Substituent Effects on Menshutkin-Type Reactions in the Gas Phase and Solutions: Theoretical Approach from the Orbital Interaction View
In this study, we developed a method to interpret the mechanism of acceleration for Menshutkin-type reactions in solutions theoretically, from the orbital interaction view, utilizing the through-space/bond (TS/TB) interaction analysis in the polarizable continuum model (PCM). Different method levels were tested to determine the substituent effects on the reactions of NH3 attacking para-substituted benzyl bromide. The geometrical structures and Mulliken charge distributions were analyzed to elucidate the substituent effects on the SN2 reaction center. The results of Mulliken charge analysis showed that the para-substituted benzyl group (-C6H4Y) received negative charge through the reaction process, and both electron-donating and electron-withdrawing substituents Y made -C6H4Y groups receive greater charges. Solvent effects on the structures of transition states (T-S(s)) were significant. The structures of T-S(s) were found to be exhibiting longer bond lengths in solutions, especially in polar solvents such as water. Our TS/TB-PCM analysis method can predict the substituent effects in solutions by evaluating contributions from orbital interactions in question. The orbital interaction analysis results revealed that the key orbital interactions for stabilizing the T-S(s) of the systems with substituents Y = NH2 and NO2 in water were n(NH2)-π*(ph) (ph = phenyl) and π(ph)-π*(NO2) interactions, respectively. Stronger interactions between π*(ph) and σ*(Cα-Br) occurred because of the n(NH2)-π*(ph) and π(ph)-π*(NO2) interactions that resulted when para-substituents -NH2 and -NO2, respectively, were added to the system. These stronger π*(ph)-σ*(Cα-Br) interactions stabilized the transition state and enabled the Br leaving group to leave more easily.