Millimeter-wave spectroscopy of H₂=CCD: Tunneling splitting and ortho-para mixing interaction

The H₂=CCD 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 K_a =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 Δ E ₀ =1187.234 (17) MHz, which is 1/14 that for H₂ CCH. The potential barrier height derived from the observed tunneling splitting by an analysis of the tunneling dynamics using a one-dimensional model is 1545 cm ^-1, consistent with the value 1568 cm^-1 obtained for the normal vinyl. The observed spectrum was found to be perturbed by a hyperfine interaction connecting ortho and para levels. The constant for the interaction, which we call the ortho-para mixing Fermi contact interaction, has been determined to be δ aF (β) =68.06 (53) MHz. This is believed to be the first definite detection of such an interaction. By this interaction the ortho and para states of H₂=CCD are mixed up to about 0.1%. The constant is more than 1000 times larger than spin-rotation interaction constants that cause ortho-para mixing in closed shell molecules and suggests extremely rapid conversion between the ortho and para nuclear spin isomers of H₂=CCD.