Section 4: Intermolecular Interactions in Crystals18: Revealing the Intermolecular Bonds in Molecular Crystals Through Charge Density Methods

This chapter is focused on the visible features associated with intermolecular interactions, in particular in crystals, which may be revealed, in the position space, through electron-density-based descriptors. Being defined in terms of a quantum observable, these interpretive tools may be applied, on the same ground, to both the ab initio and experimentally derived electron densities (EDs), thereby ensuring one of the best possible unbiased comparisons between experiment and theory. The chapter reviews how the ED is reconstructed from the X-ray data and illustrates a number of electron-density-based methods able to reveal intermolecular interactions in molecular crystals, such as those applying the Quantum Theory of Atoms in Molecules (QTAIM), the Source Function and the Reduced Density Gradient descriptors. Several applications are then illustrated, including a discussion on σ- and π-hole interactions where the charge density view seems to offer an interesting unifying scenario, along with examples of the use of QTAIM to reveal and quantify a number of subtle and challenging crystal field effects in molecular crystals. The chapter concludes by showing how the Source Function tool characterizes the diverse nature of hydrogen bonded interactions in terms of more or less localized atomic sources and the capability of the Reduced Density Gradient approach, in synergy, but sometimes also in (apparent) contrast, with the QTAIM description, to detect and classify non covalent interactions in molecular crystals.