tilde A sup 1 B sub 2 -- tilde X sup 1 A sub 1 26 sup v sub 0 transitions of sup 18 O-enriched tropolone

Laser excitation spectra with {ital v}=0, 2, 4, and 6 in the {ital {tilde A}} {sup 1}{ital B}{sub 2}--{ital {tilde X}} {sup 1}{ital A}{sub 1} 26{sup {ital v}}{sub 0} progression of jet-cooled {sup 18}O/{sup 16}O isotopomers of tropolone are reported. The isotope shift for {nu}{sub 26}, an out-of-plane deformation mode at 39 cm{sup {minus}1} in the {ital {tilde A}} state, is 2% for tropolone-{sup 18}O{sup 18}O. This large {sup 18}O isotope effect indicates that {ital Q}{sub 26} 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 {ital {tilde A}} state is quenched by exciting the 26{sup {ital v}} overtone states and a mechanism for this quenching is proposed in terms of the indicated normal coordinate. Tunneling splittings are {lt}0.3 cm{sup {minus}1} for the zero point levels of the {ital {tilde X}} state of the symmetrical isotopomers. In contrast, vibrational isotope effects dominate the tunneling interactions to split the corresponding levels of tropolone-{sup 16}O{sup 18}O by 1.7 cm{sup {minus}1}. In the {ital {tilde A}} 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-{sup 16}O{sup 18}O are smaller than those of the symmetrical isotopomers.