Molecular and Electronic Structures of the Short-Lived Excited Triplet State of Tropone

Time-resolved electron paramagnetic resonance experiments on the short-lived T 1 state of nonphosphorescent tropone were carried out using mixed crystals. Well-resolved hyperfine splittings due to allowed and forbidden transitions were observed at principal axes of the zero-field tensor. The spin density distribution, C−H bond angles, and the sign of the D value were deduced from the hyperfine spectra. It has been concluded that the T 1 state of tropone mainly has a 3 ππ* configuration and holds its molecular structure of C 2 v symmetry. The experiments and molecular orbital calculations indicate that the large Franck−Condon factor between the T 1 and the S 0 states arises from the change of the potential surfaces due to the antibonding character of the π* orbital at the C 4 −C 5 bond in the T 1 state.