Abstract
Abstract The existence of ring transformation equilibrium (bond switch) in 5-(1-aminoethylideneamino)-3-methyl-1,2,4-thiadiazole (5a) was confirmed by nitrogen-15 labeling experiment. Kinetic studies were carried out under neutral or acidic conditions. Kinetic parameters of the intramolecular equilibration of the monoprotonated 5a (5a-H+) were obtained by line shape analyses of the 1H NMR, i.e., ΔG273\eweq=14.8±0.2, ΔH\eweq=11.6±1.0 kcal·mol−1 ΔS\eweq=−12±3 eu, k273=9 s−1. Position of protonation was determined at the nitrogen of the ethylideneamino part of the amidino group. It was concluded that the central sulfur atom moves back and forth in the range of 0.38 Å nine times a second at 0 °C, that is, bond switching.
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