Polymorphism and intermolecular interactions in crystalline fluorinated alkylbenzoic acids

Thermodynamic and spectral characteristics of fluorinated derivatives of alkylbenzoic acids (FABA) were studied in the temperature range 100–550 K. Experimental values of temperatures, enthalpies and entropies of phase transitions, as well as hydrogen bond energy at different temperatures and in the various physical (phase) states were compared to those for alkyl- and fluoroalkyl-benzoic acids. IR absorption spectra of substances studied were interpreted on the basis of theoretical calculations of frequencies, shapes and intensities of normal modes of molecular vibrations using the published data for related compounds. The information derived was used to construct models of packing of dimerized molecules of FABA in the crystalline state, and possible causes of the absence of mesomorphism on complete fluorination of alkyl radical were discussed.