The specificities of application of the supermolecule method to the calculation of mechanisms of reactions in a protonodonor medium are considered using as an example the reaction of methylamine with ethylene carbonate in methanol. The energies of non-interacting solvated reactants have been used as the reference levels for calculation of relative energies. The problems that result from using the energies of the pre-reaction complexes or the sums of the energies of infinitely separated reactants as the reference levels are noted. The optimal number of the solvent molecules necessary to comprehensively model the reaction under consideration is found. Two possibilities are established for the stabilization of transition states and lowering of activation barriers due to the formation of cycles involving hydrogen bonds. The formation of eight-membered proton-transfer cycles and eight- or ten-membered stabilization cycles is preferable for the given reaction. The lower calculated magnitude of the barrier of the reaction proceeding by concert mechanism in methanol is 15.1 kcal mole−1. The reaction mechanism specificities associated with the formation of zwitterionic intermediates are discussed.
Theoretical Chemistry Accounts – Springer Journals
Published: Aug 7, 2017
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