A study of conformational equilibrium of 1,1,2-trichloroethane in FAU-type zeolites

• Equilibrium of conformers is predicted well by computations as compared with experiments.
• Molecular interactions are predicted differently by empirical and semi-empirical modeling.
• Formation of aggregates inside the zeolite cavities.
• Realization of the limitations of empirical methods to describe correctly the system.

We study the effects of loading and nature of the zeolitic framework in the trans ⇌ gauche equilibrium of 1,1,2-trichloroethane (TCE) in zeolites with Faujasite (FAU) structure including sodium Y (Na-Y) and siliceous Y (Si-Y). TCE is used as a prototype molecule for the study of zeolite–chlorinated hydrocarbon interactions that determine conformational equilibrium. Conformational changes of TCE were sampled using empirical force fields and quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) in combination with replica exchange molecular dynamics (REMD). The population ratio of the conformers at high loading was compared with that found by Fourier transform (FT)-Raman spectroscopy and the results are in very good agreement. Comparison between the infinitely dilute limit of loading, and higher loading shows that the population ratio of the conformers at equilibrium depends strongly on the Si/Al ratio and consequently, the presence of charge compensating cations as well as the loading. At very low loading, in both Na-Y and Si-Y the population of the trans conformer is the largest with the exception of the empirical modeling that predicts the gauche conformer to be dominant in Na-Y. At the higher loading, in Na-Y the population the gauche conformer constitutes the great majority. In Si-Y the population of the trans conformer continues to dominate but the percentage of the gauche conformer increases substantially relative to the loading at the infinitely dilute limit. Such different conformational behaviors provide insight into the factors responsible for diffusion mechanisms and adsorption energies in the FAU-type zeolitic frameworks.