A theoretical study on the structure of 2-amino-1,3,4-thiadiazole and its 5-substituted derivatives in the gas phase, water, THF and DMSO solutions

•Investigation of the structure of 2-amino-1,3,4-thiadiazole and its 5-substituted derivatives•Investigation of the thermodynamic parameters in the gas phase and solutions•Investigation of the effect of different solvents on geometry and stability

The results of a detailed DFT (B3LYP) and ab initio (HF and MP2) investigation on one of the amino-imino tautomers and its derivatives are presented here. The energy, geometrical parameters and vibrational frequencies of 2-amino-1,3,4-thiadiazole tautomerization in the gas phase, water, THF and DMSO solutions were calculated using 6-311++G(d,p), 6-311G(d) and 6-31+G(d,p) basis sets. The solvent effect was explored using the PCM method and the related conformers have been fully optimized at the B3LYP/6-311++G** level of theory. For the investigated molecule, proton-transfer from N to N and from N to C occurred, which leads to three tautomers: 2-amino-1,3,4-thiadiazole (A), 2(3H)-imino-1,3,4-thiadiazole (B) and 2(5H)-imino-1,3,4-thiadiazole (C). For B and C, geometrical isomerism (E and Z) was also considered. This leads to five isomers: A, B1, B2, C1 and C2 for each derivative. However, all calculations evidently show that isomer A strongly or solely dominates and dictates the structure of 2-amino-1,3,4-thiadiazoles. Substituents and solvents have no effect on the tautomeric preferences.