Using a modified Marcus model to analyze the Brønsted correlation of gas-phase reactions of the deprotonation of para-substituted toluenes by CH₂Cl⁻, H(O)C⁻, CH₃O⁻, H(F)N⁻, NH₂C≡C⁻, and СH₃C≡C⁻ anions

Abstract

Ab initio calculations of the energy parameters for a series of gas-phase reactions of the deprotonation of para-substituted toluenes (OH, CH₃, H, Cl, NC, CN, and NO₂ substituents) by CH₂Cl^–, H(O)C^–, CH₃O^–, H(F)N^–, NH₂C≡C^–, and CH₃C≡C^– anions (the conjugated acids are chloromethane (CH₃Cl), formaldehyde (Н₂CO), methanol (CH₃OH), fluoroamine (NH₂F), aminoacetylene (NH₂C≡CН), and methylacetylene (CH₃C≡CН), respectively) is performed using a technique of intramolecular reorganization proposed earlier. The equilibrium (E _a ^eq) and nonequilibrium (E ^neq_a ) contributions to energy of activation E _а are found for distance Q(C…B) = 3.0 Å (where С and В are the central atoms of the acid and base). At this value of Q, the tunneling mechanism of proton transfer predominates. The results from calculations, together with similar data obtained earlier for reactions of the deprotonation of toluenes by СН₂CN^– anions, are considered from the viewpoint of the harmonic Marcus model. It is found that in the graph ΔE ₀ − E ^neq_a (where ΔE ₀ is the internal energy of the reaction (for a collision complex)), the points for five reaction series with C-bases fall on the same quadratic (within the −11 to 12 kcal/mol range of ΔE ₀) curve; the second curve, which includes points for the series with N- and O-bases (OCH₃ ^– and H(F)N^–) is shifted below the first curve by 1.5 kcal/mol. For most reaction series (except the one with the strongest bases, CH₂Cl^– and H(O)C^–), the curvature and slope of graph ΔE ₀ − E ^neq_a are in agreement with the theory. The correlations of energies E ^neq_a , E _a ^eq, E _а, and ΔE ₀ with the energy of reaction ΔE ₀₀ are satisfactorily described by quadratic equations as well. Differences between the structures of bases are reflected most by energy E _a ^eq and the height of the proton tunneling barrier in the activated complex (AC) E _b. For the series of reactions of toluene with a set of bases, graphs ΔE ₀₀ − E _a ^eq and ΔE ₀ − E _b show good linear correlation between the values (opposite in sign) of departures from straight lines drawn through the points for two C-bases of the same type (CH₂Cl⁻ and СН₂CN⁻).