Characterization of NO non-covalent interactions involving σ-holes: "electrostatics" or "dispersion".

In this article, the existence of NO noncovalent interactions was explored in per-halo substituted ammonia-water complexes. Optimized geometry at the MP2/aug-cc-pVTZ level shows that the NO distance in all complexes is less than the sum of the vdW radii of N and O. The strength of these contacts was directly dependent on the extent of chlorine substitution on N or O atoms. Also, the level of theory and the basis set employed for the binding energy calculations have a direct effect on the strength of the NO contacts. Energy decomposition analysis reveals that dispersion was the major contributor towards the stability of these contacts followed by electrostatic energy. The topological analysis further confirmed the existence of NO contacts due to the presence of a bond critical point between the N and the O atom in all the complexes. These contacts have characteristics of a σ-hole interaction with the NBO analysis revealing that the primary charge transfer in all the complexes is occurring from O(lp) to σ*(N-X) orbitals, confirming these interactions to be predominantly in the category of pnicogen bonds.