Infrared, Raman and NMR spectra, conformational stability, normal coordinate analysis and B3LYP calculations of 5-amino-4-cyano-3-(methylthio)-1H-pyrazole-1-carbothioamide

The Raman and infrared spectra of solid 5-amino-4-cyano-3-(methylthio)-1H-pyrazole-1-carbothioamide (AMPC, C6H7N5S2) 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. Aided by normal coordinate analysis and potential energy distributions, a confident vibrational assignment of all fundamentals is proposed herein. As a result of internal rotation around CN and/or CS bonds, 32 rotational isomers are suggested for AMPC (Cs symmetry). RHF and DFT/B3LYP quantum mechanical calculations including polarization and diffusion functions up to 6-311++G(d,p) basis sets, predict that after complete relaxation of the 32 possible isomers, four structures lie within 1500 cm−1 of the lowest energy conformer. However, vibrational analysis reveals the lowest energy conformer to be the only structure giving all real frequencies. Thus, the only stable conformer of AMPC is shown to have a fully planar skeleton with the NH2 groups trans to one another. The recorded IR and Raman spectral measurements favor the calculated structural parameters which are further supported by spectral simulation. Additional support is given by 1H and 13C NMR spectra recorded with the sample dissolved in DMSO-d6 and by predicted chemical shifts at the B3LYP/6-311+G(2d,p) level obtained using the Gauge-Invariant Atomic Orbitals (GIAO) method with and without inclusion of solvent using the Polarizable Continuum Model (PCM). Finally, CH3, CH3S, and NH2 torsional barriers to internal rotation have been investigated using the optimized structural parameters from the B3LYP method with the 6-31G(d) basis set. The results are discussed herein and compared with similar molecules whenever appropriate.