Millimeter-wave spectroscopy of H2CCD: Tunneling splitting and ortho-para mixing interaction

The H2CCD isotopic species of vinyl radical produced in a supersonic jet expansion by ultraviolet laser photolysis was studied by millimeter-wave spectroscopy. Due to the tunneling motion of the α deuteron, the ground state is split into two components, 0+ and 0−. Tunneling-rotation transitions connecting the lower (0+) and upper (0−) components of the tunneling doublet were observed in the frequency region of 184–334 GHz, including three R- and two Q-branch transitions. Three and two pure rotational transitions in the Ka=0 and 1 stacks, respectively, were also observed for each of the 0+ and 0− states in the frequency region of 52–159 GHz. Least-squares analysis of the observed frequencies for the tunneling-rotation and pure rotational transitions with well resolved hyperfine structures yielded a set of precise molecular constants, among which the tunneling splitting in the ground state was determined to be ΔE0=1187.234(17) MHz, which is 1/14 that for H2CCH. The potential barrier height derived from th...