Temperature and pressure dependences of rate constants for solid phase tunneling reactions are analytically considered within the framework of modified theory of radiationless transitions, taking into account the intermolecular and soft intramolecular promotive vibrations of reagents. This treatment allows us to describe theoretically the process of atomic tunneling and the effect of temperature on the potential barrier and reorganization of the reagents. The influence of external pressure appears in our treatment as a static reduction of widths and heights of the potential barrier with hydrostatic compression of the matrix, and also as an increase of frequencies of promotive vibrational modes owing to anharmonicity. The theoretical results are used to interpret experimental data concerning the effect of temperature and pressure on the hydrogen-atom tunneling in the fluorene-acridine reaction system. It has been shown that by taking into account the contributions from reorganization of the reagents, which statically reduce the tunneling barrier and are related to four types of promotive vibrations (translational, librational, and two low-frequency intramolecular modes at 95 and 238 cm(-1)), one can reproduce the experimental data available in the literature. The parameters of the reaction system required for this analysis are calculated from two-dimensional potential-energy surfaces generated at the DFT-B3LYP/6-31G* level.

ABSTRACT:A problem of entirely theoretical evaluation of ferroelectric characteristics of H-bonded crystals is considered based on the quantum chemical calculations of different levels. This consideration employs the Ising-type pseudospin formalism and treats the "zero-dimensional" TKHS family crystals M 3 (H/D)(AO 4 ) 2 as convenient examples. Ab initio and post-Hartree-Fock schemes are applied to calculate the parameters of pseudospin Hamiltonians. The simple mean (molecular) field approximation (MFA) and Bethe clusters approach (BCA), which partly takes into account proton-proton (deuteron-deuteron) correlations, are compared. It is found that both mean field approximations describe the unusual low-temperature ferroelectric behavior of the TKHS-like materials qualitatively well. MFA overestimates significantly the critical temperature T c of the structural phase transition paraelectric phase-antiferroelectric phase. The use of BCA provides the T c value much closer to the experimental data.

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On basis of the nonempirical methods (SCF, B3LYP, MP2-MP4) and several cluster models, the ferroelectric KH 2 PO 4 (KDP) and its deuteroanalogue are studied. The tunneling integrals ⍀ and the parameters of the effective coupling of protons/deuterons U,V (the Ising parameters) are calculated for these materials with the 3d network of H/D-bonds. Using the obtained U,V and ⍀ values in the frames of molecular field approximation, it is found that the structural phase (ferroelectric) transition occurs for both crystals within the lowering of temperature. Such low-temperature behavior differs the 3d KDP-family materials from 0d systems, where the low-temperature phase transition takes place only upon deuteration. It is demonstrated that this difference is associated with an abrupt Ising parameters growth for KDP if compared with the nondeuterated 0d materials.

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