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

Abstract

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 α deuteron into two components 0+ and 0−, whose separations have been determined to be ΔE 0 = 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 , where I β1 and I β2 are spins of the two hydrogen nuclei in the β 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 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 106 times faster compared with that in closed shell molecules such as H2CO and H2CCH2.