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The Primary Process in Formaldehyde Photolysis

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The Primary Process in Formaldehyde Photolysis This dissertation has been 65-3890 microfilmed exactly as received McQUIGG, Robert Duncan, 1936- THE PRIMARY PROCESSES IN FORMALDEHYDE PHOTOLYSIS. The Ohio State University Ph.D., 1964 Chemistry, physical University Microfilms, Inc., Ann Arbor, Michigan THE PRIMARY PROCESSES IN FORMALDEHYDE PHOTOLYSIS DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of the Ohio State University By Robert Duncan McQuigg, B.S. ****** The Ohio State University 1964 Approved by Adviser Department of Chemistry ACKNOWLEDGMENT The author wishes to express his appreciation to Professor Jack G. Calvert for suggesting this problem and for his encouragement and instruction during the course of the work. Sincere appreciation is extended to the Petro­ leum Research Fund Advisory Board of the American Chemical Society for the financial support provided under PRF Grant 532-A. ii TABLE OF CONTENTS Page INTRODUCTION ................................... 1 APPARATUS............... 12 Vacuum s y s t e m ............. ................ 12 Photolysis system ......................... 12 Flash discharge system...... ................ 17 EXPERIMENTAL PROCEDURES ......................... l8 Reagents and standards............. 18 Spectral purity of formaldehydes .............. 22 Procedures for typical experiments ............ 25 Procedures for typical experiments with mixtures of CHgO and CDgO ............ 28 DATA AND R E S U L T S ................................. 31 Temperature calibration of reaction cell . 3 I Calibration of filters and cell transmission . 3 I Data from the flash photolysis of CHgO, CDgO, and C H D O ..................... 33 Data from the flash photolysis of mixtures of CHgO and C D g O .............. 38 DISCUSSION OF RESULTS ............................. 46 Photolysis of the pure formaldehydes........... 46 CHgO and C D g O ............................. 46 iii TABLE OF CONTENTS (CONTD.) Page C H D O ' . 57 Photolysis of mixtures of CHgO and CDgO . 60 The rate of photodecomposition of the pure aldehydes In the mixtures....... 71 The effect of temperature on the photodecomposition of mixtures ....... 78 Variation of photodecomposition as a function of wavelength ................... 80 Estimation of values of 0^ and 0jj- as a function of wavelength ................... 89 Calculation of theoretical 0-i/'0ij ratios . 95 BIBLIOGRAPHY............................... 10 4 AUTOBIOGRAPHY ........................... 107 Iv LIST OF TABLES Table Page 1. Photolysis of CHgO alone with Pyrex jacket filter at 80° C .................. 3^ 2. Photolysis of CD2O alone with Pyrex jacket filter at 80° 0 .......... 37 3 . Photolysis of CHDO at 80° C .............. 39 4. Photolysis of mixtures of CHgO and CDgO at 80° C, no filters............... 4l 5. Photolysis of mixtures of CHgO and CDgO at 80° 0, with a Pyrex jacket filter........ 42 6 . Photolysis of mixtures of CH2O and CDgO at 80° C, with a Pyrex jacket and one plate glass f i l t e r ........................... 43 7 . Photolysis of mixtures of CH^O and CC^O at 80° C, with a Pyrex jacket and two plate glass filters........................... 44 8. Photolysis of mixtures of CHgO and CDgO at 60^ and 100°, with a Pyrex jacket and two plate glass filters ................... 45 9. Mass spectra of CHDO and CH2O ............. 59 10. Results from the photolysis of mixtures of CHpO and C D g O ............. 63 11. Effect on photodecomposition of one aldehyde by another..................... 76 V LIST OP TABLES (CONTD.) Table Page 12. Average values of (P^^), (P2 ), and for mixtures of CHgO and CD^O . 79 13 . Estimation of and 0'^^ as a function of wavelength for C H g O .................. 91 14. Estimation of 0^ and 0^^ as a function of wavelength for C D g O .................. 93 1 5. Estimation of as a function of A by unimolecular decomposition rate theory for C H g O ......................... 99 1 6. Estimation of 0j/0jj as a function of A by unimolecular decomposition rate theory for CHgO, "bands" 5 and I5 assumed correct ................... 102 Vi LIST OP FIGURES Figure Page 1. Flash photolysis system and accessory equipment................. 13 2. Monomeric CHgO preparation apparatus .... 19 3. IR spectra of the formaldehydes............ 23 4. Transmission of glasses.................... 32 5. Total measured volume of products as a function of the aldehyde pressure for the flash photolysis of CHgO and CDgO alone, one 10,000 volt flash, t = 80° C . 51 6 . Total measured volume of products as a function of the flash voltage squared, CHgO pressure, 21 m m ................... 56 7. Effect of CDgO on the photodecomposition of C H g O ................................. 73 8. Effect of CHgO on the photodecomposition of C D g O ................................. 75 9. Relative absorbed intensities of CHgO .... 82 10. Relative absorbed intensities of CD^O .... 84 vii INTRODUCTION The photochemical decomposition of formaldehyde has been studied by several investigators under a variety of conditions. It has been shown that two primary processes are involved: HgCO + hi/ ►HCO + H (1) CO + Hg (11) At first, researchers were convinced that (ll) was the only important primary process at wavelengths of ab­ sorbed light greater than 2750 A., the onset of the predis­ sociation limit of the absorption spectrum. This was the conclusion reached by Norrish and Kirkbride,^ who studied the decomposition at 110° in the three wavelength ranges 2540-2800 A., 3030-3130 A., and 3340-3650 A. They ob­ tained an average quantum yield for decomposition in all three regions of about 1.0. This was interpreted to mean that no free hydrogen atoms were present in the system, for they assumed a chain decomposition of formaldehyde would result if H atoms were present. Thus, in the pre- dissociation region spontaneous dissociation occurred, whereas above 2750 A. in the region of fine structure the process involved activation followed by collision: CHgO* + X = Hg + CO + X (1) 1 2 ^ Patat and Patat and Locker Investigated the re­ action in the presence of oxygen. They reasoned that if hydrogen atoms were produced following light absorption, the ratio of CO/Hg would no longer be near unity, since the hydrogen atoms would react with oxygen and eventually form water. At 80°, they found the ratio was unchanged at wave­ lengths above 2750 A., but changed to as much as 1.5:1 below 2750 A. They concluded that above 2750 A., either the Norrish and Kirkbride excited molecule mechanism pro­ duced hydrogen and carbon monoxide, or that some intra­ molecular process was taking place. A different interpretation of the process occurring below 2750 A. in the CHgO - Og system was introduced by 4 Carruthers and Norrish. Their photoxidation of formalde­ hyde at 100° using a full mercury arc resulted in the for­ mation of formic acid, which then decomposed as follows: HCOgH » Hg + COg (2) ------ > HgO + CO (3) In these experiments, the direct photodecomposition of formaldehyde was small. Therefore the increase in the CO/Hg ratio found in Patat's work may not have been due to a loss of hydrogen to form water, but could arise in­ stead from the two preceding reactions. Norrish and Carruthers reported quantum yields for formic acid formation via a chain oxidation of 12.6 and 9*0 for CHgO/Og ratios of 1:1 and 2:1, respectively. It was shown at about the same time that formaldehyde photodecomposition was strongly temperature dependent. By varying the temperature at several constant pressures, 5 Akeroyd and Norrish obtained data which indicated that a chain process was active, thus pointing to the occurrence of (I), at least to some extent. Using light from a full mercury arc, they found, as an example, a chain length of 100 at 350° and 100 mm of formaldehyde. Gorin^ studied the CH^O - Ig system at the wave­ lengths 2537 A., 3130 A., and 365O A., all at 100°. The formation of HI at these wavelengths confirmed that (l) definitely occurred. His work also gave some insight into the variation of the two primary processes with the wave­ length of absorbed radiation. A very low rate of hydrogen production was obtained at 2537 A. and 3130 A., whereas, at 3650 A., the rate of Hg formation was considerable, in ad­ dition to HI. Both (l) and (II) then must be occurring at this higher wavelength. The investigation by Parkas e_t aj^, of the photode­ composition of solid formaldehyde (presumably monomeric) at 85° K does not support the evidence for the occurrence of primary process (l). Thermal conductivity measurements on the hydrogen resulting from irradiating a small deposit Ox CHgO with a full mercury arc, showed it to be 25^ . para-Hg, identical to the composition at normal tempera­ tures. These authors suggest that if free hydrogen atoms had been formed, then by the reaction: H + HgCO = Hg + HCO (4) a higher percentage of para-Hg should have been found at the low temperature. Instead, the 25^ para-Hg implies that the two atoms forming a molecule come from the same molecule of formaldehyde. Even so, in the solid state, hydrogen atom formation could be followed by: H + HCO = Hg + CO (5) where the formyl radical is a fragment of the same parent molecule of the hydrogen atom. Also, only one experiment using this technique on formaldehyde was reported. It ap­ pears that these results are inconclusive. Leighton et al.,^ applied the Paneth mirror free radical test to several organic molecules in the vapor phase. For formaldehyde using tellurium mirrors, they demonstrated that free hydrogen atoms are formed at 2750 A. and 3160 A. With little doubt remaining concerning the occurrence of both (I) and (II) as primary processes, investigations turned toward solving the kinetics of the overall photodecomposltlon. In particular, the energetics of the decomposition were not known to any degree of certainty. The dissociation energy of the two C-H bonds in formalde­ hyde is known to require 105 kcal./mole:^ HgCO = 2 H + CO A Hgng = 105 kcal. From his study of the CHgO - Ig system, Gorin^ had determined a value of 78 kcal./mole for the dissociation energy of the first C-H bond, arising from the fact that a quantum of 3^50 A.
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