Characterization of N•••O non-covalent interactions involving σ-holes: "electrostatics'' or "dispersion''

In this article, the existence of N center dot center dot center dot O noncovalent interactions was explored in per-halo substituted ammonia-water complexes. Optimized geometry at the MP2/aug-cc-pVTZ level shows that the N center dot center dot center dot O 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 N center dot center dot center dot O 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 N center dot center dot center dot O 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 sigma-hole interaction with the NBO analysis revealing that the primary charge transfer in all the complexes is occurring from O( lp) to sigma star( N-X) orbitals, confirming these interactions to be predominantly in the category of pnicogen bonds.