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

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Abstract
The S 1–S 0fluorescence 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 protontunneling. 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 S 1 state. An electron‐withdrawing character of chlorine and bromine atoms is almost canceled by a conjugative electron‐releasing effect.