Steric, quantum, and electrostatic effects on S(N)2 reaction barriers in gas phase.

Biomolecular nucleophilic substitution reactions, S(N)2, are fundamental and commonplace in chemistry. It is the well-documented experimental finding in the literature that vicinal substitution with bulkier groups near the reaction center significantly slows the reaction due to steric hindrance, but theoretical understanding in the quantitative manner about factors dictating the S(N)2 reaction barrier height is still controversial. In this work, employing the new quantification approach that we recently proposed for the steric effect from the density functional theory framework, we investigate the relative contribution of three independent effects-steric, electrostatic, and quantum-to the S(N)2 barrier heights in gas phase for substituted methyl halide systems, R(1)R(2)R(3)CX, reacting with the fluorine anion, where R(1), R(2), and R(3) denote substituting groups and X = F or Cl. We found that in accordance with the experimental finding, for these systems, the steric effect dominates the transition state barrier, contributing positively to barrier heights, but this contribution is largely compensated by the negative, stabilizing contribution from the quantum effect due to the exchange-correlation interactions. Moreover, we find that it is the component from the electrostatic effect that is linearly correlated with the S(N)2 barrier height for the systems investigated in the present study. In addition, we compared our approach with the conventional method of energy decomposition in density functional theory as well as examined the steric effect from the wave function theory for these systems via natural bond orbital analysis.