Gas-Phase Photolysis and Radiolysis of Isobutane

Abstract

The photolysis of (CH3)3CH, (CH3)3CD, and of (CH3)3CH–(CD3)3CD and (CD3)3CD–H2S mixtures has been investigated at 1470 Å (8.4 eV), 1236 Å (10.0 eV), and 1048–1067 Å (11.6–11.8 eV). Since the irradiating photons in these experiments have energies both higher and lower than the ionization energy of isobutane (10.6 eV), the modes of decomposition of the “superexcited” molecule can be compared with those of the molecule excited to states below the ionization potential. On the basis of isotopic analyses of the major products, it was determined that the following primary modes of decomposition of the excited isobutane molecule occur in the photolysis at all three energies: i-C4H10*→CH4+C3H6; i-C4H10*→C3H8+CH2; i-C4H10*→CH3+ sec-C3H7; i-C4H10*→H2+i-C4H8. Most of the propyl radicals decompose further: C3H7→H+C3H6;C3H7→CH3+C2H4. An increase in photon energy increases the relative importance of C–C cleavage processes. The parent ions formed in the photolysis with 11.6–11.8-eV photons decompose to form C3H6+ and sec-C3H7+. The C3H7+ ions react with isobutane to form sec-C3H7 radicals or propane: C3H6++i-C4H10→CH3CHCH3+C4H9+;C3H6++i- C4H10→C3H8+C4H8+. The probability of the former reaction is 1.5 times that of the latter, in agreement with a value for this ratio derived from the radiolysis of C4D10–H2S and C4H10–C4D10 mixtures. The role of superexcited molecules in the radiolysis is discussed and it is estimated that the number of dissociating neutral excited molecules per ion pair is no greater than 0.46.