Crystal packing in the 2-R,4-oxo-[1,3-a/b]-naphthodioxanes - Hirshfeld surface analysis and melting point correlation

The crystal structures of an array of 2-R,4-oxo-[1,3-b]-naphthodioxanes (R = H, Me, Et, i-Pr, n-Bu, CCl3, i-propylideneglyceral) have been determined at 100 K, together with a more limited series of the isomeric 2-R,4-oxo-[1,3-a]-naphthodioxanes (R = Et, i-Pr, n-Bu, i-propylideneglyceral), permitting a study of the effects of substituent and ring fusion variation on the crystal packing of neutral molecules that are otherwise essentially planar. We have conducted an analysis of Hirshfeld surfaces (a definition of an outer contact surface of a molecule within a crystal) for all eleven compounds and follow the changes of different properties on these surfaces upon systematic variation of the substituent R. Not only qualitative information by means of fingerprint plots, but also quantitative information by means of melting point correlations are presented. The results show the extent to which C-H center dot center dot center dot O hydrogen bonds and C-H center dot center dot center dot pi/pi center dot center dot center dot pi interactions contribute positively to the lattice energies on the one hand, and H center dot center dot center dot H contacts negatively on the other. The percentage of H center dot center dot center dot H contacts as closest contacts on the Hirshfeld surfaces is a universally applicable measure of the crystal lattice energy and can be used as a reference for the importance of other types of contacts.