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Gas-Phase Elimination Reaction of Ethyl (5-cyanomethyl-1,3,4-thiadiazol-2-yl)carbamate: A Computational Study
The gasâphase elimination reaction of ethyl (5âcyanomethylâ1,3,4âthiadiazolâ2âyl)carbamate has been studied computationally at the MP2/6â31++G(2d,p) level of theory. The values of the activation parameters and rate constants for the thermal decomposition were evaluated over a temperature range from 405.0 to 458.0 K. The temperature dependence of the rate constants was used to deduce the modified Arrhenius expression: log k405â458 K = (9.01 ± 0.49) + (1.32 ± 0.16) log T â (6946 ± 30) 1/T, which is in good agreement with the expression obtained from experimental data. The results confirm that the mechanism is a cisâconcerted elimination that occurs in two steps: The first one corresponds to the formation of ethylene and an intermediate, 5â(cyanomethyl)â1,3,4âthiadiazolâ2âylâcarbamic acid, via a sixâmembered cyclic transition state, and the second one is the decarboxylation of this intermediate via a fourâmembered cyclic transition step, leading to carbon dioxide and the corresponding 1,3,4âthiadiazole derivative (5âaminoâ1,3,4âthiadiazoleâ2âacetonitrile). The connectivity of transition states with their respective minima was verified through intrinsic reaction coordinate calculations, and the progress of the reaction was followed by means of Wiberg bond indices, resulting that both transition states have an âearlyâ character, nearer to the reactants than to the products