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KEIICHI TANAKA, MASATO HAYASHI, MITSUHIKO OHTSUKI, KENSUKE HARADA AND TAKEHIKO TANAKA, Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki, Higashiku, Fukuoka 812-8581, Japan.
\par Ortho-para mixing interaction due to the coupling of nuclear and electron spins was detected for the first time by millimeter-wave spectroscopy of deuterated vinyl radicals, H2CCD and D2CCD, of which the ground states are split by the tunneling motion of the
deuteron into two components 0+ and 0-, whose separations have been determined to be
E0 = 1186.644(16) and 771.978(18) MHz, respectively. The observed tunneling-rotation spectra are significantly perturbed by the othro-para mixing interaction expressed by \langle 0^\pm |H'|0^\mp\rangle = (
a F^(
)/2)\boldmath S
(\boldmath I
1-\boldmath I
2), where \boldmath I
1 and \boldmath I
2 are spins of the two hydrogen nuclei in the
position and \boldmath 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
a F^(
) constants for H2CCD and D2CCD have been determined to be 68.06(53) and 10.63(94) MHz, respectively, consistent each other within the isotopic mass relation. The othro and para states are mixed by about 0.097% and 0.0123% due to this interaction. The
a F constant for H2CCH should be similar to that for H2CCD because of the same probability density of the unpaired electron at the
protons, but could not be determined independently in our previous study. It is because the mixing of para - and ortho -levels of about 0.00044% is much smaller than that for H2CCD due to the large tunneling splitting of
E0=16271.8429(59) MHz.
\par The rate constant of para to ortho (I_
= 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.