Molecular electrostatic potential as reactivity index in hydrogen bond formation: an HF/6-31+G(d) study of hydrogen-bonded (HCN)n clusters, n=2,3,4,5,6,7

Abstract

Ab initio molecular-orbital calculations of (HCN)n clusters for n=2,3,4,5,6,7 were performed following the procedure of King and Weinhold [B.F. King, F. Weinhold, J. Chem. Phys. 103 (1995) 333]. Geometry optimisation and vibrational frequency calculations at the optimised geometry were carried out at HF/6-31+G(d) level of theory. The calculations confirm the known linear relations between the energy of hydrogen bond formation (ΔE(n)) and: (1) the hydrogen bond length (rN⋯H(n−1)); (2) the change of the neighbouring C–H bond length (ΔrC–H(n)); and (3) its characteristic vibrational frequency shift (ΔνC–H(n)). An excellent linear dependence is found between the energy of hydrogen bond formation (ΔE(n)) and the molecular electrostatic potential at the end nitrogen atom (VN(n−1)). A perfect linear relation also exists between ΔE(n) and the molecular electrostatic potential at the end hydrogen atom (VH(n−1)). The results obtained confirm that the molecular electrostatic potential at atomic sites can be used as a reactivity index reflecting the ability of molecules to participate in hydrogen bonding.