thus reaction 30 is included as the logical precursor of the CC^s* Reaction 27 cannot be the predominant neutralization reaction for such a reaction should not have a temperature coefficient while a positive temperature coefficient for the production of HCl is seen below 0°. One must have a reaction which produces chlorine atoms such as reaction 25 or reaction 29. The occurrence of reac tion 27 could explain somewhat the low activation energy which Werner and Firestone obtain for reaction 4, however, the yield of HCl at -196° is 0.71. If this were all attributed to reaction 27 the ap parent activation energy would not change much. If the yield of HCl of 5*4 is taken to represent the primary decompositions of the chloroform and if the spur yield of 1.1^ is attributed to excitation reactions, one has a G of 9.7 for CHCl^ molecules destroyed by ionization or by radicals which are produced by ions. If GCClg^ = 0.25 is used for G(26) and if G(CCl20 = 0.41 is a measure of G(30) then one has left 7*9 since both reac tions have resulted from or will produce the destruction of three molecules of chloroform. If reaction 25 were the only other mode of neutralization, G(e”)9J, would be 4 since the chloride ion of reaction 25 represents one molecule of chloroform decomposed and the chlorine atom which results will decompose another through reac tion 4. If reaction 27 or 28 is the neutralization mode the same arguments will lead one to Qr 28* ^ reaction 29 is considered> G(e‘)29 becomes 2.0 since four chloroform molecules are involved in the entire reaction sequence. Through any combination of these reactions one will, obtain that the yield of ions for all steps ex cept reactions 27 and 30 is between 2.0 and 4.0. Including the 0.2 of reaction 26 and 0.4 of reaction 30 the total ionization is calculated to have a G value between 2.6 and 4.6, A criticism of the above mechanism might involve the omission of the CHCl^" species. Using the same arguments which were used earlier about the stabilization of a parent ion with an increase in pressure one might conclude that the parent anion ought to be stable. Consideration of the solid phase data of Werner and Fire stone"^ indicate that this may be the case in the solid phase. Some of their data are presented in Table 10. TABLE 10 100-ev." Yields in Solid Chloroform — Temperature (°C) -196 -125 -90 -78 -67 CH2C12 0.80 0.90 1.05 0.86 0.79 - CCl^ 0.61 0.80 0.87 0.81 0.88 — Data of Werner and Firestone* cf. Ref. 1. The similarity of the yields of CHgCl.? and CCl^ and the temperature independence leads to the conclusion that the reaction which forms these two products is chci3+ + CHCl^- - ch2ci2 + CCl^ (31) The increase of the CHgClg yield from 0.23 at -62° in the liquid phase to 0.?9 at -67° in the solid shows that some reaction is occurring in the solid which did not take place in the liquid. Thus one of the participants in reaction 31 must not be present in the liquid phase. Since there is little probability of having no parent ion in the liquid phase the conclusion must be that reaction 24 occurs efficiently in the liquid. Little is known about the yields of ions in the liquid phase. Extensive work on water has given the yield of solvated electrons 40 iti as G(e*“ ) = 2.7. Geissler and Willard have studied the decom- aq position of methyl and ethyl iodide in liquid pentane and conclude *» | i O that in pentane G(e”) > 6. Freeman has made conductance measure ments on liquid hydrocarbons under radiolysis and has concluded that the G value for free electrons is approximately 0.2 and that c low (ca. 10 mole fraction) concentrations of electron scavengers 4 3 do not affect this yield appreciably. Samuel and co-workers have qualitatively observed large pulses of electrons in liquid hydro carbons under gamma radiolysis again showing the presence of electrons in liquids during radiolysis. All of this evidence is varied and does little to augment or dispute the yield of electrons which is calculated above. 40. J. Rabani, W, A. Mulac, and M. S-* Matheson,. J. Pbys. Chem. , 62, 53 (.1965). 41. P. R. Geissler and J. E. Willard, J. Am. Chem. Soc., 84, 462? (1962). \ 42. G. R. Freeman,. J. Ghem. Phvs., 39, 988 (1963). 4 3 . A. H. 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