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Spectral Lines
Published in Ronald L. Snell, Stanley E. Kurtz, Jonathan M. Marr, Fundamentals of Radio Astronomy, 2019
Ronald L. Snell, Stanley E. Kurtz, Jonathan M. Marr
Molecules!electronic angular momentum The second effect that alters the molecular rotational energy levels occurs when the molecule has a net (non-zero) electronic angular momentum in the ground electronic state. Examples of such molecules include the ethynyl radical (CCH) and the cyano radical (CN). Both of these molecules have an unpaired electron, giving rise to a net electronic spin angular momentum in the ground electronic state of the molecule. The interaction between the net electronic spin and the molecular rotation produces splitting of the rotational energy levels. The details are somewhat complicated; the interested reader should consult a book on molecular spectroscopy. Some molecules, such as CN, have both quadrupole hyperfine structure and structure produced by the non-zero electronic angular momentum.
Conclusive determination of ethynyl radical hydrogen abstraction energetics and kinetics*
Published in Molecular Physics, 2020
Michael C. Bowman, Alexandra D. Burke, Justin M. Turney, Henry F. Schaefer III
Among the most prevalent reactions for the ethynyl radical is hydrogen-atom abstraction. In reactions with hydrogen-containing molecules, hydrogen abstraction is often the main reaction pathway or at least a viable alternative. These abstractions are thermodynamically driven by the relatively large dissociation energy of acetylene's sp-hybridised C–H bond. Kinetic studies have revealed large rate constants for the reaction of CH with various saturated hydrocarbons over a wide range of temperatures, suggesting the barriers to abstraction are moderately low [13–16]. This is in agreement with theoretical results which claim moderate to low barrier heights for several H-atom donors [17,18]. In the particular case of , a slight negative temperature dependence has been observed for the hydrogen abstraction, consistent with a barrierless reaction [19,20]. These results have been corroborated by theoretical studies which have computed a submerged barrier due to the formation of strongly bound NH–CH hydrogen-bonded complex [21,22]. Furthermore, the small mass of hydrogen suggests that many of these reactions will proceed at low temperatures, regardless of the barrier height, due to tunnelling. The importance of tunnelling has been confirmed by several kinetics studies which have demonstrated a large isotopic dependence at moderate to low temperatures [13,20,23,24].