à 1B2–X̃ 1A1 26v transitions of 18O-enriched tropolone

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Abstract
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 −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.