HYDROGEN-BOND DONOR PROPERTIES OF THE DIFLUOROMETHYL GROUP

A theoretical and experimental study of the existence and the properti es of the difluoromethyl group acting as a hydrogen bond donor has bee n carried out. An intramolecular CF2H--O=C interaction was examined us ing semiempirical molecular orbital calculations of both a non-hydroge n-bonded and hydrogen-bonded conformation of the CF2H-substituted pyra zole carboxamide fungicide. results revealed a short H--O contact, a s ignificant energy stabilization, and a lowering in the IR spectrum of 22 cm(-1). The experimental IR spectrum of this molecule gave two carb onyl stretching frequencies, one lower by 18 cm(-1), very similar to t he 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 counter part. In addition, ab initio molecular orbital methods were employed t o examine inter- and intramolecular hydrogen-bonding models of the dif luoromethyl group. The results showed that the CF2H--O=C interaction h as a binding energy of similar to 1.0 kcal . mol(-1) and a H--O distan ce of similar to 2.4 Angstrom.