RADICAL IONS IN PHOTOCHEMISTRY .28. CLEAVAGE OF THE RADICAL CATIONS OF ALKENES - 1-BUTENE AND 4,4-DIMETHYL-1-PENTENE - ABINITIO CALCULATIONS ON THE INTERACTION BETWEEN THE ALLYL AND ALKYL RADICAL AND CARBOCATION MOIETIES

Carbon-carbon bond cleavage of the radical cations of 1-butene (C4H8+) and 4,4-dimethyl-1-pentene (C7H14+) will generate the allyl and alkyl radical and carbocation fragments. Alternative bonding arrangements between the allyl and methyl moieties in C4H8+ and between the allyl and tert-butyl moieties in C7H14+, possible metastable intermediates or transition states preceding complete separation of the fragments, have been investigated by ab initio molecular orbital calculations. Structures were fully optimized at the UHF/6-31G* or UHF/STO-3G levels, and some of the calculations on C4H8+ were expanded with single point MP2/6-31G*//UHF/6-31G* computations. The C4H8+ radical cation, having a structure similar to that of 1-butene, is more stable than the separated fragments: 183 kJ mol-1 lower in energy than the sum of the energies of the allyl cation and the methyl radical, and 385 kJ mol-1 lower than the sum of the energies of an allyl radical and a methyl cation, at the MP2/6-31G* level. The corresponding values at the UHF/STO-3G level are 276 and 415 kJ mol-1, respectively. There is less bonding interaction between the allyl and tert-butyl moieties in C7H14+. The summation of the energies of the allyl radical and tert-butyl cation is 123 kJ mol-1 lower than the summation of the energies of the allyl cation and tert-butyl radical, and 115 kJ mol-1 higher in energy than the bonded radical cation C7H14+, at the UHF/STO-3G level. These calculated values are compared with thermochemical data and with experimental results on the cleavage of these, and related, radical cations.