Electronic spectra of jet-cooled tropolone. Effect of the vibrational excitation on the proton tunneling dynamics

Abstract

The laser fluorescence excitation spectra and single vibronic level fluorescence spectra have been measured for jet‐cooled tropolone (–OH) and tropolone (–OD). The ν’ 11(a 1), ν12(a 1), and ν’ 19(a 2), modes have been newly identified in the excitation spectrum of tropolone (–OH), in addition to the previously observed ν13(a 1), ν’ 14(a 1), ν25(b 1), and ν’ 26(b 1) modes. The tunneling doublet splittings of these modes in the à 1 B 2 state have been determined to investigate the effect of the vibrational excitation on the protontunneling dynamics. The tunneling splitting is highly mode specific. The ν13(a 1) and ν’ 14(a 1) skeletal deformation modes are strongly coupled to the proton transfer coordinate and enhance tunneling, whereas the ν19(a 2) skeletal twisting mode as well as ν’ 25(b 1) and ν26(b 1) out‐of‐plane bending modes diminish tunneling. The ν’ 11(a 1) C–C stretching and ν12(a 1) C–C–C bending modes are considered to be weakly coupled to the proton transfer coordinate, because the tunneling splittings of these modes are similar to that of the zero‐point level. The hydrogen/deuterium isotope dependence of the tunneling splitting of the vibronic level has been studied. In contrast with the ν’ 13 mode, the isotope effect on the tunneling splitting is much smaller in the ν14 mode, although the magnitude of the tunneling splitting of ν’ 14 is very similar to ν13. This implies that both the average O–O distance and the potential energy barrier are significantly different between these modes.