A Theoretical Study of the Low-Lying Excited States of trans- and cis-Urocanic Acid
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چکیده
A multiconfigurational second-order perturbation theory (CASPT2) study of the lowest lying states in the gas-phase electronic spectra of transand cis-urocanic acid is presented. Geometries of both isomers have been optimized at the MP2/6-31G(d) and π-CASSCF/ANO-L(4s3p1d,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 ππ* 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 (ππ*) 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.
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تاریخ انتشار 1999