FT-IR, FT-Raman, NMR spectral analysis and theoretical NBO, HOMO–LUMO analysis of bis(4-amino-5-mercapto-1,2,4-triazol-3-yl)ethane by ab initio HF and DFT methods

A combined experimental and theoretical studies were conducted on the molecular structure and vibrational spectra of bis(4-amino-5-mercapto-1,2,4-triazol-3-yl) ethane (BAMTE). The FT-IR and FT-Raman spectra of BAMTE were recorded in the solid phase. The molecular geometry and vibrational frequencies of BAMTE in the ground state have been calculated by using the ab initio HF (Hartree–Fock) and density functional methods (B3LYP) invoking 6-311++G(d,p) basis set. The optimized geometric bond lengths and bond angles obtained by HF method shows best agreement with the experimental values. Comparison of the observed fundamental vibrational frequencies of BAMTE with calculated results by HF and density functional methods indicates that B3LYP is superior to the scaled Hartree–Fock approach for molecular vibrational problems. The difference between the observed and scaled wave number values of most of the fundamentals is very small. The thermodynamic functions and atomic charges of the title compound was also performed at HF/B3LYP/6-311++G(d,p) level of theories. A detailed interpretation of the FT-IR, FT-Raman, NMR spectra of BAMTE was also reported. The theoretical spectrograms for Infrared and Raman spectra of the title molecule have been constructed. Natural bond orbital analysis has been carried out to explain the charge transfer or delocalization of charge due to the intra-molecular interactions. Energy of the highest occupied molecular (HOMO) orbital and lowest unoccupied (LUMO) molecular orbital have been predicted.