Laser-induced fluorescence excitation, dispersed fluorescence, and population labeling spectra of the S 0 -S 1 transition of 5-hydroxytropolone ͑5-HOTrOH͒ have been recorded in a supersonic free jet. In the ground state, the two in-plane orientations of the 5-OH substituent produce two isomers, syn and anti relative to the 2-OH. Population labeling is used to identify transitions in the spectrum originating from the syn and anti zero-point levels. In the excited electronic state where 2-OH H-atom tunneling is expected to be more facile, the single vibronic level dispersed fluorescence spectra identify excited state levels with widely varying syn-anti character. Many of the levels in the low-energy region of the spectrum are nearly pure syn or anti in character, thereby showing little coupling to the isomerization reaction coordinate. Other levels are strongly syn-anti mixed via Fermi resonance interactions. A few levels exhibit strong syn-anti mixing despite being relatively isolated from other levels. These levels have strong syn-anti coupling matrix elements which reflect a strong coupling to the reaction coordinate. Analysis of the dispersed emission identifies a ''promoter'' vibrational mode W, with syn ͑anti͒ ground-state frequency 336 ͑337͒ cm Ϫ1 which appears to play a key role in coupling syn and anti levels. The accessibility of excited state levels of mixed character makes it possible to efficiently and reversibly isomerize 5-hydroxytropolone in a highly mode-specific fashion.

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The Letters to the Editor section is divided into four categories entitled Communications, Notes, Comments. and Errata. Communications are limited to three and one half journal pages, and Notes, Comments, and Errata are limited to one and three-fourths journal pages as described in the Announcement in the I July 1993 issue.

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ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.