Molecular Electrostatic Potential as Reactivity Index in Hydrogen Bonding: Ab Initio Molecular Orbital Study of Complexes of Nitrile and Carbonyl Compounds with Hydrogen Fluoride

Abstract

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⋮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 (ΔE) and the H−F stretching frequency shift (ΔνHF) are obtained for the two series studied. Linear dependencies are also found between ΔE and the change of H−F bond length (ΔrHF). An excellent linear dependence is found between ΔER-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 and the ab initio calculated molecular electrostatic potential at the carbonyl oxygen (VO) 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.