Periodic DFT Study of the Effects of Co-Crystallization on a N-Salicylideneaniline Molecular Switch

This work aims at better understanding the complex effects of co-crystallization on a single salicylideneaniline molecular switch, (E)-2-methoxy-6-(pyridine-3-yliminomethyl)phenol (PYV3), which can tautomerize between an enol and a keto form. A combination of periodic boundary conditions DFT and molecular wavefunction calculations has been adopted for examining a selection of PYV3 co-crystals, presenting hydrogen bonds (H-bonds) or halogen bonds (X-bonds), for which X-ray diffraction data are available. Three aspects are targeted: i) the energy (H-bond strength, enol to keto relative energy, and geometry relaxation energies), ii) the geometrical structure (PYV3 to co-crystal and enol to keto geometrical variations), and iii) the electron distribution (PYV3 to co-crystal and enol to keto Mulliken charge variations). These allow i) explaining the preference for forming H-bonds with the nitrogen of the pyridine of PYV3 with respect to the oxygens and the importance of the crystal field, ii) distinguishing the peculiar behavior of the SulfonylDiPhenol (SDP) coformer, which stabilizes the keto form of PYV3, iii) describing the relative stabilization of the enol form upon co-crystallization (with the exception of SDP) and therefore iv) substantiating the co-crystallization-induced reduction of thermochromism observed for several PYV3 co-crystals.