The laser-induced fluorescence excitation spectrum of jet-cooled 2-methylanthracene (2-MA) near the S1-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-1 has been determined for S1(p,p*). A Frank-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 p-bond order of the adjacent ring bonds upon electronic excitation, such that one of the CH methyl bonds prefer to eclipse the adjacent CC bond with the stronger p-bond character. © 1993.
|Number of pages||7|
|Journal||Chemical Physics Letters|
|Publication status||Published - 23 Jul 1993|