Photosensitization by Cd(³P₁) Atoms. II. Gas Phase Decomposition of Cyclohexane

Abstract

The photodecomposition of cyclohexane sensitized by Cd(3Pl) atoms has been studied in the vapor phase at 355 "C. The primary decomposition gives hydrogen atoms and cyclohexyl radicals. The volatile products of the decomposition are H,, cyclohexene, propylene, ethane, ethylene, methane, propane, butadiene, and methylcyclopentane. Products other than H2 and cyclo-C6Hlo arise from unimolecular reactions of cyclohexyl radicals, the most important such process being the production of propylene and allyl radicals. Hydrogen yields decrease rapidly with time because of H-atom scavenging reactions involving olefinic products. The quantum yield of molecular hydrogen formation at the shortest exposure time examined is 0.53.La photodecomposition du cyclohexane sensibiliske par des atomes de Cd(3Pl ) a kt6 Ctudiee en phase vapeur a 355 'C. La dkomposition initiale donne des atomes d'hydrogene et des radicaux cyclohexyles. Les produits volatils de la d~com~osition sont Hz, le cyclohexene, le propyl&e, I'bthane, ~'~t h~l e n e , le mbthane, le propane, le butadikne et le mbthylcyclopentane. Les produits differents de Hz et du cyclo-C6Hlo proviennent de reactions unimolkulaires a partir des radicaux cyclohexyles; le processus le plus important etant la formation de radicaux propyknes et allyles. Les rendements en hydrogene diminuent rapidement avec le temps par suite des rbactions qui piegent les atomes d'H et qui impliquent des produits olefiniques. Le rendement quantique correspondant a la formation d'hydrogene molbculaire pendant le temps d'exposition examine le plus court est de 0.53.

Reaction of excited cadmium(3PJ) atoms with alkane hydrocarbons. Quenching cross sections and primary cadmium hydride yields

“…The lower yield of CdH may result from competition between exit channels 4a and 4b at the higher energies reached with the abstractive mechanism, especially with the possibility of unimolecular energy contributions from the more complex c-C6H12 molecule. 13 Indeed, Knight and co-workers12,13 observed that extrapolation of H2 yields in Cd(3Pj)-photosensitized decomposition of both cyclopentane and cyclohexane to very low conversions led to H2 quantum yields approaching 1.0. Continuing this line of argument, it is also possible to understand why no HgH is formed in the reaction of Hg(3P0,i) with alkanes, since the total energies available are much higher, and the HgH bond energy is only 8.6 kcal as compared to 15.6 kcal in the CdH bond.…”

Photosensitization by Cd(3P0,1) atoms. III. Gas phase decomposition of cyclopentane

“…In a previous paper (1) it was shown that the deconlposition of cyciohexane by triplet cadmium atoms at 3261 A generally exhibited the same characteristics as the analogous triplet mercury sensitized system. Very few cadmium sensitized systems have been examined, however, and it is inlportant t o establish whether this similarity extends t o other cyclic hydrocarbons, as well as t o examine the cyclopentane system in its own right.…”

“…The apparatus and procedures have been described previously (1). The cadmium-containing reservoir attached to the reaction cell was maintained at 315 "C, while the cell itself was at 355 i 2 'C in the experinlents at fixed temperature.…”

Cadmium photosensitized reactions

“…Kalra and Knight used a commercially available cadmium spectral lamp and obtained an intensity of about 4 × 10 -3 t~E rain -x at 326.1 nm. 23 We have made quartz and Pyrex glass discharge lamps, in which a few pieces of cadmium metal and a few Torr of argon were sealed. These lamps were operated at 200-300°C with 1000 V AC and gave about 0.5 t~E rain-1.…”