Electronic spectra of jet-cooled isopropyltropolones in the S1–S region. Proton tunneling in the S1 state

Abstract

Laser fluorescence excitation spectra and dispersed fluorescence spectra of jet-cooled 3-, 4-, and 5-isopropyltropolone in the S1–S0 region have been measured to investigate the effect of unsymmetrical and symmetrical substitution of an isopropyl group on proton tunneling. The 0++ and 0−+ transitions have been identified in the excitation spectra of 3-isopropyltropolone and its −OD derivative. The tunneling doublet splittings in the S1 state have been determined to be 58 and 14 cm−1 for normal and deuterated species, respectively. The increase in tunneling splitting of 3-isopropyltropolone compared with that of tropolone suggests that the substitution of the isopropyl group induces an asymmetry in the double-minimum potential well. In contrast with the excitation spectrum of 3-isopropyltropolone, no 0−− and 0−+ transitions have been detected in 4- and 5-isopropyltropolone. The nonobservation of these transitions is likely to be due to substantial increase in the potential-energy barrier to tunneling in the S1 state.