Vacuum-Ultraviolet Photolysis of Solid Propane at 20° and 77°K

Abstract

The photolysis of CD₃CH₂CD₃, CH₃CD₂CH₃, and C₃H₈—C₃D₈ mixtures has been investigated at 1470 Å (8.4 eV), 1236 Å (10 eV), and 1048–1067 Å (11.6–11.8 eV) at both 20° and 77°K. On the basis of the isotopic analysis of the products it is concluded that the following primary fragmentation processes occur: $${\mathrm{C}}_3{\mathrm{H}}_8\mathrm{+hv\to }{\mathrm{C}}_3{\mathrm{H}}_7+\mathrm{H}$$ $$\to {\mathrm{C}}_3{\mathrm{H}}_6+{\mathrm{H}}_2$$ $$\to {\mathrm{C}}_2{\mathrm{H}}_6+{\mathrm{CH}}_2$$ $$\to {\mathrm{C}}_2{\mathrm{H}}_5+{\mathrm{CH}}_3$$ $$\to {\mathrm{C}}_2{\mathrm{H}}_4+{\mathrm{CH}}_4.$$ Hydrogen, methane, and ethane are mainly formed by elimination processes from a single carbon atom which results in the simultaneous formation of a carbene in each case. In this respect the primary processes occurring in the solid phase are the same as those reported to occur in the gas phase. The elucidation of the primary processes in the condensed phase is facilitated because of the inhibition of secondary decomposition of the internally excited fragments formed in Processes A to E. For instance, the ethylidene and propylidene rearrange quantitatively to ethylene and propylene, respectively. The occurrence of geminate disproportionation and combination leads to difficulties in determining the relative quantum yields of the primary processes A and D. It could, however, be concluded on the basis of isotopic analyses that the C–H and C–C cleavage processes, A and D, increase in importance when the energy of the incident photon is increased. Although propane ions, which have an ionization energy of 11.07 eV in the gas phase, must be produced at 1048–1067 Å, no concrete information could be obtained as to the fate of these ions in the solid phase. The data, however, show that decomposition of the superexcited molecules may be the major source of the hydrocarbon products formed at these wavelengths.