A Theoretical Study of the Low-Lying Excited States of trans- and cis-Urocanic Acid

A multiconfigurational second-order perturbation theory (CASPT2) study of the lowest lying states in the gas-phase electronic spectra of trans- and cw-urocanic acid is presented. Geometries of both isomers have been optimized at the MP2/6-31G(d) and jr-CASSCF/ANO-L(4s3pld, 2s) levels of theory. The geometries are found to differ considerably between the two levels. The vertical and 0-0 excitation spectra were calculated for each isomer. Both singlet and triplet states are described for each, including the lowest lying noπ* excitations and the noπ* excitations. Remarkably, in the trans spectrum, it is found that the noπ* state has a higher vertical excitation energy than the lowest ππ* (5.12 vs 4.93 eV), but a lower band origin (4.10 vs 4.66 eV). Thus, the ππ* and noπ* surfaces cross at a coordinate between that of the ground-state structure and the equilibrium excited state structure. The trans vertical spectrum consists of three intense (nn) electronic transitions at 4.93, 5.40, and 6.00 eV, whereas the cis spectrum is dominated by a single intense transition at 4.15 eV and a weaker one at 5.85 eV. The wave functions of the excitations typically show a multiconfigurational character, with the weighting of doubly excitated configurations exceeding 20% in a number of instances. The lowest lying Rydberg states (π3s) were found at 5.47 and 5.67 eV for the trans and cis isomers, respectively. © 1999 American Chemical Society.