MODEL-CALCULATIONS OF MATRIX EFFECTS ON THE CONVERSION OF PROPENE RADICAL CATIONS INTO ALLYL RADICALS IN HALOCARBON MATRICES

A comparative study has been undertaken of the conversion of propene r adical cations into allyl radicals in the gas phase and in CCl4 and CF 4 matrices, The semiempirical UHF/PM3 method, complemented with ab ini tio PMP2/3-21G and PMP2/6-31G(d, p) calculations, is employed to model this process, which is observed to occur through an ion-molecule reac tion by a proton transfer from the propene radical cation to a neutral propene molecule. Single molecules of CCl4 and CF4 are used to model the influence of the corresponding matrices, following two different p athways depending on which proton is transferred. It is found that the reaction in the gas phase occurs without activation energy. In contra st, a barrier is found on the potential energy curve of the reaction i n the CCl4 matrix. The strongest interactions are found for CCl4, resu lting in a 6.9 kcal mol(-1) activation energy (2.6 kcal mol(-1) for th e second pathway). The reaction in CF4 has an intermediate profile wit h an activation energy of only 2.0 kcal mol(-1) (no barrier for the se cond pathway). The interactions between the migrating proton and the m atrix molecule are interpreted in terms of the basicity of the matrix.