Hydrogen Bond Donor Properties of the Difluoromethyl Group

Abstract

A theoretical and experimental study of the existence and the properties of the difluoromethyl group acting as a hydrogen bond donor has been carried out. An intramolecular CF2H- -O=C interaction was examined using semiempirical molecular orbital calculations of both a non-hydrogen-bonded and hydrogen-bonded conformation of a CF2H-substituted pyrazole carboxamide fungicide. Results revealed a short H- -0 contact, a significant energy stabilization, and a lowering in the IR spectrum of 22 cm-1. The experimental IR spectrum of this molecule gave two carbonyl stretching frequencies, one lower by 18 cm-1, very similar to the calculated number. Low-temperature NMR results are also consistent with a geometry having the possibility of an intramolecular CF2H- -O=C hydrogen bond. The hydrogen bond in this system may be related to the enhanced biological activity of the CF2H compound over its CF3 counterpart. In addition, ab initio molecular orbital methods weee employed to examine inter- and intramolecular hydrogen-bonding models of the difluoromethyl group. The results showed that the CF2H- -O=C interaction has a binding energy of ˜1.0 kcal mol-1 and a H- -O distance of ˜2.4 Å. © 1995, American Chemical Society. All rights reserved.