Propynylidyne is a chemical compound that has been identified in interstellar space.
ü<sub>D</sub>=3.551 Debye
<sup>2</sup>ÃÂ electronic ground state
A rotational spectrum of the <sup>2</sup>àelectronic ground state of l-C<sub>3</sub>H can be made using the PGopher software (a Program for Simulating Rotational Structure, C. M. Western, University of Bristol, http://pgopher.chm.bris.ac.uk) and molecular constants extracted from the literature. These constants include ü=3.551 Debye and others provided by Yamamoto et al. 1990, given in units of MHz: B=11189.052, D=0.0051365, A<sub>SO</sub>=432834.31, ó=-48.57, p=-7.0842, and q=-13.057. A selection rule of ÃÂJ=0,1 was applied, with S=0.5. The resulting simulation for the rotational spectrum of C<sub>3</sub>H at a temperature of 30 K agree well with observations. The simulated spectrum is shown in the figure at right with the approximate atmospheric transmission overplotted in blue. All of the strongest simulated lines with J < 8.5 are observed by Yamamoto et al.
ü<sub>D</sub>=2.4 Debye electronic ground state
The molecule C<sub>3</sub>H has been observed in cold, dense molecular clouds. The dominant formation and destruction mechanisms are presented below, for a typical cloud with temperature 10K. The relative contributions of each reaction have been calculated using rates and abundances from the UMIST database for astrochemistry.
The C<sub>3</sub>H molecule provides the dominant pathway to the production of C<sub>4</sub>H<sup>+</sup>, and thereby all other C<sub>n</sub>H (n>3) molecules via the reactions:<br>
These reactions produce the majority of C<sub>4</sub>H<sup>+</sup>, which is necessary for the production of higher-order carbon-chain molecules. Compared to the competing reaction,<br> C<sub>3</sub>H<sub>3</sub><sup>+</sup> + C â C<sub>4</sub>H<sub>2</sub><sup>+</sup> + H, <br> also shown right, the destruction of C<sub>3</sub>H provides a much faster pathway for hydrocarbon growth.
Other molecules in the C<sub>3</sub>H family, C<sub>2</sub>H and C<sub>3</sub>H<sub>2</sub>, do not significantly contribute to the production of carbon-chain molecules, rather forming endpoints in this process. The production of C<sub>2</sub>H and C<sub>3</sub>H<sub>2</sub> essentially inhibits larger carbon-chain molecule formation, since neither they nor the products of their destruction are recycled into the hydrocarbon chemistry.
The first confirmation of the existence of the interstellar molecule C<sub>3</sub>H was announced by W.M Irvine et al. at the January 1985 meeting of the American Astronomical Society. The group detected C<sub>3</sub>H in both the spectrum of the evolved carbon star IRC+10216 and in the molecular cloud TMC-1. These results were formally published in July of the same year by Thaddeus et al. A 1987 paper by W.M. Irvine provides a comparison of detections for 39 molecules observed in cold (T<sub>k</sub> â 10K), dark clouds, with particular emphasis paid to tri-carbon species, including C<sub>3</sub>H.
Later reports of astronomical detections of the C<sub>3</sub>H radical are given in chronological order below.
In 1987, Yamamoto et al. report measurements of the rotational spectra of the cyclic C<sub>3</sub>H radical (c-C<sub>3</sub>H) in the laboratory and in interstellar space towards TMC-1. This publication marks the first terrestrial measurement of C<sub>3</sub>H. Yamamoto et al. precisely determine molecular constants and identify 49 lines in the c-C<sub>3</sub>H rotational spectrum. Both fine and hyperfine components are detected toward TMC-1, and the column density for the line of sight toward TMC-1 is estimated to be 6 trillion/cm<sup>2</sup>, which is comparable to the linear C<sub>3</sub>H radical (l-C<sub>3</sub>H).
M.L Marconi and A. Korth et al. reported a likely detection of C<sub>3</sub>H within the ionopause of Comet Halley in 1989. Using the heavy ion analyzer (PICCA) on board the Giotto spacecraft they determined that C<sub>3</sub>H was responsible for producing a peak at 37amu detected within ~4500 km of the comet nucleus. Marconi et al. argue that a gas phase progenitor molecule for C<sub>3</sub>H is unlikely to exist within the ionopause and suggest that desorption from circumnuclear CHON dust grains may have instead produced the observed C<sub>3</sub>H.
In 1990, Yamamoto et al. detected C<sub>3</sub>H toward IRC + 10216 using the Nobeyama Radio Observatory's 45-m radio telescope. They determine an upper limit for the column density of the ý<sub>4</sub> state 3 trillion/cm<sup>2</sup>. From additional laboratory measurements they determine an extremely low vibrationally excited state for the C<sub>3</sub>H radical: ý<sub>4</sub>(<sup>2</sup>ã<sup>ü</sup>)=610197(1230) MHz, caused by the Renner-Teller effect in the ý<sub>4</sub> (CCH bending) state.
J.G. Mangum and A. Wootten report new detections of c-C<sub>3</sub>H towards 13 of 19 observed Galactic molecular clouds. They measure relative abundance of C<sub>3</sub>H to C<sub>3</sub>H<sub>2</sub>: N(c-C<sub>3</sub>H)/N(C<sub>3</sub>H<sub>2</sub>) = 9.04ñ2.87 x 10<sup>âÂÂ2</sup>. This ratio does not change systematically for warmer sources, which they suggest provides evidence that the two ring molecules have a common precursor in C<sub>3</sub>H<sub>3</sub><sup>+</sup>.
L.A. Nyman et al. present a molecular line survey of the carbon star IRAS 15194-5115 using the 15m Swedish-ESO Submillimetre Telescope to probe the 3 and 1.3 mm bands. Comparing the molecular abundances with those of IRC + 10216, they find C<sub>3</sub>H to have similar abundances in both sources.
In 1993, M. Guelin et al. map the emission from the 95 GHz and 98 GHz lines of the C<sub>3</sub>H radicals in IRC+10216. This reveals a shell-like distribution of the C<sub>3</sub>H emission and time-dependent chemistry. The close correspondence between the emission peaks of C<sub>3</sub>H and the species <noautolink>MgNC</noautolink> and C<sub>4</sub>H suggests a fast common formation mechanism, suggested to be desorption from dust grains.
Turner et al. survey 10 hydrocarbon species, including l-C<sub>3</sub>H and c-C<sub>3</sub>H in three translucent clouds and TMC-1 and L183. Abundances are measured or estimated for each. The mean cyclic-to-linear abundance ratio for C<sub>3</sub>H is found to be 2.7, although a large variation in this ratio is observed from source to source.
In 2004, N. Kaifu et al. completed the first spectral line survey toward TMC-1 in the frequency range 8.8-50.0 GHz with the 45-m radio telescope at Nobeyama Radio Observatory. They detected 414 lines of 38 molecular species including c-C<sub>3</sub>H and compiled spectral charts and improved molecular constants for several carbon-chain molecules.
Martin et al. made the first spectral line survey towards an extragalactic source, targeting the starburst galaxy NGC253 across the frequency range 129.1-175.2 GHz. Approximately 100 spectral features were identified as transitions from 25 different molecular species, including a tentative first extra-galactic detection of C<sub>3</sub>H.