Laser-induced fluorescence spectroscopy of jet-cooled 2-methylanthracene S1 (π,π*). Evidence for methyl conformation change upon electronic excitation
✍ Scribed by H. Lin; J.A. Hunter; J. Pfab
- Publisher
- Elsevier Science
- Year
- 1993
- Tongue
- English
- Weight
- 608 KB
- Volume
- 210
- Category
- Article
- ISSN
- 0009-2614
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✦ Synopsis
The laser-induced fluorescence excitation spectrum of jet-cooled 2methylanthracene (2-MA) near the S,-So origin reveals a prominent torsional progression due to the internal rotation of the methyl group or its torsional vibration against the aromatic ring frame. From the torsional level spacings a low, threefold hindered internal rotation barrier of 70 cm-' has been determined for S, (a, rr*). A Franck-Condon intensity analysis of the progression in the excited state torsion suggests that the most stable rotamer of 2-MA changes from an eclipsed-staggered (e-s) conformation in the electronic ground state to a (s-e) conformation in the first singlet excited state. This 60" change in the dihedral conformation can be rationalized by the change of the n-bond order of the adjacent ring bonds upon electronic excitation, such that one of the C-H methyl bonds prefer to eclipse the adjacent C-C bond with the stronger n-bond character.