ROTATIONAL SPECTRA OF THE FREE RADICALS $C_{10}H, C_{12}H, C_{13}H$, AND $C_{14}H$ IN A SUPERSONIC JET
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Date
1998
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Ohio State University
Abstract
Four new carbon chain radicals $C_{10}H, C_{12}H, C_{13}H$, and $C_{14}H$ 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 $^{2}\Pi$ electronic ground states, and all except $C_{14}H$ have resolved lambda-type doubling. At least 10 rotational transitions, between 6 and 16 GHz, were measured in the lowest spin component --- $^{2}\Pi_{3/2}$ of $C_{10}H, C_{12}H$, and $C_{14}H$, and the $^{2}\Pi_{1/2}$ component of $C_{13}H$. Only three spectroscopic constants in the standard Hamiltonian for a molecule in a $^{2}\Pi$ state were required to reproduce the spectra to a few parts in $10^{7}$: an effective rotational constant, a centrifugal distortion constant, and a lambda-type doubling constant. Detection of these highly unsaturated carbon chains establishes that $C_{n}H$ radicals containing up to 14 carbon atoms are readily produced in a supersonic molecular beam. The relative abundance of $C_{n}H$ radicals with an even number of carbon atoms is fairly constant from $C_{n}H$ through $C_{12}H$. Although the new radicals are about two orders of magnitude less abundant than $C_{4}H$, the strong predicted $^{2}\Pi - ^{2}\Pi$ electronic transitions may be detectable in a supersonic jet by standard laser spectroscopic techniques.
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Author Institution: Harvard-Smithsonian Center for Astrophysics; Division of Engineering and Applied Sciences, Harvard University