Computational Studies of Nucleophilic Substitution at Nitrogen Center: Reactions of NH2Cl with HO−, CH3O− and C2H5O

• Published in: Chemphyschem 2024-11, Vol.25 (22), p.e202400365-n/a
• Main Authors: Dutta, Siddharth Sankar, Lourderaj, Upakarasamy
• Format: Article
• Language: English
• Subjects: Ammonia; hydride-transfer; Nitrogen; nitrogen-center; Nucleophiles; Potential energy; proton-transfer; Protons; SN2; Substitution reactions
• ISSN: 1439-4235; 1439-7641; 1439-7641
• DOI: 10.1002/cphc.202400365

The atomic‐level mechanisms of the nucleophilic substitution reactions at the nitrogen center (SN2@N) were investigated for the reactions of chloramine (NH2Cl) with the alkoxide ions (RO−, where R=H, CH3, and C2H5) using DFT and MP2 methods. The computed potential energy profiles for the SN2@N pathways involving the back‐side attack of the nucleophiles show the typical double‐well potential with submerged barriers similar to the SN2 reactions at the carbon center (SN2@C). However, the pre‐reaction and post‐reaction complexes are, respectively, the N−H⋅⋅⋅O and N−H⋅⋅⋅Cl hydrogen‐bonded intermediates, which are different from those generally seen in SN2@C reactions. The SN2@N pathways involving front‐side attack of the nucleophiles have high‐energy barriers. The potential energy surfaces (PESs) along the proton‐transfer pathways were flat. In addition to the proton‐transfer and SN2 pathways, we also observed a new path for the methoxide and ethoxide nucleophiles where a hydride‐transfer from the nucleophile to chloramine resulted in the products Cl−+R'CHO+NH3, (R’=H, CH3), and was the most exoergic. A comparison of the energetics obtained used different DFT and MP2 methods with that of the benchmark coupled‐cluster methods reveals that CAM‐B3LYP best describes the PESs. Electronic structure calculations reveal that the reactions of NH2Cl with alkoxides HO−, CH3O−, and C2H5O− follow three barrierless competing pathways: proton‐transfer, SN2, and hydride‐transfer. The three pathways occur via a common hydrogen‐bonded complex, with the hydride‐transfer pathway being the most exoergic.