The case of intramolecular hydrogen bonding, hyperconjugation and classical effects on the conformational isomerism of substituted carbonyl and thiocarbonyl compounds

摘要

The conformational isomerism of substituted (substituents = OR and SR, R=H and Me) acetaldehydes and thioacetaldehydes is described in terms of intramolecular interactions, namely hydrogen bonding (when R=H), hyperconjugation involving the carbonyl or the thiocarbonyl system, and classical effects (steric and electrostatic interactions). 3D potential energy surfaces were obtained by scanning both X-C-C=Y (alpha) and R-X-C-C (phi) dihedral angles (X/Y=O and S) and used to identify local and global minima. Geometry optimization and NBO calculations, including determination of NLMO steric energies and deletion of hyperconjugative interactions, were then performed in order to find the governing factors for these conformational equilibria. Hyperconjugative contribution for hydrogen bonding showed to be more important for thioaldehydes, while O-H showed to be a better proton donor than S-H; however, hydrogen bonding also appeared to be as of electrostatic nature. Overall, orbital interactions, particularly those involving the pi* system, and classical factors (steric and electrostatic effects) drive the conformational isomerism of the title compounds. (c) 2007 Elsevier B.V. All rights reserved.