A theoretical study of the low-lying excited states of trans- and cis-urocanic acid

A multiconfigurational second-order perturbation theory (CASPT2) study of t he lowest lying states in the gas-phase electronic spectra of trans- and ci s-urocanic acid is presented. Geometries of both isomers have been optimize d at the MP2/6-31G(d) and pi-CASSCF/ANO-L(4s3p1d,2s) levels of theory. The geometries are found to differ considerably between the two levels. The ver tical and 0-0 excitation spectra were calculated for each isomer. Both sing let and triplet states art: described for each, including the lowest lying pi pi* excitations and the n(O)pi* excitations. Remarkably, in the trans sp ectrum, it is found that the n(O)pi* state has a higher vertical excitation energy than the lowest pi pi* (5.12 vs 4.93 eV), but a lower band origin ( 4.10 vs 4.66 eV) Thus, the pi pi* and n(O)pi* surfaces cross at a coordinat e between that of the ground-state structure and the equilibrium excited st ate structure. The trans vertical spectrum consists of three intense (pi pi *) electronic transitions at 4.93, 5.40, and 6.00 eV, whereas the cis spect rum is dominated by a single intense transition at 4.15 eV and a weaker one at 5.&5 eV. The wave functions of the excitations typically show a multico nfigurational character, with the weighting of doubly excitated configurati ons exceeding 20% in a number of instances. The lowest lying Rydberg states (pi 3s) were found at 5.47 and 5.67 eV for the trans and cis isomers, resp ectively.