{\em ORTHO-PARA} MIXING INTERACTION IN THE VINYL RADICAL DETECTED BY MILLIMETER-WAVE SPECTROSCOPY AND PREDICTION OF FAST {\em ORTHO-PARA} CONVERSION RATE
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Date
2010
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Ohio State University
Abstract
\par {\em 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, H$_2$CCD and D$_2$CCD, of which the ground states are split by the tunneling motion of the $\alpha$ deuteron into two components $0^+$ and $0^-$, whose separations have been determined to be $\Delta E_0 =$ 1186.644(16) and 771.978(18) MHz, respectively. The observed tunneling-rotation spectra are significantly perturbed by the {\em othro-para} mixing interaction expressed by $\langle 0^\pm |H'|0^\mp\rangle = (\delta a_{\rm F}^{(\beta)}/2)\mbox{\boldmath $S$} \cdot(\mbox{\boldmath $I$}_{\beta 1}-\mbox{\boldmath $I$}_{\beta 2})$, where $\mbox{\boldmath $I$}_{\beta 1}$ and $\mbox{\boldmath $I$}_{\beta 2}$ are spins of the two hydrogen nuclei in the $\beta$ position and {\boldmath $S$} is the electron spin, which connects rotational levels in the $0^+$ and $0^-$ states, one being an {\em ortho} level and the other a {\em para} level. The $\delta a_{\rm F}^{(\beta)}$ constants for H$_2$CCD and D$_2$CCD have been determined to be 68.06(53) and 10.63(94) MHz, respectively, consistent each other within the isotopic mass relation. The {\em othro} and {\em para} states are mixed by about 0.097\% and 0.0123\% due to this interaction. nderline{\textbf{131,}}~111101 (2009).} The $\delta a_{\rm F}$ constant for H$_2$CCH should be similar to that for H$_2$CCD because of the same probability density of the unpaired electron at the $\beta$ protons, but could not be determined independently in our previous study. It is because the mixing of {\em para}- and {\em ortho}-levels of about 0.00044\% is much smaller than that for H$_2$CCD due to the large tunneling splitting of $\Delta E_0=16271.8429(59)$ MHz. nderline{\textbf{120,}}~3604 (2004).} \par The rate constant of {\em para} to {\em ortho} ($I_\beta$ = 0 $\rightarrow$ 1) conversion is predicted as $1.2\times 10^5$ s$^{-1}$ torr$^{-1}$ for H$_2$CCD, suggesting extremely rapid mutual conversion between {\em ortho} and {\em para} nuclear spin isomers of H$_2$CCD, which is more than 10$^{6}$ times faster compared with that in closed shell molecules such as H$_2$CO and H$_2$CCH$_2$.
Description
Author Institution: Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki, Higashiku; Fukuoka 812-8581, Japan