Propynylidyne

Propynylidyne

Names
IUPAC name 1,2-Propadien-1-yl-3-ylidene
Other names 2-Propyn-1-ylidyne; 2-Propynylidyne
Identifiers
CAS Number	53590-28-6 Y
3D model (JSmol)	Interactive image
SMILES [C]C#C[H]
Properties
Chemical formula	C3H
Molar mass	37.04 g·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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

Structure

Linear (l-C₃H)

μ_D=3.551 Debye^[1]

²Π electronic ground state

Simulated spectrum

A rotational spectrum of the ²Π electronic ground state of l-C₃H 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^[1] and others provided by Yamamoto et al. 1990,^[2] 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 C₃H at a temperature of T=30K agree well with observations.^[2] 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.^[2]

Cyclic (c-C₃H)

μ_D=2.4 Debye^[3] electronic ground state

Chemistry

The molecule C₃H 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.^[3]

Dominant formation reactions

Dominant destruction reactions

Contribution to carbon-chain molecule production

The C₃H molecule provides the dominant pathway to the production of C₄H⁺, and thereby all other C_nH (n>3) molecules via the reactions:

C₃H + C⁺ → C₄⁺ + H

C₄⁺ + H₂ → C₄H⁺ + H

These reactions produce the majority of C₄H⁺, which is necessary for the production of higher-order carbon-chain molecules. Compared to the competing reaction,
C₃H₃⁺ + C → C₄H₂⁺ + H,
also shown right, the destruction of C₃H provides a much faster pathway for hydrocarbon growth.

Other molecules in the C₃H family, C₂H and C₃H₂, do not significantly contribute to the production of carbon-chain molecules, rather forming endpoints in this process. The production of C₂H and C₃H₂ 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 C₃H was announced by W.M Irvine et al. at the January 1985 meeting of the American Astronomical Society.^[4] The group detected C₃H 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.^[5] 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 C₃H.^[6]

Subsequent astronomical detections

Later reports of astronomical detections of the C₃H radical are given in chronological order below.

In 1987, Yamamoto et al.^[7] report measurements of the rotational spectra of the cyclic C₃H radical (c-C₃H) in the laboratory and in interstellar space towards TMC-1. This publication marks the first terrestrial measurement of C₃H. Yamamoto et al. precisely determine molecular constants and identify 49 lines in the c-C₃H 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 6x10¹²cm⁻², which is comparable to the linear C₃H radical (l-C₃H).

M.L Marconi and A. Korth et al.^[8] reported a likely detection of C₃H within the ionopause of Comet Halley in 1989. Using the heavy ion analyzer (PICCA) on board the Giotto spacecraft they determined that C₃H 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 C₃H is unlikely to exist within the ionopause and suggest that desorption from circumnuclear CHON dust grains may have instead produced the observed C₃H.

In 1990, Yamamoto et al.^[2] detected C₃H toward IRC + 10216 using the Nobeyama Radio Observatory's 45-m radio telescope. They determine an upper limit for the column density of the ν₄ state 3x10¹²cm⁻². From additional laboratory measurements they determine an extremely low vibrationally excited state for the C₃H radical: ν₄(²Σ^μ)=610197(1230) MHz, caused by the Renner-Teller effect in the ν₄ (CCH bending) state.

J.G. Mangum and A. Wootten^[9] report new detections of c-C₃H towards 13 of 19 observed Galactic molecular clouds. They measure relative abundance of C₃H to C₃H₂: N(c-C₃H)/N(C₃H₂) = 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 C₃H₃⁺.

L.A. Nyman et al.^[10] 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 C₃H to have similar abundances in both sources.

In 1993, M. Guelin et al.^[11] map the emission from the 95 GHz and 98 GHz lines of the C₃H radicals in IRC+10216. This reveals a shell-like distribution of the C₃H emission and time-dependent chemistry. The close correspondence between the emission peaks of C₃H and the species <noautolink>MgNC</noautolink> and C₄H suggests a fast common formation mechanism, suggested to be desorption from dust grains.

Turner et al.^[12] survey 10 hydrocarbon species, including l-C₃H and c-C₃H in three translucent clouds and TMC-1 and L183. Abundances are measured or estimated for each. The mean cyclic-to-linear abundance ratio for C₃H 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.^[13] 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-C₃H and compiled spectral charts and improved molecular constants for several carbon-chain molecules.

Martin et al.^[14] 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 C₃H.

References