Conformational stability, theoretical and experimental vibrational spectral analysis of 2,4,6-trihydroxybenzaldehyde

The experimental and theoretical study on the structures and vibrations of 2,4,6-trihydroxybenzaldehyde (THB) is presented. The Fourier transform infrared spectra (4000–400 cm−1) and the Fourier transform Raman spectra (4000–50 cm−1) of the title molecule in the solid space have been recorded. The geometrical parameters and energies have been obtained for all 16 conformers from DFT (B3LYP) with 6-31+G(d,p) basis set calculations. There are 16 conformers, Cn = 1–16 for this molecule. The computational results identify the most stable conformer of THB as the C1 form. The vibrations of stable and unstable conformers of THB are researched with the aid of quantum chemical calculations. The complete assignments were performed on the basis of the potential energy distribution (PED) of the vibrational modes, calculated with scaled quantum mechanical (SQM) method. The molecular structures and vibrational frequencies, infrared intensities and Raman scattering actives have been calculated. The optimized bond length, bond angles and calculated frequency showed the best agreement with experimental results. The formation of the hydrogen bond was investigated using NBO calculations. The calculated HOMO and LUMO energies show that charge transfer occurs within the molecule.