Use of molecular overlap to predict intermolecular repulsion in N ··· H—O hydrogen bonds

Abstract

The overlap of the monomer charge distributions is used to derive atom-atom intermolecular repulsion models for N ··· H and N ··· O to model the hydrogen bonds formed between methylcyanide/methanol and pyridine/methanol. The use of the overlap approximation allows the repulsion contribution to be assigned to specific atomic pairs, and the anisotropic form to be derived analytically. The resulting models are tested against intermolecular perturbation theory calculations of the exchange—repulsion surface, in the regions sampled by these N ··· H—O hydrogen bonds in molecular crystal structures. Such models readily reproduce the repulsion surface within a 5% rms error, with the main error being attributable to the overlap approximation. The anisotropy of the atom-atom repulsion in these hydrogen bonds is small and the expansion converges slowly. However, the methodology shows that considering the overlap is a useful method of assigning the repulsion between organic molecules into atomic contributions and estimating the isotropic and anisotropic parameters, which can be applied to larger systems where accurate ab initio calculation of the potential energy surface is not feasible.