Theoretical studies of unimolecular reactions of C2-C5 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 pentoxy 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 surfaces and to test their applicability to transition state structures. In addition, density functional theory calculations (DFT) using the B3LYP hybrid-functional have been performed. A comparison of DFT and classical ab initio results shows that only G2(MP2,SVP) theory yields critical energy barriers of 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 energetics suffer from the UHF/3-21G* geometry optimisation. This can only partly 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 characteristics of transition states. Simultaneously, the importance of reliable zero point and thermal energy contributions is emphasised. The results obtained from the present calculations have been used in statistical kinetic rate theories (Somnitz and Zellner, Phys. Chem. Chem. Phys., 2000, 2, 1907).