Ab initio calculation of the vibrational and electronic spectra of trans- and cis-azobenzene

Heike Fliegl,*† Andreas Köhn, Christof Hättig, and Reinhart Ahlrichs
Contribution from the Institut für Physikalische Chemie, Lehrstuhl für Theoretische Chemie, Universität Karlsruhe (TH), Kaiserstrasse 12, D-76128 Karlsruhe, Germany and Institute of Nanotechnology, Forschungszentrum Karlsruhe, P.O. Box 3640, D-76021 Karlsruhe, Germany

J. Am. Chem. Soc., 125, 9821-9827 (2003).
Received January 31, 2003; E-mail: heike.fliegl@chem-bio.uni-karlsruhe.de

We report accurate geometries and harmonic force fields for trans- and cis-azobenzene determined by second-order Møller-Plesset perturbation theory. For the trans-isomer, the planar structure with C2h symmetry, found in a recent gas electron diffraction experiment, is verified. The calculated vibrational spectra are compared with experimental data and density functional calculations. Important vibrational frequencies are localized and discussed. For both isomers, we report UV spectra calculated using the second-order approximate coupled-cluster singles-and-doubles model CC2 with accurate basis sets. Vertical excitation energies and oscillator strengths have been determined for the lowest singlet * and ππ* transitions. The results are compared with the available experimental data and second-order polarization propagator (SOPPA) and density functional (DFT) calculations. For both isomers, the CC2 results for the excitation energies into the S1 and S2 states agree within 0.1 eV with experimental gas-phase measurements.

Universität Karlsruhe
Forschungszentrum Karlsruhe

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