AB-INITIO STUDY OF SOME CH(3)OCXYCH(2) RADICALS - THE INFLUENCE OF ANOMERIC EFFECTS ON THEIR STRUCTURE AND THEIR STABILITY

Optimized equilibrium geometries and rotational transition structures for CH3OCH XCH(2) (X = H, F, CH3, NH2) and CH3OCF2CH2 radicals are obtained by using unrestructed Hartree-Fock (UHF) and second-order Moller-Plesset perturbation (UMP2) theory; a standard 6-31G* basis set is used for geometry optimizations; 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 beta substituted radicals and their alpha 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) --> sigma(CX)* negative hyperconjugation in beta-substituted radicals was explored with the aid of natural bond orbital (NBO) energetic analysis; it appears that n(O) --> sigma(CF)* delocalization plays a predominant role in the conformational preference and stabilization of beta fluoro derivatives; on the other hand, the stabilization arising from the oxygen lone pair into the sigma(CN(H2))* orbital does not appear to be the key factor in the conformational preference of the CH3OCHNH2CH2 radical. (C) 1994 by John Wiley and Sons, Inc.