# Propynylidyne

Propynylidyne is a chemical compound that has been identified in interstellar space.

## Structure
### Linear (l-C$_3$H)

μ$_D$=3.551 Debye
$^2$Π electronic ground state

#### Simulated spectrum

A rotational spectrum of the $^2$Π electronic ground state of l-C$_3$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 μ=3.551 Debye and others provided by Yamamoto et al. 1990, given in units of MHz: B=11189.052, D=0.0051365, A$_{SO}$=432834.31, γ=-48.57, p=-7.0842, and q=-13.057. A selection rule of ΔJ=0,1 was applied, with S=0.5. The resulting simulation for the rotational spectrum of C$_3$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.

### Cyclic (c-C$_3$H)

μ$_D$=2.4 Debye electronic ground state

## Chemistry

The molecule C$_3$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.

### Dominant formation reactions

| Reactant 1 | Reactant 2 | Product 1 | Product 2 | Rate Constant | Contribution |
| C$_3$H$_3$$^+$ | e$^−$ | C$_3$H | H$_2$ | 1.0E-7 cm$^3$s$^{−1}$ | 81.2% |
| C | C$_2$H$_2$$^+$ | C$_3$H | H$^+$ | 2.18E-10 cm$^3$s$^{−1}$ | 18.8% |

### Dominant destruction reactions

| Reactant 1 | Reactant 2 | Product 1 | Product 2 | Rate Constant | Contribution |
| O | C$_3$H | CO | C$_2$H | 1.7E-11 cm$^3$s$^{−1}$ | 95.4% |
| N | C$_3$H | C$_3$N | H | 1.7E-11 cm$^3$s$^{−1}$ | 3.7% |
| H$_3$$^+$ | C$_3$H | C$_3$H$_2$$^+$ | H | 2.0E-9 cm$^3$s$^{−1}$ | 0.7% |
| C$^+$ | C$_3$H | C$_4$$^+$ | H | 1.0E-10 cm$^3$s$^{−1}$ | 0.2% |
| H$^+$ | C$_3$H | C$_3$$^+$ | H$_2$ | 2.0E-9 cm$^3$s$^{−1}$ | <<1% |

### Contribution to carbon-chain molecule production

The C$_3$H molecule provides the dominant pathway to the production of C$_4$H$^+$, and thereby all other C$_n$H (n>3) molecules via the reactions:
    C$_3$H + C$^+$ → C$_4$$^+$ + H
    C$_4$$^+$ + H$_2$ → C$_4$H$^+$ + H
These reactions produce the majority of C$_4$H$^+$, which is necessary for the production of higher-order carbon-chain molecules. Compared to the competing reaction, C$_3$H$_3$$^+$ + C → C$_4$H$_2$$^+$ + H,
also shown right, the destruction of C$_3$H provides a much faster pathway for hydrocarbon growth.
Other molecules in the C$_3$H family, C$_2$H and C$_3$H$_2$, do not significantly contribute to the production of carbon-chain molecules, rather forming endpoints in this process. The production of C$_2$H and C$_3$H$_2$ essentially inhibits larger carbon-chain molecule formation, since neither they nor the products of their destruction are recycled into the hydrocarbon chemistry.

## First astronomical detection

The first confirmation of the existence of the interstellar molecule C$_3$H was announced by W.M Irvine et al. at the January 1985 meeting of the American Astronomical Society. The group detected C$_3$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$_k$ ≅10K), dark clouds, with particular emphasis paid to tri-carbon species, including C$_3$H.

## Subsequent astronomical detections

Later reports of astronomical detections of the C$_3$H radical are given in chronological order below.
In 1987, Yamamoto et al. report measurements of the rotational spectra of the cyclic C$_3$H radical (c-C$_3$H) in the laboratory and in interstellar space towards TMC-1. This publication marks the first terrestrial measurement of C$_3$H. Yamamoto et al. precisely determine molecular constants and identify 49 lines in the c-C$_3$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 6x10$^{12}$cm$^{−2}$, which is comparable to the linear C$_3$H radical (l-C$_3$H).
M.L Marconi and A. Korth et al. reported a likely detection of C$_3$H within the ionopause of Comet Halley in 1989. Using the heavy ion analyzer (PICCA) on board the Giotto spacecraft they determined that C$_3$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$_3$H is unlikely to exist within the ionopause and suggest that desorption from circumnuclear CHON dust grains may have instead produced the observed C$_3$H.
In 1990, Yamamoto et al. detected C$_3$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 ν$_4$ state 3x10$^{12}$cm$^{−2}$. From additional laboratory measurements they determine an extremely low vibrationally excited state for the C$_3$H radical: ν$_4$($^2$Σ$^μ$)=610197(1230) MHz, caused by the Renner-Teller effect in the ν$_4$ (CCH bending) state.
J.G. Mangum and A. Wootten report new detections of c-C$_3$H towards 13 of 19 observed Galactic molecular clouds. They measure relative abundance of C$_3$H to C$_3$H$_2$: N(c-C$_3$H)/N(C$_3$H$_2$) = 9.04±2.87 x 10$^{−2}$. 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$_3$H$_3$$^+$.
L.A. Nyman et al. present a molecular line survey of the carbon star IRAS 15194-5115 using the 15m Swedish-ESO Submillimetre Telescope to probe the 3 and 1.3 mm bands. Comparing the molecular abundances with those of IRC + 10216, they find C$_3$H to have similar abundances in both sources.
In 1993, M. Guelin et al. map the emission from the 95 GHz and 98 GHz lines of the C$_3$H radicals in IRC+10216. This reveals a shell-like distribution of the C$_3$H emission and time-dependent chemistry. The close correspondence between the emission peaks of C$_3$H and the species <noautolink>MgNC</noautolink> and C$_4$H suggests a fast common formation mechanism, suggested to be desorption from dust grains.
Turner et al. survey 10 hydrocarbon species, including l-C$_3$H and c-C$_3$H in three translucent clouds and TMC-1 and L183. Abundances are measured or estimated for each. The mean cyclic-to-linear abundance ratio for C$_3$H is found to be 2.7, although a large variation in this ratio is observed from source to source.
In 2004, N. Kaifu et al. completed the first spectral line survey toward TMC-1 in the frequency range 8.8-50.0 GHz with the 45-m radio telescope at Nobeyama Radio Observatory. They detected 414 lines of 38 molecular species including c-C$_3$H and compiled spectral charts and improved molecular constants for several carbon-chain molecules.
Martin et al. made the first spectral line survey towards an extragalactic source, targeting the starburst galaxy NGC253 across the frequency range 129.1-175.2 GHz. Approximately 100 spectral features were identified as transitions from 25 different molecular species, including a tentative first extra-galactic detection of C$_3$H.