Infrared spectra of tropolone(OH) and tropolone(OD) obtained from vapor phase, solvated, and rare gas matrix-isolated samples, and from fluorescence dip infrared spectroscopy experiments by Frost et al. on jet-cooled samples, are analyzed with the guidance of high level ab initio molecular orbital (MO) computations. It is found that the anharmonicity of the double minimum global potential energy surface of S0 tropolone is manifested by multistate local resonance networks coupling fundamental vibrations to nearby overtone and combination states. These resonance networks pervade the IR spectrum of tropolone above 500 cm−1, and the absorbances are much more strongly perturbed from harmonic level predictions than the frequencies. Some of the IR absorbances are also sensitive to intermolecular interactions. At maximum spectral resolutions reaching ∼0.2 cm−1 only the v1 and v22 (OH stretching and nascent skeletal tunneling) vibrations show resolved vibrational state-specific tunneling doublets. The tunneling behavior of tropolone is analyzed in the accompanying article.

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Infrared-absorption profiles observed for vibrational transitions of gaseous tropolone often show sharp Q branch peaks, some of them ultranarrow spikes, indicative of the band origins for vibrational state-specific spectral tunneling doublets. In this work oxygen isotope effects for two CH wagging fundamentals, the COH torsion fundamental, and the skeletal contortion fundamental are reported. They allow considerations to be given: (1) oxygen isotope effects on the vibrational frequencies and state-specific tunneling splittings; (2) the asymmetry offset of the potential-energy minima for 16O and 18O tropolone; and (3) additional details concerning previously proposed high J rotation-contortion resonances in the contortional fundamental. The new results help to characterize the skeletal contortion fundamental and support the joint participation of skeletal tunneling with H tunneling in the vibrational state-specific tautomerization processes of tropolone in its ground electronic state.

Laser excitation spectra with v=0, 2, 4, and 6 in the Ã 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.