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The experimental and theoretical study on the structures and vibrations of 5-fluoro-salicylic acid and 5-chloro-salicylic acid (5-FSA and 5-ClSA, C 7 H 5 FO 3 and C 7 H 5 ClO 3 ) is presented. The Fourier transform infrared spectra (4000-400 cm −1 ) and the Fourier transform Raman spectra (4000-50 cm −1 ) of the title molecules in the solid phase were recorded. The molecular structures, vibrational wavenumbers, infrared intensities, Raman intensities and Raman scattering activities were calculated for a pair of molecules linked by the intermolecular O-H· · ·O hydrogen bond. The geometrical parameters and energies of 5-FSA and 5ClSA were obtained for all eight conformers/isomers from density functional theory (DFT) (B3LYP) with 6-311++G(d,p) basis set calculations. The computational results identified the most stable conformer of 5-FSA and 5-ClSA as the C1 form. 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 spectroscopic and theoretical results were compared with the corresponding properties for 5-FSA and 5-ClSA monomers and dimer of C1 conformer. The optimized bond lengths, bond angles and calculated wavenumbers showed the best agreement with the experimental results.

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