Thermal behavior of disordered phase of caffeine molecular crystal: insights from Monte Carlo simulation studies

We have studied the thermal behavior of orientationally disordered phase of caffeine molecular crystal using variable shape variable size Monte Carlo simulations in isothermal-isobaric ensemble. We have investigated the structure, especially the nature of orientational disorder of caffeine molecules as a function of temperature in the range of 400-550 K. Experimentally this system is known to undergo a phase transition at 426 K (considered to be an orientational order-disorder transition) and melt at 512 K. Our simulations reproduce these two transitions in excellent agreement with experiment. We find that the in-plane reorientational motion of molecules is restricted to small angles below 425 K, and above this temperature, molecules undergo essentially free rotations in molecular plane, and we find the melting to occur between 525 and 550 K. In the high temperature disordered phase, the disorder is mostly attributable to the in-plane orientational motion of the molecules. The potential energy profile for the in-plane reorientational rotation has six wells as a consequence of specific packing of molecules in the ab crystallographic plane. Also we find considerable out-of-plane reorientational disorder for the molecules in the high temperature disordered phase. We have also studied the structure and orientational disorder of the system that is quenched from 450 to 300 K. We find that in the quenched phase, the molecular orientational arrangement remains partially frozen.