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

摘要

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, -OCH_3, -NH_2, -NO_2, -C ident to N, -F, -C1, -CH_3, and -CF_3. 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##nu#_(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#E~(R-CN) and the ab initio calculated molecular electrostatic potential at the nitrile nitrogen (V_N) in isolated nitrile molecules. A linear dependence is also found between E~(R-CHO) and 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.