Theoretical studies of unimolecular reactions of C-2-C-5 alkoxy radicals. Part I. Ab initio molecular orbital calculations

Structures, vibrational frequencies and barrier heights characterising the unimolecular reactions of primary and secondary alkoxy radicals up to pento xy have been determined by means of quantum chemical calculations. Several additive model chemistries such as CBS-4, CBS-q, CBS-Q and G2(MP2,SVP) have been applied to obtain information on the reactive potential energy surfac es and to test their applicability to transition state structures. In addit ion, density functional theory calculations (DFT) using the B3LYP hybrid-fu nctional have been performed. A comparison of DFT and classical ab initio r esults shows that only G2(MP2,SVP) theory yields critical energy barriers o f nearly chemical accuracy. Consistently, the energy barriers obtained from the CBS-4, CBS-q and B3LYP calculations are too low. The CBS-4 and CBS-q e nergetics suffer from the UHF/3-21G* geometry optimisation. This can only p artly be improved by the use of MP2(Full)/6-31G(d) geometries. Improvements to G2(MP2,SVP) theory are proposed to better account for the characteristi cs of transition states. Simultaneously, the importance of reliable zero po int and thermal energy contributions is emphasised. The results obtained fr om the present calculations have been used in statistical kinetic rate theo ries (Somnitz and Zellner, Phys. Chem. Chem. Phys., 2000, 2, 1907).