Intermolecular interactions in molecular crystals: what's in a name?

Alison J Edwards,Peter R Spackman,Campbell F Mackenzie,Dylan Jayatilaka,Mark A Spackman

doi:10.1039/c7fd00072c

Abstract

Structure-property relationships are the key to modern crystal engineering, and for molecular crystals this requires both a thorough understanding of intermolecular interactions, and the subsequent use of this to create solids with desired properties. There has been a rapid increase in publications aimed at furthering this understanding, especially the importance of non-canonical interactions such as halogen, chalcogen, pnicogen, and tetrel bonds. Here we show how all of these interactions - and hydrogen bonds - can be readily understood through their common origin in the redistribution of electron density that results from chemical bonding. This redistribution is directly linked to the molecular electrostatic potential, to qualitative concepts such as electrostatic complementarity, and to the calculation of quantitative intermolecular interaction energies. Visualization of these energies, along with their electrostatic and dispersion components, sheds light on the architecture of molecular crystals, in turn providing a link to actual crystal properties.

Highlights

The detailed analysis of the interactions between molecules and ions in crystals plays an increasingly important role in modern solid state chemistry, and in particular crystal engineering, where the derivation of predictive structure–property relationships is key to a genuine “engineering” of crystals
We show how all of these interactions – and hydrogen bonds – can be readily understood through their common origin in the redistribution of electron density that results from chemical bonding. This redistribution is directly linked to the molecular electrostatic potential, to qualitative concepts such as electrostatic complementarity, and to the calculation of quantitative intermolecular interaction energies
Visualization of these energies, along with their electrostatic and dispersion components, sheds light on the architecture of molecular crystals, in turn providing a link to actual crystal properties

Summary

Introduction

The detailed analysis of the interactions between molecules and ions in crystals plays an increasingly important role in modern solid state chemistry, and in particular crystal engineering, where the derivation of predictive structure–property relationships is key to a genuine “engineering” of crystals. The computed electrostatic energy for this pair, À76 kJ molÀ1, re ects the strong complementarity in this case Another hydrogen bond interaction shows a similar red-blue complementarity, but over a smaller area, Fig. 4 Molecular diagrams, deformation electron density and ESP of (left to right) benzene and hexafluorobenzene. There are two other notable molecular pairs, with total CE-B3LYP energies of À14 and À18 kJ molÀ1, both between tetra uoroethane and cyanobutane molecules in adjacent “chains”, and for which the dispersion contribution exceeds À20 kJ molÀ1 in both cases These results clearly contradict the observation in ref. The CE-B3LYP estimated lattice energies are À69 (HCB) and À115 (HBB) kJ molÀ1, compared with sublimation enthalpies of 88 Æ 12 kJ molÀ1 (from 12 measurements) for HCB, and a single value of 118 kJ molÀ1 for HBB.[54] (We note that the interaction energies reported in ref. 52 for HCB are all $25% greater than the present CE-B3LYP results, but we cannot identify the origin of this difference)

Conclusions

Findings

Crystal Engineering