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

Intramolecular hydrogen bonding interactions were searched for in conformers of isolated α-phenyl-, α-pyridyl- and α-furylcinnamic acid stereoisomers. The conformers were obtained by ab initio (HF/3-21G//HF/3-21G 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 semi-empirical quantum chemical calculations. The most common intramolecular hydrogen bond was of C-H...O type. In certain conformers of α-(2-pyridyl)cinnamic acids, O-H...N_pyridyl and α-(2-furyl)cinnamic acids, O-H...O_furyl interactions were also found. In most cases, at the level of HF/3-21G calculations, these conformers were more stable than those lacking these close contacts. When the larger basis set was applied the extra stabilizing effect disappeared, nevertheless, these geometries still represented minimum structures.