Add time:10/01/2019         Source:sciencedirect.com

A theoretical analysis for the proton transfer equilibrium reaction involving the most stable unionized tautomeric conformer of N,N-dimethylglycine, IIu, and its most stable zwitterionic tautomeric conformer, Iz, has been accomplished. High-level ab initio calculations, which include geometry optimization at RHF and MP2 levels of theory, were utilized. Conformer Iz is a minimum at all levels of theory examined including geometry optimization at the MP2/6–311++G** level of theory, and is characterized as the `stretched out' form which is not stabilized by intramolecular hydrogen bonds. To the best of our knowledge, this ground state zwitterionic α-amino acid is among the first to be detected in the gas phase at such high levels of theory. The formation of Iz first involves a proton transfer from IIu to form IIz, which is a relatively unstable zwitterion conformer. IIz is transformed into the more stable zwitterion, Iz, by a C–N bond rotation. The initial proton transfer reaction does not have an activation barrier at the MP2/6–311++G** level of theory, but rotation about the C–N bond of IIz to form Iz has an activation barrier of 32.7 kJ mol−1.

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