Ortho-para mixing interaction in the vinyl radical detected by millimeter-wave spectroscopy

Ortho-para mixing interaction due to the coupling of nuclear and electron spins was detected for the first time by millimeter-wave spectroscopy of the deuterated vinyl radical H2CCD. The ground state of H2CCD is split by the deuteron tunneling into two components 0+ and 0− separated by ΔE0=1186.794(16) MHz. Rotational levels in the 0+ and 0− states, one being an ortho level and the other a para level, are coupled by the interaction expressed by ⟨0±|H′|0∓⟩=(δaF/2)S⋅(I1−I2), where I1 and I2 are spins of the two protons and S is the electron spin. The δaF constant has been determined to be 68.06(52) MHz. The para to ortho conversion rate constant is predicted to be 1.2×105 s−1 torr−1.