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| - Enthalpy Surfaces for Hydrogen Atom Transfer in a Molecular Crystal
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| - The mechanism of the intermolecular photoinitiated hydrogen transfer in fluorene crystal doped with acridinemolecules is studied theoretically. For this reaction, extensive experimental data in a wide interval oftemperatures and pressures are available in the literature. Computations of energetics for this reaction withthe explicit account of the crystalline environment are performed at atmospheric pressure, and also at 10 and20 kbar. Parameters of the fluorene crystal lattice are reported as functions of pressure. The reaction modelconsiders a large cluster of crystalline lattice arranged around the reaction pair; it includes three coordinationspheres and its structure depends on pressure. The interaction inside the chemical subsystem is calculated bya semiempirical quantum-chemical method (PM3); its interaction with the crystalline environment is treatedin terms of the atom−atom scheme. Studies of the potential energy surface (PES), as a function of pressure,showed that the tunneling transition of H-atom is essentially two-dimensional. Other modes that undergo asignificant rearrangement and determine the reaction mechanism are revealed and investigated. The mechanismof multidimensional tunneling is discussed, and the computational scheme aimed at estimating the correspondingrate constant is outlined. It includes a computation of special PES cross sections providing relatively loweffective potential barriers during the tunneling. The main visible effect of pressure on the PES is a significantdecrease of equilibrium distances between reactants, promoted by increasing pressure. This results in decreasingthe effective tunneling barriers along the reaction path and accelerating the reaction.
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