Deciphering Front-Side Complex Formation in SN2 Reactions via Dynamics Mapping.

Due to their importance in organic chemistry, the atomistic understanding of bimolecular nucleophilic substitution (SN2) 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.