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 used to investigate the hydrogen bonding between hydrogen fluoride and two series of molecules, nitrile and carbonyl compounds of the type R-CN and R-CHO, respectively, where R= -H, -OH, -SH, -OCH3, -NH2, -NO2, -C equivalent to N, -F, -Cl, -CH3, and -CF3. Geometry optimization and vibrational frequency calculations at the optimized geometry were performed for isolated and hydrogen-bonded systems. The estimated energies of hydrogen-bond formation were 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 frequency shift (Delta v(HF)) are obtained for the two series studied. Linear dependencies are also found between Delta E and the change of H-F bond length (Delta 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 found between ER-CHO,d the ab initio calculated molecular electrostatic potential at the carbonyl oxygen (V-O) in isolated carbonyl molecules. These relations show that the molecular electrostatic potential can be successfully used to predict the reactivity of the molecules studied with respect to hydrogen bonding. Significantly, a dependence that unifies the two series of proton-acceptor molecules was also found. It can be used with confidence in predicting the energy of hydrogen-bond formation when different substituents are added to the simplest member of a series.