The application of ab initio molecular orbital theory to structural moieties of carbohydrates

Ab initio molecular orbital calculations were made on methoxymethanol, a model for the hemiacetal and acetal moieties in aldopyranoses and methyl aldopyranosides, thereby improving on the previous calculations using methanediol. The new calculations confirmed the favored conformations already deduced, and gave, for the conformational-energy differences and CO bond-length variations, values more appropriate to the carbohydrate systems, as confirmed by a re-examination of the experimental data from crystal-structure determinations. From the results, it was predicted that the OCH₃ bond in methyl aldopyranosides is lengthened; this is supported by the experimental data. An examination of the angles and bond-lengths in the pyranoid ring and of the linkage bonds of oligosaccharides indicated that similar electronic effects involving the oxygen lone-pair electrons apply to oligo- and poly-saccharides.