Theoretical Investigation of the Gas-Phase SN2 Reactions of Anionic and Neutral Nucleophiles with Chloramines.

The SN2 reactions at nitrogen center (SN2@N) play a significant role in organic synthesis, carcinogenesis, and the formation of some environmentally toxic compounds. However, the SN2@N reactions specifically for neutral compounds as nucleophiles are less known. In this work, reactions of dimethylamine (DMA) and F- with NH2Cl were investigated as model reactions to validate an accurate functional from 24 DFT functionals by comparing with the CCSD(T) reference data. M06-2X functional was found to perform best and applied to systematically explore the trends in reactivity for halides (F- and Cl-) and simple amines toward the substrates NH2Cl and NHCl2 (SN2@N) as well as CH3Cl and CH2Cl2 (SN2@C). The computational results show that the backside inversion channel dominates most the SN2@N reactions except for the case of F- + NHCl2, which reacts preferentially via proton transfer. The overall activation free energies (Δ G‡) of the inversion channel for the SN2 reactions of F- and Cl- with chloramines are negative, whereas those for amines as nucleophiles are around 30-44 kcal/mol. The SN2@N reactions for all the nucleophiles investigated here are faster than the corresponding SN2@C. Moreover, amines react faster when they have a higher extent of methyl substitution. Additionally, the energy gap between the HOMO of nucleophile and LUMO of substrate generally correlates well with Δ G‡ of the corresponding SN2 reactions, which is consistent with previous results.