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 investigated. 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 center dot mol(-1) and the reaction energies (Delta E) is only 2.67 kcal center dot mol(-1). However, the reaction energy barrier for tautomerism of reaction 1 (b -> e) is 54.90 kcal center dot 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 center dot 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.
tautomerism intramolecular hydrogen shifts 5-amino-tetrazole kinetics tunneling effect