Transition-State Spectroscopy of Cyclooctatetraene

Area of Science:

• Physical Chemistry
• Spectroscopy
• Quantum Chemistry

Background:

• Cyclooctatetraene (COT) is a non-aromatic cyclic hydrocarbon with a unique electronic structure.
• Understanding the electronic states of COT is crucial for predicting its reactivity and properties.
• The radical anion (COT•−) provides a unique probe into the electronic landscape of COT.

Purpose of the Study:

• To investigate the electronic states of cyclooctatetraene (COT) using photoelectron spectroscopy.
• To determine the electron binding energies of specific electronic states of COT.
• To verify theoretical predictions regarding COT's electronic structure and adherence to Hund's rule.

Main Methods:

• 351-nanometer photoelectron spectroscopy of the planar cyclooctatetraene radical anion (COT•−).
• Analysis of observed spectral features to identify electronic transitions.
• Assignment of vibrational structure using a potential energy surface model.

Main Results:

• Photoelectron spectrum reveals transitions to two electronic states of COT: the D4h ¹A₁g (transition state for ring inversion) and the D8h ³A₂u state.
• The electron binding energy for the ¹A₁g state is 1.099 ± 0.010 eV, significantly lower than the ³A₂u state.
• Observed vibrational structure for both states was assigned, confirming theoretical predictions of Hund's rule violation.

Conclusions:

• The singlet state of D8h COT lies below the triplet state, contrary to Hund's rule.
• Photoelectron spectroscopy provides direct evidence for the electronic structure of COT and its radical anion.
• The findings validate ab initio calculations and enhance the understanding of electronic state ordering in polycyclic systems.