Unraveling the Nature of Weak Hydrogen Bonds and Intermolecular Interactions Involving Elements of Group 14–17 via Experimental Charge Density Analysis

Abstract

Mapping of charge densities in molecular crystals has been contemplated ever since it was recognized that X-rays are scattered by the electron density in the crystal. The methodology both from the experimental and theoretical perspective was standardized and applied extensively only during the last few decades, as technological advances were a prerequisite in both data collection and computation. Multipole formalism developed for accurate X-ray diffraction data is routinely utilized in conjunction with the concept of atoms in molecules to obtain quantitative estimates of the topological properties in molecular crystals which allow the evaluation of both bonded and non-bonded contacts. Recently, with the advent of quantum crystallography, combining Hirshfeld atom refinement along with libraries of extremely localized molecular orbitals, HAR–ELMOs, has emerged as an alternate approach. Apart from the weak hydrogen bonds, other highly directional non-bonded contacts like halogen, pnicogen, chalcogen and carbon bonds have been subjected to charge density analysis to experimentally observe and quantify σ-holes using experimental high-resolution X-ray diffraction data. The recognition of lack of directional preferences in hydrophobic interactions is demonstrated experimentally which might have far reaching consequences in the areas of materials and biology.