FTIR and Raman spectra and optimized geometry of 2,3,6-tri-fluorobenzoic acid dimer: a DFT and SQMFF study

Raman and FTIR spectra of the 2,3,6-tri-fluorobenzoic acid molecule have been recorded in the regions 50-4000cm⁻¹ and 400-4000cm⁻¹, respectively. Vibrational frequencies have been calculated by employing DFT method in dimeric form in optimum state. SQM force field has 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 bond lengths and angles of the title molecule have been interpreted and compared with the earlier reported experimental values for benzoic acid and some mono and di-fluorinated benzoic acids. Some of the vibrational frequencies of the title molecule 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.