Synchrotron x-ray diffraction study of liquid and glassy toluene.

We measured the X-ray diffraction data of liquid toluene at several temperatures between 300 and 180 K and those of glasses prepared by liquid quenching and vapor deposition (VD) at 3 K. The VD glasses annealed at 90 and 112 K were also measured at 3 K. Throughout the measurements, we used a custom-made cryostat for VD experiments and a high-energy X-ray diffractometer at BL04B2, SPring-8. The structure factor S(Q) of each state was successfully fitted using potential fitting with molecular dynamics simulation and reverse Monte Carlo methods. Interestingly, in both the liquid and glassy states, the most preferred intermolecular structure is not a parallel one, which is the most stable dimer in vacuo, but a T-shaped one in which two hydrogen atoms of a benzene ring point toward the benzene ring of a neighboring molecule. The abundance of the T-shaped and side-by-side structures increases with decreasing temperature and annealing of the VD glass. As one possibility, we propose that the T-shaped structure can constitute the cooperatively rearranging region conceived in the Adam-Gibbs theory.