Add time:07/28/2019         Source:sciencedirect.com

The infrared (4000–225 cm−1) and Raman (3500−60 cm−1) spectra of 2-Amino-5-(ethylthio)-1,3,4-thiadiazole (AEST; C4H7N3S2) have been recorded in the solid phase. In addition, the 1H and 13C NMR spectra of AEST were obtained in DMSO-d6. We have focused on five staggered conformers (1–5) resulting from rotations of the methyl, ethyl and thioethyl groups around CC and CS bonds with the NH2 group having a non-planar (sp3) geometry. DFT-B3LYP/6-31G(d) calculations proved that only conformers 4 and 5 produce real vibrational frequencies and conformer 5 is the favored one with an energy difference of 124 cm−1 (0.35 kcal/mol) in agreement with the observed ethyl torsion at 92 cm−1. The calculated frequencies, infrared intensities, Raman activities and potential energy distributions were compiled with spectral observations in favor of conformer 5. All vibrational modes were assigned to their corresponding vibrational bands. Applying the observed methyl torsions, the kinetic parameter F number was found to be 5.3263 cm−1 and an average V3 value of 1565 ± 14 cm−1 (4.48 ± 0.04 kcal/mol) was obtained. These values are close to the methyl barriers predicted from B3LYP/6-31G(d) potential surface scans. Moreover, the 1H and 13C NMR chemical shifts were also estimated for conformers 4 and 5 with DMSO as solvent implementing GIAO/PCM approximation utilizing B3LYP method at 6–311++G (d,p) basis set. The results are reported and discussed herein and compared with 2-Amino-5-ethyl-1,3,4-thiadiazole (AET) whenever appropriate.

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