Knowledge Bank

The free SiC radical in its $X3\Pi$ electronic state was found at millimeter-wavelength in this laborattory and in $space1$ following detection of an excited electronic transition by Bernath $etal.2$ Here we report the analysis of hyperfine structure in the spectra of two SiC isotopes with nuclear spin, $Si13C$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">29</mrow>}SiC$. The millimeter-wave rotational spectra of both radicals were observed in a low-pressure glow discharge through $SiH4$ and $CO.1$ Wel-resolved hyperfine structure in eleven rotational transitions of $Si13C$ and nine of $\mathrm{<mrow\; data-mjx-texclass="ORD">29</mrow>}SiC$ allowed determination of the magnetic hyperfine constant to an accuracy of 2% or better. A qualitative understanding of the spin density distribution was obtained from these constants. The anomolous sign of the Fermi contact parameter, b$f1$ in $\mathrm{<mrow\; data-mjx-texclass="ORD">29</mrow>}SiC$ indicate that higher-order effects are important in this molecule.