Deciphering Front-Side Complex Formation in S_N2 Reactions via Dynamics Mapping

Abstract

Due to their importance in organic chemistry, the atomistic understanding of bimolecular nucleophilic substitution (S_N2) reactions shows exponentially growing interest. In this publication, the effect of front-side complex (FSC) formation is uncovered via quasi-classical trajectory computations combined with a novel analysis method called trajectory orthogonal projection (TOP). For both F^- + CH₃Y [Y = Cl,I] reactions, the lifetime distributions of the F^-···YCH₃ front-side complex revealed weakly trapped nucleophiles (F^-). However, only the F^- + CH₃I reaction features strongly trapped nucleophiles in the front-side region of the prereaction well. Interestingly, both back-side and front-side attack show propensity to long-lived FSC formation. Spatial distributions of the nucleophile demonstrate more prominent FSC formation in case of the F^- + CH₃I reaction compared to F^- + CH₃Cl. The presence of front-side intermediates and the broad spatial distribution in the back-side region may explain the indirect nature of the F^- + CH₃I reaction.