FT-Raman, FT-IR spectra and DFT calculations on monomeric and dimeric structures of 5-fluoro- and 5-chloro-salicylic acid

• Published in: Journal of Raman spectroscopy 2010-09, Vol.41 (9), p.1085-1097
• Main Authors: Karabacak, Mehmet, Kose, Etem, Kurt, Mustafa
• Format: Article
• Language: English
• Subjects: 5-chloro-salicylic acid; 5-fluoro-salicylic acid; Bonding; DFT; Fourier transforms; intermolecular hydrogen bond; IR and Raman spectra; Mathematical analysis; Molecular structure; Monomers; Raman scattering; Vibration; Wavenumber
• ISSN: 0377-0486; 1097-4555; 1097-4555
• DOI: 10.1002/jrs.2551

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, C7H5FO3 and C7H5ClO3) 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. Copyright © 2010 John Wiley & Sons, Ltd. Experimental and theoretical studies on molecular structures of 5‐fluoro‐ and 5‐chloro‐salicylic acid are presented. The FT‐IR (between 4000 and 400 cm−1 region) and FT‐Raman (between 4000 and 50 cm−1 region) spectra have been recorded, and the detailed vibrational assignments are reported for the first time. The most stable monomer conformers of compounds were determined and according to these results dimer conformations analyzed with DFT/B3LYP using 6‐311++G(d,p) level of theory. The reliable vibrational assignments were made on total energy distribution, calculated with scaled quantum mechanics method.