Spatial structure of 2-(2´-hydroxyphenyl)-4-methylchromanes and some specific features of intramolecular hydrogen bond

Abstract

The B3LYP/6-311++G(2d,2p) method was used to study spatial structures in the gas phase of 2S*,4S*- and 2S*,4R*-2-(2´-hydroxyphenyl)-4-methylchromanes (1 and 2), which are the products of thermal dimerization of o-vinylphenol. A two-dimensional potential energy surface of the internal rotation of the cyclic fragments of the molecule around the C(2)—C(1´) bond and the OH group around the C(2´)—O(2´) bond was built for chromane 1. The appearance of the stretching vibration absorption band for free OH groups in the IR spectra of solutions of chromanes 1 and 2 in low polar solvents allowed us to draw a conclusion that, despite the presence of a "classic" strong enough intramolecular hydrogen bond (IMHB) O—H...O between the OH group and the O atom of heterocycle, some molecules 1 and 2 exist in solutions as rotamers with the free OH group. Theoretical evaluation shows that the amount of such rotamers in the gas phase is ~8%, while in solution, according to the IR spectroscopy data, it is ~12%. Theoretical evaluation of the IMHB enthalpy for o-phenylchromanes 1 and 2 gives the value of 1.9 kcal mol−1.