APPROXIMATELY-A-1B2-APPROXIMATELY-X-1A1260V TRANSITIONS OF O-18-ENRICHED TROPOLONE

Laser excitation spectra with v= 0, 2, 4, and 6 in the à 1B2-X̃ 1A1 260 v progression of jet-cooled 18O/16O isotopomers of tropolone are reported. The isotope shift for v26, an out-of-plane deformation mode at 39 cm-1 in the à state, is 2% for tropolone-18O18O. This large 18O isotope effect indicates that Q26 for tropolone resembles the analogous normal mode of tropone, which is a ring deformation towards the boat conformation of 2, 4, 6-cycloheptatriene accompanied by a large O atom displacement. Tunneling by tropolone in the à state is quenched by exciting the 26v overtone states and a mechanism for this quenching is proposed in terms of the indicated normal coordinate. Tunneling splittings are <0.3 cm-1 for the zero point levels of the X̃ state of the symmetrical isotopomers. In contrast, vibrational isotope effects dominate the tunneling interactions to split the corresponding levels of tropolone- 16O18O by 1.7 cm-1. In the à state of this isotopomer the tunneling interactions are dominant. Because they are determined by the overlap between localized and delocalized wave functions, the Franck-Condon factors of tropolone-16O18O are smaller than those of the symmetrical isotopomers. © 1990 American Institute of Physics.