Contention Over Carbyne | November 23, 2015 Issue - Vol. 93 Issue 46 | Chemical & Engineering News
Volume 93 Issue 46 | pp. 30-31
Issue Date: November 23, 2015

Contention Over Carbyne

Some say carbon’s 1-D allotrope is impossible to synthesize, others say they’ve already made it
Department: Science & Technology
News Channels: Materials SCENE, Nano SCENE
Keywords: Carbyne, carbon, allotrope
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HYBRIDIZE IT
Carbyne is hard to define, but researchers agree that it’s linear and its carbons are sp-hybridized.
Credit: Sci. Adv
An illustration shows different forms of carbon.
 
HYBRIDIZE IT
Carbyne is hard to define, but researchers agree that it’s linear and its carbons are sp-hybridized.
Credit: Sci. Adv

No one debates that researchers at Sun Yat-sen University have created interesting carbonaceous crystals. It’s the name that they’re using to describe those crystals that’s controversial: carbyne.

The controversy stems from the formal definition of the word itself, or the lack thereof.

Researchers agree that carbyne is a one-dimensional string of carbon atoms, or a crystallized collection of these strings. But that definition permits enough semantic uncertainty and scientific ambiguity to allow for contention.

Some carbon chemists argue that truly pure carbyne cannot exist. Other researchers believe it has already been synthesized. The recent report out of Sun Yat-sen highlights the disagreement among those trying to synthesize this potentially dangerous substance and study its unique properties.

“You could ask 10 different people to define carbyne and get 10 different answers,” says Rik R. Tykwinski, a physical organic chemist at Friedrich-Alexander University, Erlangen-Nuremberg. Tykwinski’s group made news several years ago when it reported synthesizing 1-D chains of up to 44 carbon atoms, setting a new record. These chains are polyynes, however, not carbyne, Tykwinski says.

To Tykwinski, carbyne is a material whereas polyyne is a molecule. The distinction comes down to the chain length. A polyyne chain is short enough that adding carbon atoms to it would change its properties, thermally, spectroscopically, or otherwise. Adding carbons to the chains in carbyne would change nothing, he explains. Tykwinski thinks one of the alluring mysteries of carbyne is unraveling how many carbons it takes to hit that threshold.

Carbyne is also attractive from a materials science standpoint. Computational studies suggest that the material would be incredibly strong and that its electronic band gap could be tuned by bending or compressing it.

But carbyne also represents a void in carbon chemistry. Carbon atoms hybridize into sp3 orbitals to form tetrahedral bonds with four partners in three dimensions to create diamond. The atoms assemble into the familiar, flat hexagons of graphite and graphene by bonding with three neighbors in the same plane using sp2 orbitals. But pure carbon has yet to line up into stable, sp-hybridized 1-D structures.

“Long polyynes are, in principle, not stable,” Tykwinski explains. Polyyne chains want to cross link, and when they do, they form something that is not carbyne, he says. Furthermore, that cross-linking is extremely energetic. A report from the early 1950s documents polyyne-like molecules causing explosions in the absence of air, Tykwinksi says. “These molecules can be dangerous as hell.”

Harold W. Kroto agrees. Kroto, who earned a share of the 1996 Nobel Prize in Chemistry for the discovery of carbon fullerenes, tells C&EN he does not believe the carbyne claim from the Sun Yat-sen researchers, in part, because the substance didn’t explode. “I know that they can’t have made carbyne because they are still alive,” he says.

Researchers like Tykwinski are building toward carbyne by making longer and longer polyyne chains and capping the ends of the molecules with large, stabilizing groups to prevent cross-linking. But some chemists would argue that chains with large end caps cannot even be considered carbyne.

Yet researchers led by Guowei Yang at Sun Yat-sen have reported synthesizing carbyne crystals without building up increasingly long polyyne chains. Instead, the team has directly produced crystals using pulsed laser light, Yang says (Sci. Adv. 2015, DOI: 10.1126/sciadv.1500857).

The team fired nanosecond laser pulses at a gold target submerged in ethanol. This creates a plasma in the liquid immediately above the target, where gold ions can dehydrogenate the alcohol and allow carbon atoms to bond into linear chains. These chains condense into nanoscopic crystals adorned with gold nanoparticles. The researchers synthesized enough material to obtain a white powder, which Yang says is stable, much like graphite and diamond.

The researchers characterized this powder with various spectroscopic techniques and say the results are consistent with the expected spectroscopic signatures of 1-D carbon chains held together by alternating single and triple bonds.

Yang stresses that the crystalline substance his group has created is the solid-state phase of carbyne, not polyyne. Carbyne condenses into inorganic crystals, Yang argues, whereas polyyne usually becomes an organic polymer.

The synthesis method is also safe, Yang says. The laser-growth method creates extreme thermodynamic conditions capable of creating carbyne only in the immediate vicinity of where the laser strikes the target, he explains. The experimental setup itself is kept in air at standard temperature and pressure. Yang adds that this marks the first time carbyne has been created in a lab in ambient conditions.

“What’s neat about this study, from my perspective, is it confirms that laser heating provides a mechanism for forming these structures,” says Nir Goldman, a physical chemist at Lawrence Livermore National Laboratory, who was not involved in the study.

Goldman recently performed computer simulations that demonstrated bombarding graphite with laser light could also create carbyne fibers. The thermodynamic conditions he observed in his study are likely similar to what Yang’s group created in the lab, Goldman says.

Although Goldman states he’s not an expert on carbon material nomenclature, he thinks “carbyne-like material” is a fair label for the structures made by Yang’s team. He is curious to see whether these newly synthesized crystals exhibit the electrical properties expected of carbyne. Still, he understands the reluctance other scientists might have to christening the substance as such.

“I think some skepticism is warranted,” he explains. “But this is still a very neat result that could point the way to synthesizing carbyne in a highly controlled way.”

C&EN spoke with several researchers skeptical of the carbyne claim, but perhaps no one has been more outspoken than the Nobel Laureate Kroto.

“The existence of carbyne is myth based on bad science and perhaps even wishful thinking,” Kroto wrote five years ago in Chemistry World. The new report, which he calls “total nonsense,” has done nothing to change his mind.

It cites an old phase diagram that purportedly represents carbyne, Kroto says, but that claim was shown to be erroneous decades ago. The report also includes a nuclear magnetic resonance spectrum showing that the crystalline material created by Yang’s team is not a 1-D allotrope of carbon, according to Kroto.

The spectrum basically contains one chemical shift that appears as a spike on the plot, he says. “We know where the carbon shifts should be. They should be all over the place.” He adds that this study should not have been published.

Yang disagrees, arguing that “we have provided substantial and definitive evidence” of carbyne. Myth has become a reality in his lab, he says.

The journal that published the study, Science Advances, likewise stands behind the new report. The report from Yang’s team was rigorously assessed and deemed to fit the journal’s goal of enhancing the amount of outstanding research that reaches the scientific community, says Marcia McNutt, editor-in-chief for the Science family of journals.

For his part, Tykwinski doesn’t call the new substance carbyne. Current spectroscopic techniques can’t resolve whether there are cross-links that make the difference between sexy carbyne and more pedestrian polyyne material, he says.

“We’re probably talking semantics,” he concedes. “But if you want to say you have crystalline carbyne, then you need to have crystalline carbyne.”  


Quiz: What Is Carbyne?

Whether or not carbon's 1-D allotrope has been synthesized—or even can be synthesized—depends on how researchers define carbyne. C&EN asked four carbon researchers to tell us what carbyne is (or should be).

Can you match the definition to the scientist? (Drag the definition to the scientist)

 
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Comments
Yuri Prazdnikov (November 23, 2015 3:32 AM)
The existence of a stable oriented carbyne form is protected by international patent law:
http://advances.sciencemag.org/content/1/9/e1500857.e-letters
Statements that carbyne does not exist - attempt to wishful thinking. Because to say that, you must first win the Patent Court, and refute our publications (for example in Carbon and in the book of Robert Heimann). I have samples of carbyne synthesized decades ago and they all have unique properties. I can easily prove it in any laboratory.
Matt Davenport (December 2, 2015 2:57 PM)
Thanks for your comments, Yuri. I was wondering if you've measured the electronic properties of your samples, specifically if the samples are conducting/semi-conducting/insulating. Thanks again!
Yuri Prazdnikov (December 15, 2015 3:41 AM)
Most interesting and prospective property is abnormal injection (emission) properties of oriented carbyne films (1-d conductance along chains is high as in metal, but transport transverse the chains is very strange). It was the aim of my investigation work to explain this. My be it seems a little bit fantastic but carbyne is Dirac material in my theory.[https://www.researchgate.net/publication/273145295_Oriented_Carbyne_-the_First_Manufacturable_Topological_Insulator]
Matt Davenport (December 2, 2015 2:58 PM)
From Andrew R. McGhie at UPenn:

Crystalline Carbyne?

A new controversy has flared
And the whole world wants to know
Has crystalline carbyne been prepared?
It sets us all aglow
For Guowei Yang claims true carbyne
Harry Kroto says it’s not
And with both camps scientists align
With counter claims the field is fraught
Now this would really be unique
Sp hybridized crystalline carbon
Can we tell from a simple nmr peak
And cut out all the jargon?
Afraid not says Kroto to the world
Of peaks there should be many
This work should never have been unfurled
It isn’t worth a penny
Yang, clearly, has made something fine
And prepared a little white powder
But is it true carbyne or polyyne
The claims are getting louder
Is it a 1-D carbon allotrope
The holy grail in this field
Or is it more an expression of hope
A result only time will yield
For a true scientific conclusion
Soon will, of course, be found
We’ll be under no illusion
And by no dogma, we’ll be bound
But there remains for me this question
With a little strain can a polyyne
Like a buckyball undergo a resurrection
And ring close to form a molecular carbyne?

Andrew Roxburgh McGhie 12.02.2015
Yusheng Xiong (December 8, 2015 12:54 PM)
I suggest the authors do some 13C isotope lebeling experiments using 1-13C Ethanol, 2-13C-ethanol, and 1,2 13C-ethanol mix with unlabelled ethanol to study the NMR of the product. NMR coupling between the labeled 13C should shed light on the arrangement of the c atoms.
Sergey Evsyukov (December 8, 2015 5:56 PM)
Oh, no! Not again, not from the very beginning, please! Are we going to step on the same rake over and over again?
First of all, let’s agree about the terms once and for all. Firstly, carbyne, if it does exist, is not a wisp of non-interacting carbon chain-like molecules. Carbyne was and still is claimed to be a carbon allotrope, so it is supposed to be a material, as we know other allotropes like graphite or diamond. Secondly, the chain length within the crystal should be long enough to make the contribution of the end groups or atoms negligible. Thirdly, it should be stable enough at least to be isolated and explored. One can argue that transactinides existing for milliseconds are still considered as chemical elements. Nevertheless, we are talking about allotropes here, right? So let me make a couple of additional comments.

1. The existence of many hundreds of publications, dozens of filed inventions and re-filed re-inventions, as well as several books on carbyne (including one we published back in 1999), no matter how thick they are, does not provide any reliable and unambiguous evidence for the existence of carbyne, since all analytical methods used so far are either indirect or ambiguous. So I would suggest to keep considering carbyne a hypothetical allotrope until we finally will have seen a clear-cut x-ray single crystal analysis. (For more details see also my last outcry at this site:
http://cen.acs.org/articles/91/web/2013/10/Carbyne-Predicted-Strongest-Known-Material.html )

2. Concerning the recent study by Yang et al. [1], I have got too many questions and cannot agree with many claims and interpretations made in the paper. Apparently, the Chinese group have synthesized very short and, therefore, soluble polyynes using a "novel" but well-known since a couple of decades method of laser ablation of carbon in a submerged electrical arc (see chapters by Tsuji et al., Wakabayashi et al., and Cataldo in the book Polyynes ed. by F. Cataldo, CRC Press, 2006). After that they managed to cast the solution onto a glass substrate, evaporate the solvent, and claim to have synthesized carbyne. The fact that the molecules are short is well-documented by their own mass-spectrometric data presented in the supplement (Fig.S6 (A)). The IR spectrum shown in Fig.2 triggers further questions. On the one hand long polyynes are known to be inactive in IR, and the peak shown in Fig.2B looks typically for short polyynes or even unsymmetrically substituted isolated triple bonds. On the other hand, why did the authors cut the spectrum off, having shown just a narrow range of 2000-2400 cm(-1)? I would like to take a look at the low- and high-frequency regions as well. I would also prefer to have data from classical elemental analysis rather than from EDX.
The authors also measured a 13C-NMR spectrum and wrote that "the chemical shift of the sp-hybridized carbon is in the range of 60 to 95 ppm". Yes, it is, but they reported their signals to lie in the range of 90-95 ppm, which is typical for isolated triple bonds, whereas conjugated polyynes are commonly known to resonate in a 55-70 ppm region [2-21].
The real advance of the study by Yang et al. seems to be the preparation of the crystalline stuff. The XRD and SAED definitely show a crystalline order, but the interpretation of nanorods based on "parallel arrays of helices" covered with gold clusters is rather curious. Having started with the known kinked-chain model postulated to stabilize carbon chains, the authors suddenly invoked the bending effect, which indeed is known from crystallographic studies of end-protected polyynes, and ended up with a new concept of wavy chains.
What is really puzzling in this study is the alleged stability of polyynes in the solid state. It has been proved many times (see for instance studies by Milany et al. [21, 22]) that unprotected carbon chains are extremely reactive and unstable (their collapse can be induced even by the electron beam during STEM measurements). The stability of crystals prepared by Yang et al. suggests either some special stabilization mechanism (gold atoms?) or just absence of carbon chains in the sample (cross-linking in the solid state, something like topochemical polymerization of diacetylenes).
Finally, the reasoning about ethanol as an ideal building block for carbyne, just because it has two carbon atoms in a "unique configuration" and thus provides a kind of a template for the formation of a triple bond, does not stand up under scrutiny. What about other diatomic molecules?
Summing up, one can conclude that structural evidence reported in the paper by Yang et al. can be called carbyne.
There are dozens of excellent synthetic studies by the research groups of Gladisz [3-7], Hirsch [8, 9], and Tykwinski [14-20], who developed exquisite syntheses of well-defined end-protected polyynes with up to 44 cabon atoms. Still, even longest molecules are just individual chemical substances, i.e. polyynes being far away from carbyne, and there are no examples of the successful transformation of organic polyynes into carbyne that could be proven with a clear-cut x-ray analysis.
Even if the authors will have provided clear structural evidence unambiguously confirming their samples to be crystalline polyynes, they still cannot be considered carbyne. In much the same way nobody calls naphthalene crystals graphite, and adamantane can hardly be called diamond.

P.S. The poetry above is almost lachrymatory cute…

REFERENCES:
[1] B. Pan, et al., Sci. Adv. 2015;1:e1500857; doi: 10.1126/sciadv.1500857.
[2] E. Kleinpeter, R. Borsdorf, 13C-NMR-Spektroskopie in der Organischen Chemie, Akademie, Berlin, 1981.
[3] H.-O. Kalinowski, S. Berger, S. Braun, 13C-NMR-Spektroskopie, G. Thieme, Stuttgart, 1984.
[4] T. Bartik, et al., Angew. Chem. Int. Ed. Engl. 35 (1996) 414.
[5] R. Dembinski, et al., J. Am. Chem. Soc., 2000, 122 (5), 810.
[6] W. Mohr, et al., Chem. Eur. J., 2003, 9 (14), 3324.
[7] Q. Zheng & J.A. Gladysz, J. Am. Chem. Soc., 2005, 127 (30), 10508.
[8] Q. Zheng, et al., Chem. Eur. J., 2006, 12 (25), 6486.
[9] G. Schermann, et al., Chem. Eur. J., 1997, 3 (7), 1105.
[10] T. Gibtner, et al., Chem. Eur. J., 2002, 8 (2), 408.
[11] T.N. Hoheisel & H. Frauenrath, Org. Lett., 2008, 10 (20), 4525.
[12] A. Sarkar, et al., Macromolecules, 1998, 31 (17), 5624.
[13] M. Kijima, et al., Chem. Lett., 1999, (6), 531.
[14] R. Zeisberg & F. Bohlmann, Chem. Ber., 1974, 107 (12), 3800.
[15] S. Eisler, et al., J. Am. Chem. Soc., 2005, 127 (8), 2666.
[16] Th. Luu, et al., Org. Lett., 2005, 7 (1), 51.
[17] J. Kendall, et al., Org. Lett., 2008, 10 (11), 2163.
[18] W.A. Chalifoux & R.R. Tykwinski, Compt. Rend. Chim., 2009, 12 (3-4), 341.
[19] W.A. Chalifoux, et al., Angew. Chem., Int. Edition, 2009, 48 (42), 7915.
[20] R.R. Tykwinski, et al., Pure Appl. Chem., 2010, 82 (4), 891.
[21] W.A. Chalifoux & R.R. Tykwinski, Nature Chem., 2010, 2 (11), 967.
[22] L. Ravagnan, et al., Chem. Commun., 2011, 47 (10), 2952.
[23] C.S. Casari, et al.: Carbon, 2004, 42 (5-6), 1103.
Sergey Evsyukov (December 8, 2015 6:08 PM)
Sorry, I have found a typing error in the first sentence of the last paragraph.
It shoud be read as:

Summing up, one can conclude that structural evidence reported in the paper by Yang et al. is not sufficient to call the reported stuff carbyne.
Yuri Prazdnikov (December 14, 2015 2:44 AM)
The problem is not in carbyne, but much deeper.
There are two "realities" now. Carbyne "does not exist" and "it does" simultaneously. Moreover, stable form of carbyne not only exists but was patented and transistor based on it was already produced [http://advances.sciencemag.org/content/1/9/e1500857.e-letters].
To put it simply the "skeptic reality" was established by H.Kroto, when he wrote in 2010 "carbyne is a myth". It became the position of "official science" because he has great authority. Therefore, the authors of new rediscovery carbyne [Science Advances, "Carbyne with finite length: The one-dimensional sp carbon"] wrote: "There is a recognized understanding that definitive evidence for the existence of carbyne has not been obtained ". However, they did not review all of carbyne's paper, but only those in which it is stated "carbyne does not exist". So we claim that this is nothing else than wishful thinking. Obviously there is no such private opinion that should be the reason to ignore the numerous publications that claimed opposite point of view.
One could say that the problem is a terminological. Proponents of the "skeptic reality" usually write: "show us the carbyne crystal that can be kept in hands." However, when H.Kroto discovered fullerenes, no one has asked him to show a fullerene having size of the ball - that "can be kept hands". We wrote that carbyne can be stable in thin film form only. Additionally carbyne cannot exist in a pure carbon form - it always contains hydrogen as a stabilizer. It seems that pure stable carbyne will never be produced – we share the opinion of H.Kroto. One would conclude that it does not exist, but there are various carbyne-like polymorphic forms. However, proponents of "skeptic reality" ignore the publications about these carbyne-like forms. Moreover justified claims can be seen now in "Science Advances" [http://advances.sciencemag.org/content/1/9/e1500857.e-letters] and here. As you can see the authors' reply is not tenable - the campaign of ignoring continues. Thus the carbyne's problem is not terminological only.
If we accept that H.Kroto's opinion determines the "official science", then we should say that "carbyne doesn't exist" for this reason only. One his argument is indicative: "..they can’t have made carbyne because they are still alive". But now, with the advent of the simple synthesis technique collision of two "realities" will intensify. We can assume that tomorrow carbyne will be synthesized in hundreds of laboratories worldwide. We are far from thinking that H.Kroto will change his opinion. However, there will be a lot of publications about stable carbyne form and its applications in nanoelectronics. As a result, the contradiction will reach point of absurdity, which will be obvious even for people far from science.

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