Identification of singlet excited electronic states of hydroxybenzoic acid isomers

The photoelectron spectra of benzoic and para-, meta-, and ortho-hydroxybenzoic acids and their UV absorption spectra in ethanol are obtained. The photoelectron spectra in the energy region up to 15.5 eV are interpreted based on the B3LYP/6-311+G(d, p) calculations. Based on the TDDFT B3LYP/6-311+G(d, p) calculations and taking into account the obtained interpretation of photoelectron spectra, the UV absorption bands are assigned to particular singlet transitions. For each transition, the electron configuration that makes the main contribution to the transition is determined. It is shown that the transition energy depends on the energy gap between the corresponding occupied and unoccupied molecular orbitals.