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

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journal contribution
posted on 02.03.2018, 00:00 by Jieqing Liu, Meng Dong, Shuo Zhang, Yong Dong Liu, Rugang Zhong
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 (ΔG) 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 ΔG of the corresponding SN2 reactions, which is consistent with previous results.

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