Ab initio study of some CH3OCXYCH2 radicals: The influence of anomeric effects on their structure and their stability

Optimized equilibrium geometries and rotational transition structures for CH₃OCH XCH₂ (X = H, F, CH₃, NH₂) and CH₃OCF₂CH₂ radicals are obtained by using unrestructed Hartree-Fock (UHF) and second-order Møller-Plesset perturbation (UMP2) theory; a standard 6-31G* basis set is used for geometry optmizations; single-point energies for all stable rotamers are obtained at the UMP4/6-31 + G*//UMP2/6-31G* level. By analysis of rotamers, it is apparent that an anomeric effect exists for X = F and to a lesser extent for X = NH₂. Several isodesmic reactions have been studied for the purpose of obtaining theoretical heats of formation and stabilization energies (SE) of these β substituted radicals and their α isomers; the examination of computed SE shows that in the case of CH₃OCHFCH₂ and CH₃OCF₂CH₂ radicals, a significant extra stabilization induced by the anomeric effect occurs. The question of n_O → σ negative hyperconjugation in β-substituted radicals was explored with the aid of natural bond orbital (NBO) energetic analysis; it appears that n_O → σ delocalization plays a predominant role in the conformational preference and stabilization of β fluoro derivatives; on the other hand, the stabilization arising from the oxygen lone pair into the σ orbital does not appear to be the key factor in the conformational preference of the CH₃OCHNH₂CH₂ radical. © 1994 by John Wiley & Sons, Inc.