Positional and Conformational Isomerism in Hydroxybenzoic Acid: A Core-Level Study and Comparison with Phenol and Benzoic Acid

Three positional isomers of hydroxybenzoic acid, as well as phenol and benzoic acid, were studied using core-level photoemission and X-ray absorption spectroscopies, supported by quantum chemical calculations. While 2-hydroxybenzoic (salicylic) acid exists as a single conformer with an internal hydrogen bond, 3- and 4-hydroxybenzoic acids are mixtures of multiple conformers. The effects due to isomerism are clearly seen in the C 1s and O 1s photoelectron spectra, whereas the conformational effects on the binding energies are less pronounced. The O 1s photoelectron spectrum of salicylic acid is significantly different from that of the other two isomers, providing a signature of the hydrogen bond. In contrast, the oxygen K edge X-ray absorption spectra of the three hydroxybenzoic acids show only minor differences. The salicylic acid absorption spectrum at the carbon K edge shows a more resolved vibrational structure than the spectra of the other molecules, which can be explained in part by the existence of a single conformer. Our theoretical study of vibrational excitations in the lowest C 1s absorption bands of salicylic and 4-hydroxybenzoic acids indicates that the observed structure can be assigned to 0-0 lines of various electronic transitions since most of the totally symmetric vibrational modes with sufficiently large frequencies to be resolved are predicted to be inactive. Significant sensitivity of the C 1s excitations in 3-hydroxybenzoic acid to rotational conformerism was predicted but not observed due to spectral crowding.