Theoretical studies on the tautomerism and intramolecular hydrogen shifts of 5-Amino-tetrazole in the gas phase

The tautomerism and intramolecular hydrogen shifts of 5-amino-tetrazole in the gas phase were studied in the present work. The minimum energy path (MEP) information of 5-amino-tetrazole was obtained at the CCSD(T)/ 6-311G**//MP2/6-311G** level of theory. The six possible tautomers of 1H, 4H-5-imino-tetrazole (a), 1H-5-amino-tetrazole (b), 2H-5-amino-tetrazole (c), 1H, 2H-5-imino-tetrazole (d), the mesoionic form (e) and 2H, 4H-5-imino-tetrazole (f) were inve stigated. Among these tautomers, there are 2 amino- forms, 3 imino- forms, and 1 mesoionic structure form. In all the tautomers, 2-H form (c) is the energetically preferred one in the gas phase. In the imino- tautomers, the energy value of the compound d is similar as that of the compound f but it is higher than the energy value of the compound a. The potential energetic surface (PES) and kinetics for five reactions have been investigated. Reaction 2 (b→c) was hydrogen shifts only in which the 1-H and 2-H rearrangement. This means that the reaction 2 (b→c) is energetically favorable having an activation barrier of 45.66 kcal·mol^-1 and the reaction energies (ΔE) is only 2.67 kcal·mol^-1. However, the reaction energy barrier for tautomerism of reaction 1 (b→e) is 54.90 kcal·mol^-1. Reaction 1 (b→a), reaction 3 (c→d), and reaction 5 (c→f) were amino- →imino- tautomerism reactions. The energy barriers of amino- →imino- tautomerism reactions required are 59.39, 65.57, 73.61 kcal·mol^-1 respectively in the gas phase. The calculated values of rate constants using TST, TST/Eckart, CVT, CVT/SCT and CVT/ZCT methods using the optimized geometries obtained at the MP2/ 6-311G** level of theory show the variational effects are small over the whole temperature range, while tunneling effects are big in the lower temperature range for all tautomerism reactions.