Ab initio and DFT calculations of some weakly bound dimers and complexes. I. The dimers of ammonia and phosphine

Abstract

Ab initio and density functional theory calculations are presented of the structures and vibrational properties of the weakly-bound hydrogen-bonded species, (NH₃)₂ and (PH₃)₂. Earlier studies of the ammonia dimer have indicated that the structure of the most stable species, and the question of whether a particular structure is a genuine minimum or a transition state, is largely determined by the combination of the theory employed and the basis set used. This paper explores the performance of a variety of basis sets, used in conjunction with second-order Møller–Plesset perturbation theory (MP2), and with density functional theory (DFT), in an attempt to achieve convergence on the most favourable structures of these two aggregates. The ammonia dimer is found to contain a linear hydrogen bond in the eclipsed conformation, while the phosphine dimer is characterized by a blue-shifting hydrogen bond, in a centrosymmetric doubly bifurcated structure.

Acknowledgements

This material is based on work supported by the National Research Foundation of South Africa under Grant Number 2053648. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Research Foundation. The authors also acknowledge the financial support of the University of KwaZulu-Natal Research Fund, and the invaluable technical assistance of Mr. Kishore Singh.