(334p) Simulating Polymorphic Phase Behavior Using Reaction Ensemble Monte Carlo

Brennan, J. K., U.S. Army Research Laboratory
Lisal, M., Academy of Sciences of the Czech Republic

We present an application of the reaction ensemble Monte Carlo (RxMC) method [J.K. Johnson, A.Z. Panagiotopoulos and K.E. Gubbins, Mol. Phys. 81, 717 (1994); W.R. Smith and B. Tøíska, J. Chem. Phys. 100, 3019 (1994)] for predicting solid-state structural transitions, namely conformational polymorphism. The RxMC technique provides a means of overcoming high-energy transition barriers which may be caused, for example, by sterically-hindered atoms or molecules. We demonstrate the method on nitromethane (NM), CH3NO2, whose behavior has been studied extensively. There are many morphologies of crystalline NM created by the rotation of the methyl group about the C-N bond. In applying the RxMC method to the phase transition behavior of crystalline NM, we treat the rotation of the methyl group as a conformational isomerization reaction. We then predict the equilibrium concentrations of the rotamers as a function of pressure and temperature. We find that the simpler, less-computationally expensive rigid model of nitromethane used in the RxMC simulation can capture the same behavior as the fully-flexible model. Investigations of such systems using the RxMC method allow for the identification of the key components of the mechanisms through the decoupling of the molecular degrees of freedom. In the scope of other similar solid-state transitions, the RxMC method can be a powerful tool in the interpretation of experimental measurements as well as in the confirmation of experimental findings.