FT-Raman, ATR-FTIR and FT-NMR spectra, vibrational assignments, normal coordinates analysis, force constants, barriers to internal rotations, and DFT computations for 4-(Trifluoromethyl)pyrimidine-2-thiol conformers and its thione tautomers

A combined experimental, structural and theoretical investigation of 4-(Trifluoromethyl)pyrimidine-2-thiol (C₅H₃F₃N₂S; 4TFP2T) has been carried out to elucidate its conformational preferences, tautomeric stability, and spectroscopic signatures. In the solid phase, the Raman (50-4000 cm⁻¹) and attenuated total reflectance (ATR) infrared (380-4000 cm⁻¹) spectra were recorded, while solution-phase ¹H, ¹³C, ¹⁵N, ¹⁹F, and DEPT (45, 90, 135) NMR and UV-Vis. spectra were obtained in deuterated DMSO. Density Functional Theory (B3LYP, P3PW91, ωB97XD) optimizations and frequency calculations were performed on six thio-enol (thiol) conformers and six thio-keto (thione) tautomer's utilizing 6-311+G(d) and 6-311++G(d,p) basis sets. Two isoenergetic thiol conformers, namely trans-cis (S2-TC) and trans-trans (S4-TT) were identified, with S4-TT less favored by 46-49 cm⁻¹ (0.12-0.14 kcal/mol), and S2-TC favored over the lowest thione forms (S7-T, S8-T) by ∼ 1281-1905 cm⁻¹ (3.66-5.45 kcal/mol), consistent with the solid-phase IR signature of the thiol. The predicted/observed¹H, ¹³C, ¹⁵N and ¹⁹F NMR chemical shifts (B3LYP/ωB97XD) provide further support for thiol predominance (S2-TC, S4-TT). Normal coordinate analysis (NCA) and potential energy distributions (PEDs) were used to assign all IR and Raman bands and to derive internal-coordinate force constants (FCs). Moreover, complementary natural bond orbital (NBO) and Mulliken population analyses reveal pronounced intramolecular charge transfer and key electrophilic/nucleophilic sites, while frontier molecular orbital (HOMO-LUMO) energy gap and molecular electrostatic potential (MEP) mapping characterize reactivity descriptors. While, time-dependent DFT simulations reproduce the observed UV–Visible transitions. These results provide a detailed understanding of 4TFP2T's structure, bonding, and spectroscopic behavior.