1,4-Dithiine - Puckered in the Gas Phase but Planar in Crystals: Role of Cooperativity

Tricyclic C₆S₈ is known to exist in two different polymorphic phases, namely, alpha (α) and beta (β) forms with planar and puckered conformations, respectively. Recently, it has been shown that the individual molecule undergoes spontaneous symmetry breaking due to pseudo-Jahn-Teller (PJT) distortion resulting in a puckered conformation at the ground state. Here, on the basis of solid-state dispersion-corrected DFT, DFT-D2, as well as the localized Gaussian basis calculations, on periodic systems, we have compared the relative stabilities and structural preferences for α and β polymorphs in a systematic way, starting from the monomers to different forms of higher aggregates in both α and β crystals. From the molecular viewpoint, puckered conformations of the β form are found to be more stable compared to that of planar ones in the α form. Nevertheless, it is shown that PJT distortion can be suppressed by increasing the π-stacking interactions in its aggregates along the crystallographic c-axis and, therefore, eventually in the α crystal form. The same general principle is also shown to occur in the form-II of the dithianon polymorph. Unlike the C₆S₈ molecule of α form, dithianon molecules require only a dimer aggregate to suppress PJT distortion due to the shorter π-stacking distance. It has been shown that environmental or cooperativity effects as found in crystalline phases play a crucial role to quench the PJT distortion in the molecule. The computed IR spectra for both the molecular conformations as well as crystalline phases show good agreement with the experimental spectra.