The simplest carbon growth mechanism by atomic carbon insertion reactions
and the source of DIB
Teruhiko Ogata and Yoshio Tatamitani
Department of Chemistry, Shizuoka University, Shizuoka 422-8529, Japan
Carbon growth reactions represent oneof the most fundamental chemicalprocesses. The simplest example is theatomic carbon insertion reaction (ACIR).
In ACIR, atomic carbon, :C, or its
precursors, :CCO and C3O2, jump into
the CC bond without cleaving the overall
structure. As a result, no side-products
are produced from this reaction (Figure1).

a) Potolysis of C3O2 [1a]O C C C O
C C + C C OhνC C O + COhνC C Ohνhν-CO(1)

C + 2COC

C C C(2)b) Ion + C3O2 reactions [1b,1c ]CnO+ + C3O2+CnO2 + C3O2+CnH6 + C3O2

Cn+3O3+Cn+3O4+-2COCn+3H6O2+-2CO-2COCn+1O+ (n = 1-6)

(3)

+Cn+1O2 (n = 1-18)(4)+Cn+1H6 (n = 1-4)(5)c) Discharge of C3O2 [ 2 ]n C3O2

C +CO +C2O +C3O +C4O +C5O +C6O +C7O +C8O +C9O (6)The mechanism of linear-carbond) Radiation-induced reaction in CO ice [ 3 ]monoxides formation, CnO (n = 2 - 9),
Cn+1O(X 3Σ −/ 1Σ+) (n = 1 - 4) (7)CnO(X 1Σ+/ 3Σ −) + C(3P /1D)
which is synthesized in the discharge ofC3O2, has been investigated based on thedetailed analyses of FTMW
Figure 1: Some linear-carbon-chain growths by atomicspectroscopic data [2]. The relative
carbon-insertion reactions.
abundances of the CnO products,determined from their rotational spectrum intensities, agree with those for the CnO+ ions,which were known to be lengthened by successive mono-carbon insertions [1b]. The active
chemicals in the reaction system include only :C and :CCO. Moreover, since the observed
products consist exclusively of CnO, the reaction occurs via the ACIR mechanism (Figure 2).
C
C C C OC C C C O
Nearly all of theC6O
C C C C C O
linear-carbonC
613C C O
7chain molecules
CC2O
C C C OC C C C C OCdetected inC8OC
52C
laboratories or inC
C C C C O
space, which areC9O
available on Prof. Sucsessive mono carbon insertions
4COkabayashi’sFigure 2: The proposed formation mechanism for CnO (n = 2 - 9) by atomic
homepage [4],carbon insertion reactions.
can be interpretedby this formation
mechanism. The exception is (c-H2C3)=C=C, which requires isomerization from (c-HC3)C≡CH.

ACIR is simple, as it only requires the presence of :C and a CC bond, and efficient due to its lowor zero activation energy. Hence, this reaction scheme is applicable to a wide-range of carbonbased processes, in particular, to ultra low temperature or incomplete combustion conditions [5].
It is commonly thought that long carbon chains and atomic carbon primarily exist in a relatively
diffuse part of the molecular cloud in space, which suggests that carbon chains are formed byACIR.
Based on the following evidence, we can also imagine that a huge amount of the atomic carbon
inserted clusters form a kind of “amorphous carbon” in interstellar space (reminiscent of thesource of DIBs):
1. “Amorphous carbon” can be formed by ACIR, which is the dominant reaction at ultra
low temperatures [5b].

2. While all “amorphous carbon” species have distinct structures from each other, theirstructures may be partially similar.
3. “Amorphous carbon” is black, implying that many absorption features fall in the UVvisible light region.
4. “Amorphous carbon” can withstand bombardment with ultraviolet photons in space.
Previous work has reported that the absorption curves for soots and quenched carbonaceouscomposites agree well to the absorption curves from interstellar extinction [6].
Presumably, “amorphous carbon” has a highly strained filled interior with many dangling
bonds. The possible structure and formation mechanism of “amorphous carbon” will be
proposed at the workshop.
References:[1] (a) K. D. Bayes, 1962, JACS, 84, 4077; (b) S. M. Schildcrout, et. al., 1970, JACS, 92, 251;(c) O.Bortolini, et al., 1999, Int. J. Mass Spectrom., 190/19, 171.
[2] (a) T. Ogata, et al., 1995, JACS, 117, 3593; (b) Y. Ohshima, et al., 1995, JCP, 102, 1493.
[3] C. S. Jamieson, A. M. Mebel, & R. I. Kaiser, 2006 , ApJ Supp. 163, 184.
[4] Prof. T. Okabayashi’s homepage, http://www.ipc.shizuoka.ac.jp/~sctokab/Okab.htm.
[5] (a) T. Ogata, et al., Proceedings of the Carbon2007 Conference, B083, Seattle, USA (2007); (b) Proceedingsof the Carbon2008 Conference, P0469, Nagano, Japan (2008).
[6] (a) A.T. Tokunaga and S. Wada, 1997, Adv. Space Res.,19, 1009; (b) Y. Kimura, T. Sato, and C. Kaito, 2004,Carbon, 42, 33.