Millimetre-wave spectroscopy of deuterated vinyl radicals, observation of the ortho-para mixing interaction and prediction of fast ortho-para conversion rates

Ortho-para mixing interaction due to the coupling of nuclear and electron spins was detected for the first time by millimetre-wave spectroscopy of deuterated vinyl radicals, H₂CCD and D₂CCD, of which the ground states are split by the tunnelling motion of the deuteron into two components 0⁺ and 0^-, whose separations have been determined to be ΔE₀ = 1186.644(16) and 771.978(18) MHz, respectively. The observed tunnelling-rotation spectra are significantly perturbed by the ortho-para mixing interaction expressed by 0 ^±|H′|0±, = (δa_F ^(β))S · (I_β1-I_β2), where I_β1 and I_β2 are spins of the two hydrogen nuclei in the position and S is the electron spin, which connects rotational levels in the 0+ and 0- states, one being an ortho level and the other a para level. The δa_F(β) constants for H₂CCD and D₂CCD have been determined to be 68.06(53) and 10.63(94) MHz, respectively. The ortho and para states are mixed by about 0.097% and 0.0123% due to this interaction. The rate constant of para to ortho (I_β = 0 → 1) conversion is predicted as 1.2 × 10⁵ s^-1 torr^-1 for H₂CCD, suggesting extremely rapid mutual conversion between ortho and para nuclear spin isomers of H₂CCD, which is more than 10 ⁶ times faster compared with that in closed shell molecules such as H₂CO and H₂CCH₂.