Vibrational analysis and conformational stability of 5-methoxy- and 5-methylthio-1,2,3-triazines: a combined Raman,...

Raman spectra (3700-100 cm^-1) and diffuse reflectance infrared (DRIFT) spectra (4000-400 cm^-1) of 5-methoxy-1,2,3-triazine (MeOTAz, C₄H₅N₃O) and 5-methylthio-1,2,3-triazine (MeSTAz, C₄H₅N₃S) were recorded in the solid state. Their vibrational and conformational properties were investigated using density functional theory (DFT) calculations, primarily employing the B3LYP functional in conjunction with the 6-311++G(d,p) and aug-cc-pVTZ basis sets. To improve the description of the vibrational spectra, anharmonic frequency calculations and solid-state density functional perturbation theory (DFPT) calculations were also performed. The computational results, together with potential energy surface scans, indicate that the trans conformer is the global minimum, whereas the cis and orthogonal conformers are less stable by approximately 2.5-7.5 kcal/mol. Normal coordinate analyses were carried out to obtain force constants in internal coordinates and potential energy distributions (PEDs), providing a rigorous basis for complete vibrational assignments and a detailed assessment of substituent effects. Natural Bond Orbital (NBO) analysis revealed that replacement of oxygen by sulfur significantly modifies the local electronic structure and donor-acceptor interactions while preserving the extensive π-electron delocalization within the triazine ring. The results demonstrate that n → π* conjugative interactions play an important role in stabilizing the nearly planar molecular geometries, whereas sulfur substitution increases conformational flexibility and alters the vibrational force field.