Gas-Phase Photolysis of Isopentane at Photon Energies below and above the Ionization Energy

The photolysis of (CH₃)₂CDCH₂CH₃, (CH₃)₂CHCH₂CD₃, and of equimolar $({\mathrm{CH}}_3{)}_2{\mathrm{CHCH}}_2{\mathrm{CH}}_3\mathrm{-(}{\mathrm{CD}}_3{)}_2{\mathrm{CDCD}}_2{\mathrm{CD}}_3$ mixtures has been investigated in the presence of NO at 1470 Å (8.4 eV), 1236 Å (10 eV) and at 1048–1067 Å (11.54–11.72 eV). Furthermore, in order to obtain quantitative information concerning the free radicals produced in the fragmentation processes, (CD₃)₂CDCD₂CD₃ has been photolyzed at 1236 Å in the presence of various concentrations of H₂S. Fragmentation of the excited isopentane molecule produced at 8.4 or 10 eV, as well as of the superexcited molecule formed at 11.54–11.72 eV, yields mainly olefinic products and the following radicals, in order of decreasing yield: H>CH₃>C₂H₅>C₃H₇. In addition, on the basis of the various isotopically labeled products, it is concluded that methane, ethane, and propane are split off from the excited isopentane molecule by formation of (a) the corresponding carbene (1,2 elimination), or (b) the corresponding olefin (1,3 elimination). When the energy of the photon is increased from 8.4 to 10.0 eV, the 1,2‐elimination processes become more important relative to the 1,3‐elimination processes in all cases. At 11.54–11.72 eV, the parent ion decomposes to form methane and ethane mainly by a 1,3‐elimination mechanism. It is derived from the results that, in addition to the C₄H₈⁺ and C₃H₆⁺ ions formed by the latter fragmentation processes, the ions C₄H₉⁺ and C₃H₇⁺ are produced with ion‐pair yields (M/N) of 0.15 and 0.06, respectively. The ion‐pair yield of the parent ion is estimated to be 0.35. In contrast to observations derived from an earlier investigation of photoionization of n‐butane at 11.54–11.72 eV, an increase in pressure from 0.15 to 15 torr has only a minor effect on the fragmentation of the parent ion.