Enthalpy surfaces for hydrogen atom transfer in a molecular crystal

The mechanism of the intermolecular photoinitiated hydrogen transfer in flu orene crystal doped with acridine molecules is studied theoretically. For t his reaction, extensive experimental data in a wide interval of temperature s and pressures are available in the literature. Computations of energetics for this reaction with the explicit account of the crystalline environment are performed at atmospheric pressure, and also at 10 and 20 kbar. Paramet ers of the fluorene crystal lattice are reported as functions of pressure. The reaction model considers a large cluster of crystalline lattice arrange d around the reaction pair; it includes three coordination spheres and its structure depends on pressure. The interaction inside the chemical subsyste m is calculated by a semiempirical quantum-chemical method (PM3); its inter action with the crystalline environment is treated in terms of the atom-ato m scheme. Studies of the potential energy surface (PES), as a function of p ressure, showed that the tunneling transition of H-atom is essentially two- dimensional. Other modes that undergo a significant rearrangement and deter mine the reaction mechanism are revealed and investigated. The mechanism of multidimensional tunneling is discussed, and the computational scheme aime d at estimating the corresponding rate constant is outlined. It includes a computation of special PES cross sections providing relatively low effectiv e potential barriers during the tunneling. The main visible effect of press ure on the PES is a significant decrease of equilibrium distances between r eactants, promoted by increasing pressure. This results in decreasing the e ffective tunneling barriers along the reaction path and accelerating the re action.