A 1B2–X 1A1 26v transitions of 18O-enriched tropolone

Laser excitation spectra with v=0, 2, 4, and 6 in the à 1B2–X̃ 1A1 26v0 progression of jet-cooled 18O/16O isotopomers of tropolone are reported. The isotope shift for ν26, 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.