Conformational stability of ibuprofen: assessed by DFT calculations and optical vibrational spectroscopy

A thorough conformational analysis of ibuprofen [2-(4-isobutylphenyl) propionic acid] was carried by out, using density functional theory (DFT) calculations coupled to optical vibrational spectroscopy (both Raman and FTIR). Eight different geometries were found to be energy minima. The relative orientations of the substituent groups in the ibuprofen molecule, which can be considered as a para-substituted phenyl ring, were verified to hardly affect its conformational stability. The internal rotations converting the calculated conformers of ibuprofen were studied and the intramolecular interactions governing the conformational preferences of the molecule were analyzed by quantitative potential energy deconvolution using Fourier type profiles. The harmonic vibrational frequencies and corresponding intensities were calculated for all the conformers obtained, leading to the assignment of the spectra, and evidencing the sole presence of one of the lowest energy conformers in the solid state. Vibrational spectroscopic proof of intermolecular hydrogen bonds between the carboxylic groups of adjacent ibuprofen molecules, leading to the formation of dimers, was also obtained.