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

• Published in: Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy Vol. 79; no. 5; pp. 1722 - 1730
• Main Authors: Mohamed, Tarek A., Hassan, Ali M., Soliman, Usama A., Zoghaib, Wajdi M., Husband, John, Hassan, Saber M.
• Format: Journal Article
• Language: English
• Published: England Elsevier B.V 01.09.2011
• Subjects: DFT calculations; Infrared; Magnetic Resonance Spectroscopy; Molecular Conformation; Molecular Structure; Nitriles - chemistry; NMR spectra; Pyrazoles - chemistry; Raman; Spectrophotometry, Infrared; Spectrum Analysis, Raman; Stereoisomerism; Thermodynamics; Thiocarbamates - chemistry; Vibration; Vibrational assignments; Raman; Vibrational assignments; NMR spectra; Infrared; DFT calculations
• ISSN: 1386-1425; 1873-3557
• DOI: 10.1016/j.saa.2011.05.044

[Display omitted] ► We have carried out B3LYP calculations for methyl-5-amino-4-cyano-3-(methylthio)-1H-pyrazole-1-carbodithioat. ► We have interpreted Raman, infrared, 1H NMR and 13C NMR spectra in the solid phase. ► For C s conformer, the two methyl groups are directed towards nitrogen atoms of the pyrazole ring and C S, while the –NH 2 group is triagonal planar. ► Simulated Raman, infrared, 1H NMR and 13C NMR spectral interpretations reveal a relatively good correlation with the experimental data. ► The CH 3 barriers to internal rotations were also investigated. 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 C S, while the –NH 2 group retains sp 2 hybridization and C–C N 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.