Computation of the tunneling H-transfer reaction kinetics in the fluorene molecular crystal
Titel:
Computation of the tunneling H-transfer reaction kinetics in the fluorene molecular crystal
Auteur:
Basilevsky, Mikhail V. Tikhomirov, Viacheslav A.
Verschenen in:
Molecular physics
Paginering:
Jaargang 106 (2008) nr. 20 pagina's 2391-2405
Jaar:
2008-10
Inhoud:
The semi-empirical computation of the rate constant is fully performed for the photochemical bimolecular hydrogen atom transfer reaction in condensed phase, proceeding in the deep tunneling regime. For the system considered (fluorene + acridine in its first triplet state, inside the fluorene molecular crystal) the kinetics have been thoroughly investigated experimentally over wide temperature range (http://bernd-prass.de). In the present theoretical work the potential energy surface is constructed by means of the reported earlier QM/MM approach, covering the interaction of the reaction pair with the crystalline environment. Direct evaluation of two-dimensional tunneling amplitudes on this surface is made by means of the accurate quantum-mechanical numerical method. The kinetic part of the rate calculation invokes Golden Rule methodology in its recently elaborated modification while consistently accounting for the dynamical energy exchange between the tunneling pattern and the medium. The careful calibration of transition amplitudes for H- and D-transfer reactions provided the parameters controlling their exponential dependence on the promotion mode (i.e. the distance between the terminal heavy atoms of the reaction centre), which establishes the effective tunneling length of the transfer process. The experimental temperature dependence of the reaction rates for both cases is well reproduced by tuning the parameters of the phonon frequency spectrum projected on the promotion mode. The fitted values of these spectral density parameters are contained within the range located in the preliminary independent computation of the lattice dynamics for the fluorene crystal. Most difficult is the part of the computation concerned with the absolute values of rate constants for both H- and D-reactions. The magnitude of the isotope effect can be reasonably interpreted at a qualitative level by complementing the amplitude quantum-mechanical data with empirical Frank-Condon (FC) factors inherent to the reorganization of several intra-molecular modes, which is known to accompany the background transfer reaction. However, this treatment, based on the oversimplified one-dimensional model of such reorganization, is not fully consistent, since the pertaining FC parameters were found to differ significantly for the cases of H- and D-transfer.
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