Computation and Simulation Unit (Analytical Discipline and Centralized Instrument Facility), CSIR-Central Salt and Marine Chemicals Research Institute, Bhavnagar-364002, Gujarat, India.
Phys Chem Chem Phys. 2018 Nov 14;20(44):28049-28058. doi: 10.1039/c8cp03963a.
We have examined 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 processes in (1-5), which lead 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. 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 barriers for the amino-imino tautomerization of 5, aziridine-2,3-diimine, and one water assisted, 5-W, are 58.1 kcal mol-1 and 14.8 kcal mol-1, respectively and the lowest among the 3-membered rings examined. The classical over the barrier rate constant (kCVT) obtained by QMT calculation for 5-W→5-W-P is 10.6 s-1 at 273 K. Inclusion of 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 indicates a pronounced contribution of tunneling toward the tautomerization process. The two-water assisted tautomerization of 3 has the highest activation free energy barrier in the series indicating a nonclassical contribution to 3-2W→3-2W-P. These results suggest that the tautomerization processes of 1-5 are experimentally feasible by tunneling and aromaticity.
我们研究了在没有和存在水分子的情况下，三个一组的环系统 (1-5) 中的酮-烯醇和氨-亚胺互变异构。芳香性控制着 (1-5) 中的酮-烯醇和氨-亚胺互变异构过程，导致烯醇和亚胺衍生物的形成。在三个一组的环系统的互变异构过程中，量子力学隧道效应（QMT）的可能性尚未报道。M062X/6-311+G(d,p) 理论水平的 QMT 计算表明，由于非常高的自由能活化能垒，隧道在水辅助过程中是不可能的。氨基-亚氨基互变异构的活化自由能垒，5，氮丙啶-2,3-二亚胺，和一个水辅助的 5-W，分别为 58.1 kcal mol-1 和 14.8 kcal mol-1，是所研究的三个一组的环中最低的。通过 QMT 计算获得的 5-W→5-W-P 的经典过垒速率常数（kCVT）在 273 K 时为 10.6 s-1。在 273 K 时，包括小曲率隧道（SCT）在内，可使经典过垒速率常数提高 15.1 倍，即 kCVT+SCT 为 160 s-1，表明 5-W 的互变异构为非经典行为。在 4-W 和 5-W 的互变异构过程中，较高的动力学同位素效应也表明隧道对互变异构过程有明显的贡献。在该系列中，具有两个水分子辅助的 3 的互变异构的活化自由能垒最高，表明 3-2W→3-2W-P 的互变异构具有非经典贡献。这些结果表明，通过隧道和芳香性，1-5 的互变异构过程在实验上是可行的。
