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 K-a=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 DeltaE(0)=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, epsilon(ab)+epsilon(ba) for the spin-rotation interaction and T-ab for the hyperfine interaction of the alpha (CH) proton. The hyperfine interaction constants, due to the alpha proton and the beta (CH2) protons, are consistent with those derived from electron spin resonance studies. (C) 2004 American Institute of Physics.