The spectroscopic (FT-IR and FT-Raman) and theoretical studies of 5-bromo-salicylic acid

The experimental and theoretical study on the structures and vibrations of 5-bromo-salicylic acid (5-BrSA, C7H5BrO3) are presented. The Fourier transform infrared spectra (4000–400 cm−1) and the Fourier transform Raman spectra (4000–0 cm−1) of the title molecule in the solid phase have been recorded. The geometrical parameters and energies have been obtained for all eight conformers from DFT (B3LYP) with 6-311++G(d,p) basis set calculations. There are eight conformers, Cn, n = 1–8 for this molecule. The computational results identify the most stable conformer of 5-BrSA as the C1 form. The vibrations of stable and unstable conformers of 5-BrSA are researched with the aid of quantum chemical calculations. The complete assignments were performed on the basis of the total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics (SQM) method. The molecular structures, vibrational frequencies, infrared intensities and Raman scattering activities were calculated a pair of molecules linked by the intermolecular OH…O hydrogen bond. The spectroscopic and theoretical results are compared to the corresponding properties for 5-BrSA monomers and dimer of C1 conformer. The optimized bond lengths, bond angles and calculated frequencies showed the best agreement with the experimental results.