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.