CNT-Confinement Effects on the Menshutkin SN2 Reaction: The Role of Nonbonded Interactions

We investigated the effects of CNT confinement ((6,6) tube) on the model Menshutkin reaction H3N + H3CCl = H3NCH3(+) + Cl(-), which is representative of chemical processes involving developing of charge separation along the reaction pathway. We used either a full QM approach or a hybrid QM/MM approach. We found that the CNT significantly lowers the activation barrier with respect to the hypothetical gas-phase reaction: The activation barrier Ea varies from 34.6 to 25.7 kcal mol-1 (a value similar to that found in a nonpolar solvent) and the endothermicity ΔE from 31.2 to 13.5 kcal mol-1. A complex interplay between C-H⋯π, N-H⋯π, and Cl⋯π nonbonded interactions of the endohedral system with the CNT wall explains the lower barrier and lower endothermicity. The hybrid QM/MM approach (MM = UFF force field) does not reproduce satisfactorily the QM energy ΔE (18.1 vs 13.5 kcal mol-1), while optimum agreement is found in the barrier Ea (25.8 vs 25.7 kcal mol-1). These results suggest that the simple Qeq formalism (included in the MM potential) does not describe properly the effect of CNT polarization in the presence of the net charge separation featuring the final product. A more accurate estimate of the tube polarization was obtained with single-point QM/MM computations including PCM corrections (using the benzene dielectric constant) on the QM/MM optimized structures. After PCM corrections, Ea changes slightly (from 25.8 to 24.5 kcal mol-1), but a more significant variation is observed for ΔE that becomes 13.1 kcal mol-1, in rather good agreement with the full QM. This level of theory (QM/MM with PCM correction, MM = UFF) represents a more general approach suitable for describing CNT-confined chemical processes involving significant charge separation. QM/MM computations were extended to CNTs of different radii: (4,4), (5,5), (7,7), (8,8), (9,9), (10,10), (12,12), (14,14) CNTs and, as a limit case, a graphene sheet. The lack of space available in the small tube (4,4) causes a strong structural distortion and a consequent increase in Ea and ΔE (40.8 and 44.0 kcal mol-1, respectively). These quantities suddenly decrease with the augmented volume inside the (5,5) tube. For larger tubes, different structural arrangements of the endohedral system are possible, and Ea and ΔE remain almost constant until the limiting case of graphene.