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

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• 作者：Tarek A. Mohamed^a ; ^{tarek_ama@hotmail.com"" rel=""nofollow} ; Ali M. Hassan^a ; Usama A. Soliman^a ; ¹ ; Wajdi M. Zoghaib^b ; John Husband^b ; Saber M. Hassan^a
• 关键词：Raman ; Infrared ; NMR spectra ; Vibrational assignments ; DFT calculations
• 刊名：Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
• 出版年：2011
• 出版时间：September 2011
• 年：2011
• 卷：79
• 期：5
• 页码：1722-1730
• 全文大小：510 K

文摘

The Raman and infrared spectra of solid methyl-5-amino-4-cyano-3-(methylthio)-1H-pyrazole-1-carbodithioate (MAMPC, C₇H₈N₄S₃) 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 –CH₃ groups are directed towards the nitrogen atoms (pyrazole ring) and CS, while the –NH₂ group retains sp² 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 CH₃ barriers to internal rotations were investigated. The results are discussed herein are compared with similar molecules whenever appropriate.