Theoretical (DFT) and experimental (FT-IR & FT Raman) approach to investigate the molecular geometry and vibrational properties of 2,5- and 2,6-dihydroxytoluenes

•Measured FT-IR and FT Raman spectra for 5DHT and 6DHT.•Torsional scans were made and barrier heights were evaluated.•Theoretical structure of 5DHT is Cs with OH moieties in anti-anti conformation.•Unambiguous Vibrational assignments were made for 5DHT and 6DHT.•Trimers of both the molecules stabilize in C1 structure. Fourier Transform infrared spectra were measured for 2, 5- dihydroxytoluene (5DHT) and 2, 6- dihydroxytoluene (6DHT), in the spectral range 4000–400 cm−1. Fourier Transform Raman spectra were also recorded for the same molecules, in the spectral region 3500–100 cm−1. Torsional potential energy scans were attempted around ring – ortho hydroxyl C-O bonds, and ring-methyl C-C bond, for the two molecules. Barrier heights were computed for the above bonds for both the target molecules. Optimized molecular geometry, general valence force constants, vibrational wave numbers (harmonic), infrared intensities, and Raman scattering intensities were calculated, employing density functional theory and using B3LYP/6-311++G(d,p) level of formalism. Observed and measured frequencies agreed with an rms error 8.6 and 8.3 cm−1, for 5DHT and 6DHT, respectively, on scaling. In addition, experimental infrared and Raman spectra agreed with their simulated counterparts with good accuracy. Using potential energy distribution (PED) and eigenvectors, all fundamentals were assigned unambiguously for both molecules. Geometry optimization was made for trimers of the two molecules at the same level of theory as used for the monomers. Existence of inter-molecular hydrogen bond was predicted. [Display omitted]