Molecular electrostatic potential as reactivity index in hydrogen bonding:Ab initio molecular orbital study of complexes of nitrile and carbonyl compounds with hydrogen fluoride

Ab initio molecular orbital calculations at the HF/6-31+G(d,p) level were u sed to investigate the hydrogen bonding between hydrogen fluoride and two s eries of molecules, nitrile and carbonyl compounds of the type R-CN and R-C HO, respectively, where R= -H, -OH, -SH, -OCH3, -NH2, -NO2, -C equivalent t o N, -F, -Cl, -CH3, and -CF3. Geometry optimization and vibrational frequen cy calculations at the optimized geometry were performed for isolated and h ydrogen-bonded systems. The estimated energies of hydrogen-bond formation w ere corrected for zero-point Vibrational energy and basis set superposition error (including the relaxation correction). Linear relations between the energy of hydrogen-bond formation (Delta E) and the H-F stretching frequenc y shift (Delta v(HF)) are obtained for the two series studied. Linear depen dencies are also found between Delta E and the change of H-F bond length (D elta r(HF)) An excellent linear dependence is found between Delta ER-CN and the ab initio calculated molecular electrostatic potential at the nitrile nitrogen (VN) in isolated nitrile molecules. A linear dependence is also fo und between ER-CHO,d the ab initio calculated molecular electrostatic poten tial at the carbonyl oxygen (V-O) in isolated carbonyl molecules. These rel ations show that the molecular electrostatic potential can be successfully used to predict the reactivity of the molecules studied with respect to hyd rogen bonding. Significantly, a dependence that unifies the two series of p roton-acceptor molecules was also found. It can be used with confidence in predicting the energy of hydrogen-bond formation when different substituent s are added to the simplest member of a series.