Comparison of the nature of pi and conventional H-bonds: a theoretical investigation

Abstract

The interactions of the first-row hydrides (NH3, H2O, HF) with ethene have been investigated by carrying out calculations, at the second order Moller-Plesset (MP2) level of theory using both the 6-31 + G* and aug-cc-pVDZ basis sets. Unlike previous investigations of these systems, the geometries and vibrational frequencies in the present study were obtained by carrying out explicit counterpoise corrected optimizations. In an effort to understand the nature of the H...pi interactions prevalent in these complexes, the interaction energies were decomposed into individual energy components using the symmetry adapted perturbation theory. Given the goals of the present investigation, the geometries, vibrational frequencies and interaction energy components of the water dimer have also been evaluated. While the interaction energy of the conventional H-bond is dominated by electrostatic interactions, electrostatic, dispersive and inductive interactions are important in the description of the pi H-bond. An important distinction between conventional H-bonded complexes and the pi H-bonded complexes is that the inductive interaction gets magnified at the MP2 level. Thus, the inclusion of electron correlation is an important prerequisite both for the magnification of the inductive interaction and to obtain an accurate estimate of the dispersion energies. It is observed that changes in various geometrical and vibrational parameters of these pi H-bonded complexes can be correlated to the magnitude of either the individual or a combination of various interaction energy components. (C) 2002 Elsevier Science B.V. All rights reserved.