Ab initio study of some CH 3 OC X YCH 2 radicals: The influence of anomeric effects on their structure and their stability

Optimized equilibrium geometries and rotational transition structures for CH 3 OCH X CH 2 ( X = H, F, CH 3 , NH 2 ) and CH 3 OCF 2 CH 2 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 = NH 2 . 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 CH 3 OCHFCH 2 and CH 3 OCF 2 CH 2 radicals, a significant extra stabilization induced by the anomeric effect occurs. The question of n O → σ   CX *negative hyperconjugation in β‐substituted radicals was explored with the aid of natural bond orbital (NBO) energetic analysis; it appears that n O → σ   CF *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   CN(H2) *orbital does not appear to be the key factor in the conformational preference of the CH 3 OCHNH 2 CH 2 radical. © 1994 by John Wiley & Sons, Inc.