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

The conformational isomerism of substituted (substituents = OR and SR, RH and Me) acetaldehydes and thioacetaldehydes is described in terms of intramolecular interactions, namely hydrogen bonding (when RH), hyperconjugation involving the carbonyl or the thiocarbonyl system, and classical effects (steric and electrostatic interactions). 3D potential energy surfaces were obtained by scanning both XCCY (α) and RXCC (ϕ) dihedral angles (X/YO 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 OH showed to be a better proton donor than SH; however, hydrogen bonding also appeared to be as of electrostatic nature. Overall, orbital interactions, particularly those involving the π∗ system, and classical factors (steric and electrostatic effects) drive the conformational isomerism of the title compounds.