Intramolecular hydrogen bonding in alpha-phenylcinnamic acids and their heteroatom-containing derivatives studied by ab initio quantum chemical methods

Intramolecular hydrogen bonding interactions were searched for in conformer s of isolated alpha -phenyl-, alpha -pyridyl- and alpha -furylcinnamic acid stereoisomers. The conformers were obtained by ab initio (HF/3-21G//HF/3-2 1G and HF/6-31G(d,p)//HF/6-31G(d,p)) quantum chemical methods using initial geometries corresponding to the global minima determined at the level of s emi-empirical quantum chemical calculations. The most common intramolecular hydrogen bond was of C-H circle dot circle dot circle dotO type. In certai n conformers of alpha-(2-pyridyl)cinnamic acids, O-H circle dot circle dot circle dotN(pyridyl) and alpha-(2-furyl)cinnamic acids, O-H circle dot circ le dot circle dotO(furyl) interactions were also found. In most cases, at t he level of HF/3-21G calculations, these conformers were more stable than t hose lacking these close contacts. When the larger basis set was applied th e extra stabilizing effect disappeared, nevertheless, these geometries stil l represented minimum structures. (C) 2001 Elsevier Science B.V. All rights reserved.