Electronic spectra of jet-cooled 3-chlorotropolone. Proton tunneling in the S 1 state

The electronic spectra have been measured for jet-cooled 3-chlorotropolone in the S1–S0 region to investigate the effect of an asymmetric substitution on proton tunneling in tropolone. Both the 0++ and 0−− transitions have been identified in the fluorescence excitation spectrum, while no 0−+ or 0+− transition has been detected. This suggests that the double minimum potential energy function is almost symmetric for both S1 and S0. The tunneling doublet separations for the origin bands of 3-chlorotropolone and its OD derivative have been measured to be 23 and 3 cm−1, respectively. These values are very similar to the corresponding values of 19 and 2 cm−1 for tropolone and its OD derivative, indicating that the substitution of a chlorine atom influences only a little on the strength of the intramolecular hydrogen bond.