FTIR and Raman spectra, DFT and SQMFF calculations for geometrical interpretation and vibrational analysis of some trifluorobenzoic acid dimers

Raman and FTIR spectra for 2,4,5- and 3,4,5-trifluorobenzoic acid molecules have been recorded in the regions 50–4000 cm−1 and 400–4000 cm−1, respectively. Vibrational frequencies have also been calculated in optimum state by employing DFT method in both the dimeric and monomeric forms. SQM force fields have also been used to calculate potential energy distributions in order to make conspicuous vibrational assignments. Raman activities calculated by DFT method have been converted to the corresponding Raman intensities using Raman scattering theory. Optimized geometries of both the molecules have been interpreted and compared with the earlier reported experimental values for benzoic acid and some mono and difluorinated benzoic acids. Some of the vibrational frequencies of the title molecules are effected upon profusely with the fluorine substitutions in comparison to benzoic acid and these differences have been interpreted. The strong doubly hydrogen-bonded interface of the dimerized system is well demonstrated by the red shift in OH stretching frequency concomitant with the elongation of bond length.