Abstract
According to experimental data, gas-phase SN2 reactions have low efficiency characterized by the ratio k(T)/kc, where kc is the collision rate constant. The energy profile of the reaction pathway is a double-well curve, and the top of the central barrier may be located either above or below the energy level of the reagents. Nonempirical calculations of the potential surfaces for SN2 reactions have shown that their dynamics may be described in the collinear collision approximation. The probability of a reactive collision is determined by the interference of direct processes and processes of decay of quasibound states arising upon formation of the pre-reaction complex. The reflection probability in the direct process is determined by the degree of excitation of the reactive system in channels that are closed at the central barrier point, which depends on the curvature of the reaction pathway and the depth of the potential well for the pre-reaction complex. Depending on these factors, the direct process may correspond either to total transmission or to total reflection (low reaction efficiency). In the first case, interference of direct processes with decay processes decreases the reaction probability; in the second case, such interference increases the reaction probability. Calculations of the thermal reaction rate constants k(T) have shown good agreement with experimental data.
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Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 4, pp. 435–443, July–August, 1986.
The author acknowledges M. V. Bazilevskii, A. M. Berezhkovskii, V. A. Tikhomirov, and G. É. Chudinov for useful discussions.
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Ryaboi, V.M. Quantum dynamic calculation for gas-phase nucleophilic substitution (SN2) reactions. Theor Exp Chem 22, 417–424 (1987). https://doi.org/10.1007/BF00523819
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DOI: https://doi.org/10.1007/BF00523819