Insights into Host-Guest Binding in Hydroquinone Clathrates: Single-Crystal X-ray and Neutron Diffraction, and Complementary Computational Studies on the Hydroquinone-CO2 Clathrate

High-resolution 100 K X-ray and neutron single-crystal diffraction data of the non-stoichiometric hydroquinone-CO2 (HQ-CO2) clathrate are combined, with the aim of providing further insight into host-guest binding in hydroquinone clathrates, measuring the electrostatic nature of the cavity formed by the HQ host and, for the first time, estimating the quadrupole moment of the CO2 guest molecule via diffraction techniques. The experimental electron density reveals the cavity in the beta-HQ structure to be moderately electronegative and largely featureless, but this does not mean that guest molecules are merely trapped. Calculated binding energies for a series of HQ clathrates reveal strong interactions with the host system and, in the case of CO2, a thermodynamic stability comparable to, or exceeding, that of many molecular cocrystals. The remarkable flexibility of the beta-HQ host structure is explored through an analysis of its available clathrate structures at 100 K as well as calculated elastic tensors for crystals of beta-HQ and the HQ-CO2 clathrate. Establishing the CO2 quadrupole moment from this analysis of the experimental diffraction data proves challenging, but the sign and estimated range of its magnitude are in agreement with spectroscopic measurements in the gas phase.