A Theoretical Study of the Low-Lying States of the Anionic and Protonated Ionic Forms of Urocanic Acid

A multistate second-order perturbation theory (MS-CASPT2) study of the lowest lying states in the electronic spectra of urocanic acid in vacuo is presented. The anionic trans and cis isomers, as well as the biologically important trans protonated ionic structure, are considered. The vertical and 0-0 excitation spectra were computed for each system at the MS-CASPT2/ANO-L level, describing the lowest lying ππ* and ππ* singlet . and triplet states. In all three systems, a weakly absorbing ππ* singlet state was observed at ∼4.0 eV in the vertical excitation spectrum, suggesting both a novel assignment and an alternative explanation for the previously described wavelength dependent photochemistry of this molecule. The trans anion vertical spectrum otherwise comprises three intense ππ* transitions, at 4.14, 4.59, and 5.00 eV, whereas that of the cis anion shows two, at 4.52 and 5.01 eV. Conversely, the vertical spectrum of the protonated trans system is dominated by a single, intensely absorbing ππ* state, computed at 4.76 eV. The wave functions of the ππ* states all show a multiconfigurational character, which is most pronounced in the protonated ionic structure. The lowest vertical singlet states are of ππ* character (trans anion, 4.13 eV; cis anion, 3.81 eV; trans protonated, 3.19 eV), although the lowest triplet states are due to ππ* transitions (trans anion, 3.40 eV; cis anion, 3.39 eV; trans protonated, 2.56 eV). In each of the anionic systems, the origin of the most intense singlet ππ* transition in the vertical spectrum (trans anion, 4.03 eV; cis anion, 3.81 eV) lies below the corresponding lowest singlet ππ* vertical excitation energy, further suggesting that the latter state may not be directly and efficiently populated. Good agreements with available experimental data are noted throughout. © 2000 American Chemical Society.