Blundgr2

Structure:

Linear (l-C₃H)

μ_D=3.551 Debye ^[[1][13]]
²Π electronic ground state

Cyclic (c-C₃H)

μ_D=2.4 Debye ^[2]
²B₂ 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 (Woodall et al. 2006^[[3][14]]).

Dominant Formation Reactions:

Reactant 1	Reactant 2	Product 1	Product 2	Rate Constant	Contribution
C₃H₃⁺	e^-	C₃H	H₂	1.0E-7 cm³s^-1	81.2%
C	C₂H₂⁺	C₃H	H	2.18E-10 cm³s^-1	18.8%

Dominant Destruction Reactions:

Reactant 1	Reactant 2	Product 1	Product 2	Rate Constant	Contribution
O	C₃H	CO	C₂H	1.7E-11 cm³s^-1	95.4%
N	C₃H	C₃N	H	1.7E-11 cm³s^-1	3.7%
H₃⁺	C₃H	C₃H₂⁺	H	2.0E-9 cm³s^-1	0.7%
C⁺	C₃H	C₄⁺	H	1.0E-10 cm³s^-1	0.2%
H⁺	C₃H	C₃⁺	H₂	2.0E-9 cm³s^-1	<<1%

Contribution to Carbon-Chain Molecule Production:

The relationship of C₃H to nearby carbon-chain molecules is illustrated well in Figure 4.3 of the PhD Thesis of Ben McCall (2001) ^[[4][15]] shown right.
<img src="%ATTACHURLPATH%/mccall_image.jpg" alt="mccall_image.jpg" width='279' height='250' align='right' align='top'/>

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 99% of C₄H⁺, which is necessary for the production of higher-order carbon-chain molecules. Compared to the competing reaction shown below, the destruction of C₃H provides a much faster pathway for hydrocarbon growth.
C₃H₃⁺ + C → C₄H₂⁺ + H

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^[[5][1]]. 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.^[[6][2]]. A 1987 paper by W.M. Irvine^[[7][3]] 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.

Subsequent Astronomical Detections:

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

In 1987, Yamamoto et al.^[[8][4]] 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.^[[9][5]] 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 ~4500km 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.^[[10][6]] 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 (1990)^[[11][7]] 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. (1992)^[[12][8]] 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.^[[13][9]] 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. (2000)^[[14][10]] 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.^[[15][11]]  completed the first spectral line survey toward TMC-1 in the frequency range 8.8-50.0GHz 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. (2005)^[[16][12]] made the first spectral line survey towards an extragalactic source, targeting the starburst galaxy NGC253 across the frequency range 129.1-175.2GHz. Approximately 100 spectral features were identified as transitions from 25 different molecular species, including a tentative first extra-galactic detection of C₃H.

References

[[17][1]]   *Irvine, W. M.; Friberg, P.; Hjalmarson, Å.; Johansson, L. E. B.; Thaddeus, P.; Brown, R. D.; Godfrey, P. D.* _"Confirmation of the Existence of Two New Interstellar Molecules: C₃H and C₃O"_ 1984, Bulletin of the American Astronomical Society, 16, 877
[[18][2]]   *Thaddeus, P.; Gottlieb, C. A.; Hjalmarson, A.; Johansson, L. E. B.; Irvine, W. M.; Friberg, P.; Linke, R. A.* _"Astronomical identification of the C3H radical"_ 1985, Astrophysical Journal, Part 2 - Letters to the Editor, 294, L49
[[19][3]]   *Irvine, W.M.* _"The chemistry of cold, dark interstellar clouds"_ 1987, Astrochemistry, 120, 245
[[20][4]]   *Yamamoto, S., Saito, S., Ohishi, M., Suzuki, H., Ishikawa, S.-I., Kaifu, N., & Murakami, A.* _"Laboratory and astronomical detection of the cyclic C₃H radical"_ 1987, Astrophysical Journal, Part 2 - Letters to the Editor 322, L55
[[21][5 ]]   *Marconi, M.L., Korth, A., Mendis, D.A., Lin, R.P., Mitchell, D.L., Reme, H., & D'Uston, C.* _"On the possible detection of organic dust-borne C₃H⁺ ions in the coma of Comet Halley"_ 1989, Astrophysical Journal, Part 2 - Letters 343, L77
[[22][6]]   *Yamamoto, S., Saito, S., Suzuki, H., Deguchi, S., Kaifu, N., Ishikawa, S.I., & Ohishi, M.* _"Laboratory microwave spectroscopy of the linear C₃H and C₃D radicals and related astronomical observation"_ 1990, Astrophysical Journal, Part 1 348, 363
[[23][7]]   *Mangum, J.G. & Wootten, A.* _"Observations of the cyclic C₃H radical in the interstellar medium"_ 1990, Astronomy and Astrophysics, 239, 319
[[24][8]]   *Nyman, L.A., Olofsson, H., Johansson, L.E.B., Booth, R.S., Carlstrom, U., & Wolstencroft, R.* _"A molecular radio line survey of the carbon star IRAS 15194-5115"_ 1993, Astronomy and Astrophysics 269, 377

[[25][9]]   *Guelin, M., Lucas, R., \& Cernicharo, J.*  _"<noautolink>MgNC</noautolink> and the carbon-chain radicals in IRC+10216"_ 1993, Astronomy and Astrophysics 280, L19

[[26][10]]   *Turner, B.E., Herbst, E., & Terzieva, R.* _"The Physics and Chemistry of Small Translucent Molecular Clouds. XIII. The Basic Hydrocarbon Chemistry"_ The Astrophysical Journal Supplement Series, 126, Issue 2, 427
[[27][11]]   *Kaifu, N., et al.* _"A 8.8--50GHz Complete Spectral Line Survey toward TMC-1 I. Survey Data"_ 2004, Publications of the Astronomical Society of Japan, 56, No.1, 69
[[28][12]]   *Martin, S., Mauersberger, R., Martin-Pintado, J., Henkel, C., \& Garcia-Burillo, S.* _"Extragalactic molecular line surveys: the starburst galaxy NGC253"_ 2005, IAU Symposium, 235, 265P
[[29][13]]   *Woon, D.E.* _"A correlated ab initio study of linear carbon-chain radicals C_nH (n = 2‑7)"_ 1995, Chemical Physics Letters, 244, 45
[[30][14]]    *Woodall, J., et al.* _"The UMIST database for astrochemistry 2006"_ 2007, Astronomy and Astrophysics 466, 1197
[[31][15]]    *McCall, B.J.* _"Spectroscopy of trihydrogen(+) in laboratory and astrophysical plasmas"_ 2001, PhD Thesis

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