State-Specific Spectral Doublets in the FTIR Spectrum of Gaseous Tropolone †

The infrared absorption spectrum of tropolone vapor at 25 °C, ∼0.01 Torr, and 32 m path length has been recorded from 960 to about 700 cm - 1 at a resolution of 0.0025 cm - 1 . Twenty-nine cold band and hot band spectral tunneling doublets marked by sharp type A or type C Q branch apexes protruding from the congested vibration-contortion-rotation absorption profiles are assigned. Twenty-six vibration-contortion state-specific splittings are estimated for tropolone in its ground electronic state. This number is currently unprecedented in the literature for any sizable molecule. About half of these quasiharmonic vibrational states show an appreciable quenching of tunneling due to increased effective barriers and/or path lengths and, in the case of the nominal COH torsion, the tunneling is reduced to 0.07 of the zero-point (ZP) value of 0.974 cm - 1 . The analysis is guided by predictions of the independent {tunneling skeleton}{tunneling H atom} tautomerization model that was previously applied to the vibrational spectrum and tautomerization mechanism of tropolone. The increase of effective tunneling path length by a few percent over the ZP path length is attributed to dynamical complexity arising from an atom-to-atom exchange of unequal vibrational displacements as a part of the tautomerization process. This aspect of tunneling quenching behavior can arise for quasiharmonic vibrations lacking direct contact to the OH...O group. For the case of tropolone, the tautomerization model advocates heavy atom tunneling as equal in importance to H atom tunneling. Heavy atom tunneling is supported by the observation of a (perturbed) spectral tunneling doublet at 754 cm - 1 with the separation 0.80 cm - 1 . This doubling of the high frequency component of the previously observed 11 cm - 1 doublet observed using Ne matrix-isolation sampling provides evidence for the "doublet of doublet" quartet structure predicted for the ν 3 7 nascent skeletal tunneling (contortion) vibration. The compilation of numerous vibrational state-specific tunneling doublings for a 15-atom nonrigid molecule invites further experimental and theoretical research aimed at advancing the understanding of multidimensional intramolecular tunneling dyanmics, vibrational energy redistribution, and unimolecular reaction kinetics. Several strong parallels are seen between the vibrational interactions arising in our studies of the tautomerization of tropolone and those appearing in recent articles discussing possible behaviors in the active sites of enzymatic H tranfer reactions.