Vibrational assignments, normal coordinate analysis, B3LYP calculations and conformational analysis of methyl-5-amino-4-cyano-3-(methylthio)-1H-pyrazole-1- carbodithioate

The Raman and infrared spectra of solid methyl-5-amino-4-cyano-3- (methylthio)-1H-pyrazole-1-carbodithioate (MAMPC, C ₇H ₈N ₄S ₃) were measured in the spectral range of 3700-100 cm ^-1 and 4000-200 cm ^-1 with a resolution of 4 and 0.5 cm ^-1, respectively. Room temperature ¹³C NMR and ¹H NMR spectra from room temperature down to -60 °C were also recorded. As a result of internal rotation around C-N and/or C-S bonds, eighteen rotational isomers are suggested for the MAMPC molecule (Cs symmetry). DFT/B3LYP and MP2 calculations were carried out up to 6-311++G(d,p) basis sets to include polarization and diffusion functions. The results favor conformer 1 in the solid (experimentally) and gaseous (theoretically) phases. For conformer 1, the two -CH ₃ groups are directed towards the nitrogen atoms (pyrazole ring) and CS, while the -NH ₂ group retains sp ² hybridization and C-CN bond is quasi linear. To support NMR spectral assignments, chemical shifts (δ) were predicted at the B3LYP/6-311+G(2d,p) level using the method of Gauge-Invariant Atomic Orbital (GIAO) method. Moreover, the solvent effect was included via the Polarizable Continuum Model (PCM). Additionally, both infrared and Raman spectra were predicted using B3LYP/6-31G(d) calculations. The recorded vibrational, ¹H and ¹³C NMR spectral data favors conformer 1 in both the solid phase and in solution. Aided by normal coordinate analysis and potential energy distributions, confident vibrational assignments for observed bands have been proposed. Moreover, the CH ₃ barriers to internal rotations were investigated. The results are discussed herein are compared with similar molecules whenever appropriate.