Theoretical study on anomalous kinetic isotope effects in H/Mu addition to pyrazine: Effects of water microsolvation in solution kinetics

摘要

Previous experimental studies have shown that addition of H and Mu atoms to pyrazine in aqueous solution leads to different reaction products. H adds to the N atom of the pyrazine ring, whereas Mu adds preferentially to the C atom. In this work, to explain this anomalous isotope effect, we performed reaction path analyses for the H/Mu + pyrazine(H2O) (n = 0-2) reactions at the density functional theory level by considering microsolvation effects. Without water microsolvation, the barrier height of the addition to N is always lower than that of the addition to C. However, the barrier height of the addition to N is slightly higher than that of the addition to C when the N atom of the pyrazine ring is pre-solvated by water through an N center dot center dot center dot H-O hydrogen bond. Thermal rate constants are calculated within a variational transition state theory framework including quantum tunneling corrections and are compared with the experimental results. (C) 2017 Elsevier B.V. All rights reserved.