Electronic spectra of jet-cooled 5-bromotropolone and 5-chlorotropolone. Influence of symmetrical substitution on proton tunneling in the S1 state

The S1–S0 fluorescence excitation and dispersed fluorescence spectra have been measured for jet-cooled 5-bromotropolone (5BTR-h), 5-chlorotropolone (5CTR-h), and their OD derivatives in order to investigate substituent effects on proton tunneling. Both the 0++ and 0−− transitions were identified in the fluorescence spectra of these molecules. The tunneling doublet splittings in the electronic origin band of 5BTR-h (16 cm−1) decreased by 4 cm−1, whereas that of 5CTR-h (23 cm−1) increased by 3 cm−1 as compared with tropolone (TRN-h). Relatively small deviation in the tunneling separations for these molecules from that for tropolone is ascribed to conjugation in the S1 state. An electron-withdrawing character of chlorine and bromine atoms is almost canceled by a conjugative electron-releasing effect.