Novel tools for visualizing and exploring intermolecular interactions in molecular crystals

TLDR

Hirshfeld surfaces partition crystal space by comparing a molecule’s promolecule electron distribution to that of the entire crystal, delineating regions dominated by the molecule. The study introduces Hirshfeld‑surface–based tools to analyze packing modes and compile a pictorial glossary of intermolecular interactions in molecular crystals. The authors employ Hirshfeld surfaces with color‑coded distance and curvature metrics, plus a 2D color plot of contact types, to identify and quantify intermolecular interactions for building a pictorial glossary. The resulting surfaces give a novel, visually striking depiction of intermolecular interactions and their relative strengths within the crystal.

Abstract

A new way of exploring packing modes and intermolecular interactions in molecular crystals is described, using Hirshfeld surfaces to partition crystal space. These molecular Hirshfeld surfaces, so named because they derive from Hirshfeld's stockholder partitioning, divide the crystal into regions where the electron distribution of a sum of spherical atoms for the molecule (the promolecule) dominates the corresponding sum over the crystal (the procrystal). These surfaces reflect intermolecular interactions in a novel visual manner, offering a previously unseen picture of molecular shape in a crystalline environment. Surface features characteristic of different types of intermolecular interactions can be identified, and such features can be revealed by colour coding distances from the surface to the nearest atom exterior or interior to the surface, or by functions of the principal surface curvatures. These simple devices provide a striking and immediate picture of the types of interactions present, and even reflect their relative strengths from molecule to molecule. A complementary two-dimensional mapping is also presented, which summarizes quantitatively the types of intermolecular contacts experienced by molecules in the bulk and presents this information in a convenient colour plot. This paper describes the use of these tools in the compilation of a pictorial glossary of intermolecular interactions, using identifiable patterns of interaction between small molecules to rationalize the often complex mix of interactions displayed by large molecules.