Ab Initio Molecular Orbital Calculation of Carbohydrate Model Compounds. 6. The Gauche Effect and Conformations of the Hydroxymethyl and Methoxymethyl Groups

Conformational properties and the gauche effect of the two functional groups, hydroxymethyl and methoxymethyl, in hexopyranosides have been studied with ab initio methods using the methyl 2,3,4-trideoxy-α-d-glycero-hexopyranoside (1), methyl 2,3,4-trideoxy-β-d-glycero-hexopyranoside (2), methyl 6-O-methyl-2,3,4-trideoxy-α-d-glycero-hexopyranoside (3), and methyl 6-O-methyl-2,3,4-trideoxy-β-d-glycero-hexopyranoside (4) as models. The geometry of the conformers around the C5−C6 bond for the methyl 2,3,4-trideoxy-d-glycero-hexopyranosides was determined by gradient optimization at the SCF level using the 6-31G*, 6-31+G*, and 6-311++G** basis sets. The optimized geometries were used to calculate the energy of the gauche−trans (gt), trans−gauche (tg), and gauche−gauche (gg) conformers with the 6-31G*, 6-31+G* 6-31+G**, 6-31G**, 6-311G*, and 6-311++G** basis sets. Electron correlation effects were accounted for at the second-order Moller−Plesset perturbation (MP2) level using the 6-31G* basis set and using the ...