The application of ab initio molecular orbital theory to the anomeric effect. A comparison of theoretical predictions and experimental data on conformations and bond lengths in some pyranoses and methyl pyranosides

Abstract Ab initio molecular orbital calculations on methanediol have been used to predict the favored orientations and interatomic distances of the C-O-C-O-R portion in pyranoses. The results found for methanediol suggest, for the sugars, favored conformations that are consistent with the observed anomeric and exo -anomeric effects. The calculations also show that shortenings of the C-O bond of the order of 0.01 to 0.04 A, relative to methanol, are to be expected, and that the bond lengths have a strong conformational dependence. A comparison with the experimental data from the X-ray crystal-structure determinations, both for conformational angles and bond lengths, of eighteen pyranoses and methyl pyranosides shows agreement with the theory that is surprisingly good when consideration is taken of the experimental errors, the limitations of the theoretical model, and the expected differences in the structures of the crystal and the isolated molecule.

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