Determination of the proton tunneling splitting of tropolone in the ground state by microwave spectroscopy

Rotational spectra of tropolone in the ground vibronic state were measured by microwave spectroscopy. Due to the proton tunneling motion, the ground state is split into a doublet, of which the lower and upper components are denoted by 0+ and 0−, respectively. In the frequency region 28–84 GHz, more than 150 pure rotational transitions obeying a-type selection rules were observed for each of the 0+ and 0− states. Additionally, tunneling-rotation transitions connecting the lower (0+) and upper (0−) components of the tunneling doublet were observed by pulsed Fourier transform microwave spectroscopy. Twenty-three P- and Q-branch lines were observed in the frequency region of 10–18 GHz, and analyzed combined with the pure rotational transitions for each of the 0+ and 0− states. The proton tunneling splitting in the ground state, Δ0=29 193.788±0.026 MHz, and the tunneling-rotation interaction constant F=16.456±0.015 MHz, were determined, as well as the rotational and centrifugal distortion constants. The dipole moment along the a axis, responsible for the rotational transitions, was determined to be 3.428±0.050 and 3.438±0.050 D for the 0+ and 0− states, respectively.