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

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 (1945) 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 (Delta E-(n)) and: (1) the hydrogen bond length (r(N ... H)((n-1))); and (2) the change of the neighbouring C-H bond length (Delta r(C-H)((n))); and (3) its characteristic vibrational frequency shift (Delta nu(C-H)((n))). An excellent linear dependence is found between the energy of hydrogen bond formation (Delta E-(n)) and the molecular electrostatic potential at the end nitrogen atom (V-N((n-1))). A perfect linear relation also exists between Delta E-(n) and the molecular electrostatic potential at the end hydrogen atom (V-H((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. (C) 2000 Elsevier Science B.V. All rights reserved.