Valence and low-lying Rydberg states of acetylene (C 2 H 2 ) are reexamined in the singlet as well as in the triplet manifold. The major goal of this work is a better understanding of the valence states that contribute to the low-energy electron-energy-loss spectrum recorded under conditions where transitions to triplet states are enhanced. An appropriate theoretical treatment of these states has to include the low-lying Rydberg states because of their energetic proximity to some of the valence states. The CASSCF/CASPT2 method provides a suitable framework for such a task. For some important states the geometry was optimized at the CASPT2 level to allow a comparison with the results of other highly accurate methods that have been applied to acetylene in the past.

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The contribution deals with the theoretical and experimental examination of the force field of porphycene ([18]porphyrin-(2.0.2.0))a structural isomer of porphyrin. Special care was taken for a possible tautomerism of the hydrogen atoms in the cavity of the molecule. We report IR, Raman, UV, as well as site-selective fluorescence and fluorescence excitation spectra. Most spectra were measured under matrix isolation conditions. Quantum chemical calculations based on density functional theory methods were used to derive force fields and IR intensities for the most probable tautomeric forms.

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