Refractive Indices and Dispersions of Volatile Compounds of Fluorine and Boron

REFRACTIVE INDICES AND DISPERSIONS OF VOLATILE COMPOUNDS OF FLUORINE AND BORON.

Carbon Tetrafluoride, Nitrogen Trifluoride, Fluoroform, Carbon-Nitrogen Compound of Fluorine (CF3N)2, Boron Hexahydride, and Nitrogenous Boron Hydride B~NaH~.

BY K. L. RAMASWAMY. (From the Departments of General Chemistry and Phys,cs, Indian Institute of Science, Bangalore.)

Received November 23, 1935. (Communicated by Sir C. V. Raman, Kt., F.R.S, I~.L.)

I~r an earlier communication,1 the results of measurements of dielectric coefficients of the above-mentioned gases at different temperatures have been recorded. Simultaneous measurements of refractive indices and dispersions on the same specimens of gases were carried out, with a view to compare the electronic polarisations at long wave-lengths with the corresponding dielectric polarisations. From the data obtained, the atomic polarisations for these gases have been calculated as will be explained subsequently. The apparatus, method of measurement, and the procedure adopted were practically the same as described in a separate communication by Watson and Ramaswamy.* One additional precaution to make sure of the zero reading of the compensator was taken. For this purpose the reading when both the gas tubes were empty was first taken, the gas was then introduced and fractionated if necessary and the bands counted until zero pressure in each case. The last stage of pressure reduction was done by slow condensation of the gas either by using a liquid air or a solid carbon dioxide bath. As in the earlier investigations,2 low pressure counts for one or two other wave-lengths were made to enable easy computation of the total number of bands. All measurements were made at or nearly at the prevailing laboratory temperature. The details concerning the methods of purification and the limits of purity have already been given in the earlier communication?

1 K. L. Ramaswamy, Proc. Ind. Acad. Sci., A, 1935, 4, 364-377. 2 H. E. Watson and K, L. Ramaswamy, "Refractive indices and dispersions of gases," (under publication). 630 Re/mctive Indices ~ Dispersions o/ F/zwrine dr Boron Compou~Ms 631

Results. Since details of calculation illustrating the accuracy of the results will be given in another communication,~ it seems unnecessary to repeat them here. In the following tables I-VI, the values of (n-I) x 106 (where n represents the refractive index) calculated to 760 millimeters pressure and 25 ~ .0 Centigrade and corrected for compressibility, have been given. Column 1 gives the wave-lengths A x 10-8 in centimetres corrected to vacuum, 2 tile

TABLE I. Carbon tetrafluoride.

A• -s (n--l) obs. (n-l) cal. Difference t, Pm and comp.

6440.24 440.87 440.87 0.0

5462.25 442.58 442.53 +0.05 t=27~ C.

5087.23 443.46 443.44 +0.02 pm=342 ram.

4801.25 444.29 444.28 +0.01 comp. -= O. 66 ~ 4359.56 445.94 445.94 0.0

The dispersion for carbon tetrafluoride can be summarised by the Sellmeier 9 -9384 • 1027 formula n--1 • 106 = (22759 "6 • 102~ --v2)" TABL~ II. Nitrogen trifluoride. i A • 10 -s (n--l) obs. (n--l) cal. Difference t, Pm and eomp. !

6440.24 436.73 436.75 --0.02

5462.25 439.11 439.11 0.0 t=27 ~ .0 C.

5087.23 440.43 440.42 +0.01 Pm= 403.5 ram. 4801.25 441.66 441.63 +o .03 comp. ----0 .67%

4359.56 444.00 444.01 --0.01

The dispersion for nitrogen trifluoride is best expressed by the Sellmeier 6.8443 • 1O~7 formula n--1 X 10 e ---= (15888 • 102~" 632 K. L. Ramaswamy

TABLE III. Fluoroform.

• -s (n--l) obs. ] (n--I) cal. Difference t, Pm and comp.

6440.24 433.33 433.34 --0.01

5i62.25 435.38 435.37 +0.01 t~27 ~ .0 C.

5087.23 436.48 436.48 0.0 pt~=334.0 mm.

4801.25 437.51 437.52 --0.01 eomp.=O .77%

4359-56 439.52 439.55 -0.03

The results of fluoroform are best expressed by the Sellmeier formula 7.8660 • 10 ~7 n--1 • 108 = (18368.8• TABLE IV. Carbon-nitrogen compound o/fluorine (CF3N)2.

• 10 -s (n--l) obs. (n--l) cal. Difference t, Pm and comp.

6440.24 997.52 997.47 --0.05

5462.25 1002.51 1002.47 --0.04 t=28~ (7.

5087.23 1005.25 1005.31 +o .06 p~==310.0 ram.

4801.25 1007.80 1007.77 - 0.03 comp.= l . 7,~%

4359.56 1012.81 1012-83 +0.02

The results of dispersion are best summarised in the expression 16.9275 • 1027 n--1 • l0 s = (17186.5 • 10~V~- v2)" observed value of (n--l) x 106, 3 the value calculated from the dispersion formula given at the end of each table, 4 the difference between the observed and the calculated values of (n--l) • l0 s. In the last column are mentioned the actual temperature t of measurement in degrees Centigrade, the mean pressure Pm in each experiment and the compressibility of the gas investigated expressed as a percentage correction for a mean pressure of 380 millimeters or for a change of pressure from 0 to 760 millimeters. Refractive Indices dY Dispersions of Fluorine dY ]3oron CompomMs 633

TABLE V. Boron hexahydride.

• 10 -8 (n-- 1) obs. (n--l) cal. Difference t, Pm and comp.

6440.24 810.53 810.53 0.0

5462.25 818.22 818.32 --0.10 t=25 ~ .0 C.

5087.23 822.70 822-63 +0.07 pro=202 "5 mm.

4801.25 826,65 826.66 --0 .Ol comp. =0.97%

4359.56 834.63 834.63 0.0

The dispersion can be summarised in the Sellmeier form by the expression 7 -1942 • 1027 n--I • 10 e = (9092.7 • i0"7-- v2)"

TABLE VI. Nitrogenous boron hydride.*

)~• I (n--I) obs. (n--l) cal. Difference t, Pm and comp.

6440.2 1275.7 1276-0 --0.3

5462.3 1291.4 1291.4 0.0 t=24 ~ .8 C.

5087.2 1300.1 1300.0 +0.1 pm-~8" 5 ram.

4801.3 1308.2 1308.0 +o.2 comp.= 40.4%

4359.6 1323.7 1324.0 -o .3

* Tentative. The results are expressed by the Sellmeier formula 9.0178 X 1037 n--1 • 10 6 = (7284.3 • 1027 - v2)"

The usual Sellmeier formula was employed to summarise the results of dispersions and in all cases a single term proved sufficient, viz., n--1 = c/vo2-v ~, where c is a constant, v0 and v are the free frequencies of the dispersion electrons in a molecule and of the incident radiations respectively. 634 K.L. Ramaswamy

v The value of v is obtained from the expression u -- ~ where V is the velocity of light and h the wave-length of the incident radiation. The individual values call for a little comment. The two gases carbon tetrafluoride and nitrogen trifluoride have been previously examined 2 and the results given as somewhat provisional. The present value for carbon tetrafluoride is about 2.7 per cent. lower than the value previously given ~ and it may be mentioned that Klemm and HenkeP have obtained a value about one per cent. higher than the present value. The case of nitrogen trifluoride is interesting. The value of refractive index for the mercury green line 5462 ~_, obtained in the present investiga- tion is about 0-9 per cent. lower than the figure given in an earlier communication. * If one assumes an impurity of about 2 per cent. of air in the present sample of gas, the discrepancy can be easily explained. But then a difficulty arises with regard to the value of the dielectric coefficient, which according to this assumption should be about 1256 (calculated from the value obtained with the previous sample of gas) as against the value of 1220 given in the previous paper. 1 It appears, therefore, that probably traces of other fluorides having nearly the same value of refractive index but differing widely in the values of dielectric coefficients, are generated in quantities depending on the method and conditions of preparations. 4 For the other gases, measurements by other authors do not seem to have been made. It is necessary to mention that the values for the nitrogenous boron hydride are given with reserve and need further confirmation. Electronic and Atomic Polarisations. The wave-lengths at which the measurements of dielectric coefficients are measured, can be considered infinite compared with the wave-length of light employed in the measurements of refractive indices. If one considers the effect of radiations of very long wave-lengths on the refractive index, the Sellmeier expression approximates to the form n--1 = V02_co since v becomes practically zero. In the following table vn, the electronic polarisations (represented by PE) have been calculated using the Lorenz-Lorentz relation n2-1 . M n2+2 -~- = M P~:, and the values of noo--1 obtained as explained above. Since ~- is the molecular volume which for an ideal gas is 22.412 litres at N.T.P. and n ~ + 2

a W. Klemm and P. Henkel, Zeit. Anorg. und Angew. Chem., 1933, 213, 115-125. 4 Otto Ruff and L. Staub, Zeit. Anorg. Allgem. Chem., 1931, 198, 32-8. Refractive Indices d_r Dis#ersions of Flztorine d_~ Boron Compounds 635 for most gases is not far from 3, the value of PE at 25 ~ .0 Centigrade is easily obtained by multiplying nee2 _ 1 by the factor .008155. The correction when n 2 +2 exceeds 3, has been made wherever necessary. In column 2, the values of distortion polarisation PI) (represented by A in tile Debye's equation) given in the previous paper 1 are reproduced. The columns 3 and 4 give the electronic polarisation PE and the atomic polarisation PA (the difference between PI) and PE) respectively.

TABLE VII. Values of PI), PE, PA.

Substance PD PE PA

Carbon tetrafluoride 9.72 7.12 2.60 l~itrogen trifluoride .. 9.08 7 -03 2.05

Fluoroform .. 8.86 6.98 1.88

Compound (CF~N)2 .. 21.35 16.06 5 -29

Boron hexahydride .. 14.46 12.91 1.55 l~itrogenous boron hydride* 23.76 20.18 3.58

* Tentative. Discussion. The figures given in Table VII definitely indicate that the atomic polarisations in the case of all fluorine compounds investigated, are surprisingly large being about 30 per cent. of the corresponding electronic polarisations. No clear relationship can be traced between molecular structure and atomic polarisation. It may be mentioned that the data from a systematic study of both dielectric and optical polarisations, are not sufficiently extensive for attempting to establish any relationship between PA and P~;, although it is generally known that PA increases with the size of the molecule. It has been the usual practice in the approximate calculations of dipole moments, to take PA as 10-12 per cent. of PE. This, however, will not be far from correct when the moment is sufficiently large, say of the order of one or more Debye units. From the results given in this communication, it appears that such an assumption is not applicable to the case of fluorine compounds and complex compounds of boron. 636 K.L. Ramaswamy

It is, therefore, intended to extend these investigations to more compounds and then critically examine the results to see if any definite relation_ ships can be traced. Summary. 1. The refractive indices and dispersions of carbon tetrafluoride, nitrogen trifluoride, fluoroform, nitrogen-carbon compound of fluorine (CF3N)2, boron hexahydride and nitrogenous boron hydride BsNsH~, have been measured at the ordinary temperature using a Zeiss Interferometer of the Rayleigh type. 2. The compressibilities of the gases mentioned have been determined by studying th~ variation of refractive index with pressure. 3. The atomic polarisations of these molecules have been calculated from the dielectric and optical polarisations. It has been found that the fluorine compounds and the complex compound of boron B3NsH6, have appre- ciable values of atomic polarisations. Conclusion. In conclusion I wish to acknowledge my indebtedness to Prof. Dr. Otto Ruff, Breslau, and to Prof. Dr. Alfred Stock, Karlsruhe, for the gift of the fluorine and boron compounds respectively. My grateful thanks are due to Prof. Sir C. V. Raman, Kt., F.R.S., N.L., for his kind encouragement and keen interest in the work.