Determination of the proton tunneling splitting of the vinyl radical in the ground state by millimeter-wave spectroscopy combined with supersonic jet expansion and ultraviolet photolysis

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Abstract
The vinyl radical in the ground vibronic state produced in a supersonic jet expansion by 193 nm excimer laser photolysis of vinyl bromide was investigated by millimeter-wave spectroscopy. Due to the proton tunneling, the ground state is split into two components, of which the lower and higher ones are denoted as 0+ and 0−, respectively. Eight pure rotational transitions with Ka=0 and 1 obeying a-type selection rules were observed for each of the 0+ and 0− states in the frequency region of 60–250 GHz. Tunneling–rotation transitions connecting the lower (0+) and upper (0−) components of the tunneling doublet, obeying b-type selection rules, were also observed in the frequency region of 190–310 GHz, including three R- and six Q-branch transitions. The observed frequencies of the pure rotational and tunneling–rotation transitions were analyzed by using an effective Hamiltonian in which the coupling between the 0+ and 0− states was taken into account. A set of precise molecular constants was obtained. Among others, the proton tunneling splitting in the ground state was determined to be ΔE0=16 272(2) MHz. The potential barrier height was estimated to be 1580 cm−1 from the proton tunneling splitting, by an analysis using a detailed one-dimensional model. The spin–rotation and hyperfine interaction constants were also determined for the 0+ and 0− states together with the off-diagonal interaction constants connecting the 0+ and 0− states, εab+εba for the spin–rotation interaction and Tab for the hyperfine interaction of the α (CH) proton. The hyperfine interaction constants, due to the α proton and the β (CH2) protons, are consistent with those derived from electron spin resonance studies.

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