THE MILLIMETERWAVE SPECTROSCOPY OF THE VINYL-2-$d_1$ RADICAL (HDC=CH)

Abstract

The vinyl-2-$d_1$ radical HDC=CH, mono-deuteride of the methylenic (CH$2$) proton, may have two forms, $cis$- and $trans$-HDCCH; for $cis$-HDCCH an unpaired electron is located on the same side of deuterium with respect to the C=C double bond, while on the opposite side for $trans$-HDCCH. If the barrier hight $h$ between the $cis$- and $trans$-form is sufficiently low and the zero point energy differece between the two forms $\Delta{tc0}$ is small enough ($\Delta_{tc0} \ll 1$ cm$^{-1}$), the proton on the acetylenic side (CH) will easily transfer through the barrier by tunneling effect, giving us a chance to observe the tunneling transitions between the two forms ($0^+$ and $0^-$). If not, the two forms will remain as two toutomers, $cis$-HDCCH and $trans$-HDCCH, and will give us a chance to observe the $b$-type rotational transitions within the $cis$ and $trans$ toutomers instead of the tunneling transitions. In our previous study }, TJ14(2006)}, we have observed the $a$-type rotational spectra of $cis$-HDCCH by millimeter wave spectroscopy in a supersonic jet, while no lines for $trans$-HDCCH were observed. In the present study, we have searched for either the tunneling transitions or the $b$-type rotational transitions of HDCCH generated by 193 nm excimer laser photolysis of HDCCHCl. The $b$-type rotational transitions (\textit{N${K{a}K_{c}}$}=1${11}$$-$0${00}$, 2${12}$$-$1${01}$, 1${10}$$-$1${01}$ and 2${11}$$-$2${02}$) only for one form of HDCCH were observed in the frequency region of 158$-$258 GHz, but no the tunneling transitions. The rotational and hyperfine interction constants, }{\bf 120}, 3604 (2004) } %}, WJ04(2004)}, %} \textbf{94}, 5950 (1972)}, indicate that the observed toutomer is not $trans$-HDCCH but $cis$-HDCCH. Our {\it ab initio} calulation performed in the CCSD(T)/cc-pVQZ level supports the present observation. Althogh the calculated barrier hight between the $cis$- and $trans$-form is as low as $h$ = 1770 cm$^{-1}$w, the zero point energy gap between the two toutomers $\Delta_{tc0}$ = 30.7 cm$^{-1}$ is large, as the result we have two toutomers, $cis$-HDCCH and $trans$-HDCCH, to observe the $b$-type rotational transitions instead of the tunneling transitions. The zero point energy gap $\Delta_{tc0}$ 30.7 cm$^{-1}$ is large enough to populate only to the vibrational ground state of $cis$-HDCCH in the ultra low temperature $T = 15 K$ in the supersonic jet giving us no $trans$-HDCCH spectra.