Potential Energy Surfaces for Intermolecular Proton Transfers in Crystalline Aspirin: A Computational Study
Ab initio molecular orbital methods and density functional theory were used to model the potential energy surface of intermolecular proton transfers in crystalline aspirin. Using these surfaces the importance of intermolecular interactions and excited states to the proton transfer were also explored. In addition, the ability of ab initio molecular orbital methods and density functional theory to model such a system was examined. First an aspirin homodimer was modeled with various basis sets and levels of theory to develop an efficient theoretical model to use in modeling the proton transfer in a larger crystal model. Both ab initio molecular orbital models and density functional theory using small basis sets were found to be suitable for modeling the homodimer. Second, a C++ program was created to build a model crystal fragment of aspirin from the experimental crystallographic data. Next, this fragment model was used with the chosen theoretical model to compute a potential energy surface of the proton transfer. Using this potential energy surface, reaction coordinates for the proton transfer were examined. The ab initio and density functional theory calculations both displayed a double well potential, but followed two different pathways: ab initio following a pair of symmetrical pathways while density functional theory followed a single linear pathway between the reactant and product well. The curvature of the two wells were then used to compute the vibrational excited states and their relative populations at different temperatures, as well as to valuate the possibility of tunneling. The likelihood of tunneling in the ground state was found to be insignificant, but in excited vibration levels could contribute to the hydrogen transfer observed in neutron diffraction experiments. Changes in the structure of the crystal at high temperatures could affect the process, however, further work in modeling the transition state are required.
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