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.

A vibronic level study of the spectroscopy and photophysics of the C6H6–CHCl3 complex has been carried out using a combination of laser-induced fluorescence and resonant two-photon ionization (R2PI). In C6H6-CHCl3, the S1–S0 origin remains forbidden while the 1610 transition is weakly induced. Neither 610 nor 1610 are split by the presence of the CHCl3 molecule. On this basis, a C3vstructure is deduced for the complex, placing CHCl3 on the six-fold axis of benzene. The large blue-shift of the complex’s absorption relative to benzene (+178 cm–1) and the efficient fragmentation of the complex following one-color R2PI reflect a hydrogen-bonded orientation for CHCl3 relative to benzene’ π cloud. Dispersed fluorescence scans place a firm upper bound on the ground state binding energy of the complex of 2,024 cm–1. Both the 61and 61 11 levels do not dissociate on the time-scale of the S1 fluorescence and show evidence of extensive state mixing with van der Waals’ levels primarily built on the 00 level of benzene. The C6H6–(CHCl3)2 cluster shows extensive intermolecular structure beginning at +84 cm–1, a strong origin transition, and splitting of 61. A structure which places both CHCl3 molecules on the same side of the benzene ring is suggested on this basis. The vibronic level scheme used to deduce the structure of C6H6–CHCl3 is tested against previous data on other C6H6–X complexes. The scheme is found to be capable, in favorable cases, of deducing the structures of C6H6–X complexes based purely on vibronic level data. Finally, the results on C6H6–CHCl3 are compared with those on C6H6–HCl and C6H6-H2O to evaluate the characteristics of the n hydrogen bond.

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