Dimerisation and vibrational spectroscopic properties of (RS)-phenylsuccinic acid

Abstract

Single crystals of (RS)-phenylsuccinic acid (RSPSA) were grown by the slow evaporation technique and vibrational spectral analysis was carried out using near-IR Fourier transform Raman and Fourier transform IR spectroscopy. The density functional theoretical (DFT) computations were also performed at the B3LYP/6–31G(d) level to derive the equilibrium geometry, vibrational wavenumbers, intensities and atomic charges. Vibrational spectral investigation confirmed the formation of cyclic dimers in the crystal, with the carboxyl groups of each acid molecule being hydrogen bonded to those of the adjacent molecules. Each molecule is connected to two other molecules, forming infinite chains. The wavenumber difference observed between the carbonyl modes in the IR and Raman spectra might be attributed to the intramolecular association based on C O··· H type hydrogen bonding in the molecule. In a hydrogen-bonded carbonyl, resonance can occur, which places a partial negative charge on the oxygen atom accepting the hydrogen bond and a positive charge on the atom donating the hydrogen. The resulting partial ‘transfer of allegiance’ of the proton enhances resonance and lowers the carbonyl stretching wavenumber. The OH stretching vibrational wavenumber is red-shifted owing to the formation of strong OH··· O hydrogen bonds by hyperconjugation between carbonyl oxygen lone electron pairs and OH σ* antibonding orbitals. Vibrational analysis was used to investigate the CH··· O interaction and electronic effects, induction of the methylene group by an adjacent electronegative atom and the resulting increase of stretching mode wavenumbers and decrease of infrared intensities. Copyright © 2006 John Wiley & Sons, Ltd.