Absolute Rate Calculations. Proton Transfers in Solution

Abstract

The reaction path of the intersecting-state model is used in transition-state theory with the semiclassical correction for tunneling (ISM/scTST) to calculate the rates of proton-transfer reactions from hydrogen-bond energies, reaction energies, electrophilicity indices, bond lengths, and vibration frequencies of the reactive bonds. ISM/scTST calculations do not involve adjustable parameters. The calculated proton-transfer rates are within 1 order of magnitude of the experimental ones at room temperature, and cover very diverse systems, such as deprotonations of nitroalkanes, ketones, HCN, carboxylic acids, and excited naphthols. The calculated temperature dependencies and kinetic isotope effects are also in good agreement with the experimental data. These calculations elucidate the roles of the reaction energy, electrophilicity, structural parameters, hydrogen bonds, tunneling, and solvent in the reactivity of acids and bases. The efficiency of the method makes it possible to run absolute rate calculations through the Internet.

A chemical understanding for the enhanced hydrogen tunnelling in hydroperoxidation of linoleic acid catalysed by soybean lipoxygenase‐1

“…The tunnelling correction is probably overestimated at low temperatures because the experimental KIE increase from 6.0 AE 0.5 at À30 8C to 6.8 AE 0.6 at À60 8C, whereas the calculated values increase from 8 to 13, when the difference in frequency factors between the isotopes is neglected. [18,39] The agreement between calculated and experimental activation energies is within the average error of 0.96 kcal/mol previously obtained for 100 H-atom transfers in the gas phase and in solution. [9] The H-atom transfer rate constant for this reaction must be calculated with Eq (7), which requires the calculation of partition functions of polyatomic species.…”

“…It increases with the propensity of the 'ligands' A and C to participate in partial electron transfer with the transition state (low I P and/or high E A ), but otherwise leads to no stabilization (high I P and/or low E A ). The electronic saturation further stabilizes the transition state and was introduced in the ISM reaction coordinate through a modification of the reactant and product Morse curves [9,17,18,25,26] V…”

“…We have recently shown that the Intersecting/Interacting-State Model (ISM) associated with semiclassical TST provides a reliable way to calculate absolute rates of elementary chemical reaction, notably hydrogen-atom transfers, proton transfers and methyl transfers, [9,[17][18][19][20] and offers a chemical understanding of reactivity in polyatomic systems. ISM deals in a compact manner with the chemical bonds that are broken and formed in a chemical reaction, characterizing them by their Morse potentials and equilibrium bond lengths, and associates their interactions along a unidimensional reaction coordinate with the reaction energy and an electronic parameter, the electrophilicity index proposed by Parr to measure the saturation of the electron inflow between interacting atoms.…”

Understanding Chemical Reactivity: The Case for Atom, Proton and Methyl Transfers

“…The reaction coordinate can be further sophisticated to include zero-point energy corrections and the change of m from unity in the reactants, to the value given by Equation (14) at the transition state, and back to unity in the products. [34] …”

“…[39] and proton transfers from Table 3 of ref. [34] lozenges: methyl transfers from Figure 6 of ref. [38]; *: enzyme catalyzed H-atom transfers from ref.…”