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DFT Study on Quantum Mechanical Tunneling in Tautomerization of Three-membered Rings

Abstract

We have examined the keto-enol and amino-imino tautomerization in a set of three-membered ring systems (1-5) in the absence and presence of water molecules. Aromaticity governs the keto-enol and amino-imino tautomerization process in (1-5), which leads to the formation of enol and imine derivatives. The possibility of quantum mechanical tunneling (QMT) has not been reported in the tautomerization processes of three-membered ring systems. The M062X/6-311+G(d,p) level of theory QMT calculations reveal that tunneling is not possible in the water unassisted processes because of very high free energy activation barriers. The activation free energy barrier for the amino-imino tautomerization of 5, aziridine-2,3-diimine, and one water assisted, 5-W, are 58.1 kcal/mol and 14.8 kcal/mol, respectively and lowest among the 3-membered rings examined. The classical, over the barrier rate constant (kCVT) obtained by QMT calculation for 5-W5-W-P is 10.6 s-1 at 273 K. Inclusion small curvature tunneling (SCT) enhances the classical, over the barrier rate constant by 15.1 times at 273 K, i.e., kCVT+SCT is 160 s-1 and reveals nonclassical behaviour for the tautomerization of 5-W. A higher kinetic isotope effect in the tautomerization process of 4-W and 5-W also shows the noteworthy contribution of tunneling toward the tautomerization process. The nonclassical manifestation in 3-2W3-2W-P, the two-water assisted tautomerization of 3 having highest activation free energy barrier in the series, suggests that the tautomerization processes of 1-5 are experimentally feasible by tunneling and aromaticity.

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Publication details

The article was received on 22 Jun 2018, accepted on 11 Oct 2018 and first published on 12 Oct 2018


Article type: Paper
DOI: 10.1039/C8CP03963A
Citation: Phys. Chem. Chem. Phys., 2018, Accepted Manuscript
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    DFT Study on Quantum Mechanical Tunneling in Tautomerization of Three-membered Rings

    K. Jana and B. Ganguly, Phys. Chem. Chem. Phys., 2018, Accepted Manuscript , DOI: 10.1039/C8CP03963A

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