Anab initio interpretation in gas phase and aqueous solution of the generalized anomeric effect in R?O?CR2?NR2 (R = H, CH3)

Abstract

The conformational stability of aminomethanol and its methylated derivatives has been investigated by means of ab initio methods in the gas phase and aqueous solution. Among the computational levels employed, HF/6-31G**//HF/6-31G** calculations correctly describe the conformational features of this series of compounds, and agree well with the results obtained using larger basis sets and including ZPE or electron correlation corrections. Calculated energies and geometries follow the known trends associated to the generalized anomeric effect. Thus, the most stable conformers exhibit preferences for the trans orientations of the LpN CO and LpOCN moieties. However, reverse anomeric effects are observed when a methyl group is bonded to the oxygen, because the LpOCN unit prefers a gauche orientation (that is, trans MeOCN). The natural bond orbital (NBO) method was employed to explain the cited conformational preferences. According to the NBO results, trans arrangements are preferred because the stabilization due to charge delocalization is more important than electrostatic and steric contributions. This explanation agrees with the conclusions obtained by other independent procedures based on energy decomposition schemes. The NBO method was also used to explain the origin of the rotational barriers around the C O and CN bonds in terms of the balance between unfavorable hyperconjugation and electrostatic and steric effects. Changes in conformational stability caused by methylations in different molecular positions were also explained by the influence of the methyl groups on lone-pair delocalization and on steric effects. Finally, the effect of solvation was studied by means of the ab initio PCM method, and the significant changes on relative energies found were analyzed. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 462–477, 2000