Quantum chemical conformational analysis and X-ray structure of 4-methyl-3-thiosemicarbazide

We present X-ray crystallographic results and gas-phase electronic structure calculations of the geometry of 4-methyl-3-thiosemicarbazide. Using the Hartree-Fock theory with a 6-31G* basis set, we calculated relative energies for eight different conformations. For the lowest-energy conformations of each of the four possible combinations of rotamers about the two C-N bonds, we also included electron correlation by Møller-Plesset second-order (MP2) perturbation theory with the same basis set. From these calculations, we selected the lowest-energy structure and calculated structural parameters at the MP2 level of theory with the larger correlation-consistent cc-pVDZ basis set. The geometry of the minimum-energy gas-phase structure is in good agreement with the structure observed experimentally in the crystal.