DFT simulation of barrier heights, infrared and Raman spectra, and investigation of vibrational characteristics of 2-((2-aminopyridin-3-yl) methylene) hydrazinecarbothioamide and its N-methyl variant

ABSTRACT

Fourier transform infrared spectra, for 2-((2-aminopyridin-3-yl) methylene) hydrazinecarbothioamide (APHT) and 2((2-aminopyridin-3-yl) methylene)-N-methylhydrazinecarbothioamide (APMHT), were recorded in the spectral range 4000–400 cm⁻¹. Their Raman counterparts were measured in the spectral region 4000–50 cm⁻¹. Preliminary values of dihedral angles around five rotating bonds C–NH₂ pyridyl C–CN, N–N, N–CS and C–NH₂ aliphatic in APHT, required for initiating geometry optimization, were obtained by pairing successive bonds and evaluating torsional potential energy for various values of dihedral angle around these bonds in the entire conformational space spanning 0° to 360°. Barrier heights, around five rotating bonds in APHT and six rotating bonds in APMHT were computed, making torsional scans in conformational space from 0° to 360°. This indicated existence of two rotational isomers for APMHT. General valence force field, harmonic vibrational fundamentals, potential energy distribution (PED), along with infrared and Raman intensities were determined using DFT/B3LYP/6-311++G(d,p) level of theory for both the molecules. Good agreement was found between measured and simulated spectra, for APHT and APMHT. This was also true for corresponding Raman spectra. The rms error between experimental and theoretical vibrational frequencies was 9.00 and 6.70 cm⁻¹, for APHT and APMHT, respectively. All vibrational fundamentals were assigned unambiguously, on the basis of computed PED, eigenvectors, and literature range for the first time. These assignments were further supported by comparing with attribution of corresponding bonds in the parent molecule pyridine and a related molecule 2-((2-aminopyridin-3-yl)methylene)-N-ethylhydrazinecarbothioamide, wherever possible.

KEYWORDS:

• Hydrazinecarbothioamides
• barrier heights
• FT-IR
• FT Raman
• DFT

Acknowledgements

The authors sincerely acknowledge financial support, under SAP-DRS-II, extended by the University Grants Commission, New Delhi, India through their Lr.No.F.530/24/DRS-II, 2015 (SAP). They are thankful to Sophisticated Analytical Instrumentation Facility (SAIF), IIT Madras, Chennai, India (Raman) and NIT-Warangal, (IR) for spectral measurements. The first author (KSS) is grateful to the management of SR University Warangal, India, for permitting him to undertake the reported investigations. The Second author (KR) is thankful to the CVR College of Engineering, Hyderabad, for motivating and providing the resources and constant support and encouragement to complete this research work.

Disclosure statement

No potential conflict of interest was reported by the author(s).