Knowledge Bank

Four new carbon chain radicals $C10H,C12H,C13H$, and $C14H$ have been observed in a pulsed supersonic molecular beam with a Fourier transform microwave spectrometer. The radicals were produced in a discharge through a dilute diacetylene/neon mixture in the throat of a supersonic nozzle. All are found to be linear with $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi$ electronic ground states, and all except $C14H$ have resolved lambda-type doubling. At least 10 rotational transitions, between 6 and 16 GHz, were measured in the lowest spin component --- $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi 3/2$ of $C10H,C12H$, and $C14H$, and the $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi 1/2$ component of $C13H$. Only three spectroscopic constants in the standard Hamiltonian for a molecule in a $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi$ state were required to reproduce the spectra to a few parts in $107$: an effective rotational constant, a centrifugal distortion constant, and a lambda-type doubling constant. Detection of these highly unsaturated carbon chains establishes that $CnH$ radicals containing up to 14 carbon atoms are readily produced in a supersonic molecular beam. The relative abundance of $CnH$ radicals with an even number of carbon atoms is fairly constant from $CnH$ through $C12H$. Although the new radicals are about two orders of magnitude less abundant than $C4H$, the strong predicted $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi -2\Pi$ electronic transitions may be detectable in a supersonic jet by standard laser spectroscopic techniques.