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Analytical Chemistry

A New Form of Carbon

Here’s today’s weird molecule, for sure. A collaboration between IBM-Zürich and Oxford has reported a new allotrope of carbon, this one an 18-membered ring of alternating triple and single bonds (!) People have been speculating about such structures for years, but they appear to be too reactive to spot easily in the wild. There’s some evidence in the gas phase, but not enough to settle arguments about whether such structures even exist, and if they do whether they’re pure alternating-ynes or have some cumulene character (double bond attached right on to double bond, perhaps all the way around the ring). Theoretical calculations are of limited help, since different approaches land on both of the different answers.

As many will have guessed from the IBM team, though, this new work was done at low temperature via scanning-tunneling and atomic-force microscopy. The synthetic scheme is shown at right, with the three remaining alkynes formed via the masking cyclobutenediones. These sorts of compounds had already been prepared twenty years ago in studies towards making the C18 ring by more conventional means, but the cold and delicate touch of the AFM tip (and the isolation of the resulting molecule) are what did the trick this time. The starting material was sublimed onto a copper surface that had “islands” of bilayer NaCl scattered on it, and the individual molecules that landed on these inert salt rafts were the ones selected for the AFM step. There were some partially decarbonylated species found by imaging even at that point, apparently having been produced during the sublimation, but the triangular parent molecule was quite clear (the carbonyls stand out because of their high electron density). The molecule appears to be puckered and not lying completely flat on the surface.

It was then decarbonylated by voodoo. Well, pretty close, anyway. The team raised the AFM tip about three Ångstroms from the molecule and briefly increased the voltage, which provided enough energy (via inelastic tunneling?) for the cyclobutenediones to rearrange and spit out CO molecules. The main species formed were the intermediates where one or two of those events had happened, as in the scheme above. Out of ninety attempts, though, they did get about a dozen molecules of C18 via the triple rearrangement. The triple bonds have higher electron density themselves, of course, and could be seen alternating in both the intermediates and in the final symmetric ring. So we can say that at low temperature, adsorbed onto an inert surface, the alkyne structure is the correct one. You do wonder, though, how much higher in energy the cumulene stuff is, and whether it starts rearranging to that form as it warms up and starts to fall apart. Interestingly, the C18 molecules themselves were pretty weakly bound to the surface. The paper notes that they often jumped around during the manipulations, and were often found having landed on step edges of the surface or next to adsorbed CO molecules, presumably because there was a bit more to grab onto (even at 5 degrees K!)

So put down another weirdo allotrope for carbon. This one (so far) has only been seen as twelve individual molecules, but it can and does exist. Perhaps it (or its higher-ring homologs) are floating around in cold interstellar clouds or something? If you want to see them down here on Earth, though, you’re going to have to work for it. I’m sure it has very interesting properties (the second link in the first paragraph has more on that), but investigating those is not going to be so easy!

37 comments on “A New Form of Carbon”

  1. Barry says:

    Shall we attribute the final step of the Paquette/Ternansky dodecahedrane synthesis to “voodoo” too?

    1. Bob Ternansky says:

      there was no vodoo with the final step of the DDH synthesis…. I can assure you of that…but a hell of a lot of luck !

      1. Anonymous says:

        For those interested in dodecahedrane, see this:

        A Genius, Yet Out of Contention: DuPont’s Howard E. Simmons, Jr.
        The Posthumous Nobel Prize in Chemistry. Volume 1. Correcting the
        Errors and Oversights of the Nobel Prize Committee
        Chapter 13, pp 283–344
        DOI: 10.1021/bk-2017-1262.ch013
        ACS Symposium Series, Vol. 1262
        ISBN13: 9780841232518 ISBN: 9780841232501

        There is a photocopy of HES Jr.’s original 1956 proposal to DuPont management to prepare triquinacene and its dimerization to DDH. To repeat: 1956, several years before Woodward’s 1964 synthesis of triquinacene.

        1. Magrinho says:

          Yes! Howard Simmons was a terrific scientist! Greatly respected by his peers and by academics.

          1. Anonymous says:

            HES Jr. did his PhD with JD Roberts at MIT. One of their key publications (JACS, 1953) provided strong proof of the intermediacy of benzyne in various reactions of aryl halides. The review cited above also provides some insight into that work. I always wondered which genius, JDR or HES, came up with the idea. (From my memory of the review,) HES Jr. conceived of and carried out the experiments on his own while JDR was out of town and not even there to supervise him or, more importantly, to hold him back.

            Benzyne has been used in quite a few syntheses of natural products and med chem projects.

    2. Barry says:

      I confess, there’s something both spooky and gratifying about pointing an (itsy bitsy) wand at a chosen C-C bond and exciting it to rupture (even if it only works 12% of the time). Also cool that on a NaCl heat-sink, a thermally labile molecule can be born in a thermally-excited state (surely the COs don’t carry away all the energy?) and yet survive long enough to characterize.

  2. Old Timer says:

    Does it do a Bergman cyclization or hexa-hydroDA? Man, I’d expect more followup from a group like this! 🙂

    1. Marcus Theory says:

      It’s not every day your yields are quantized, in intervals of 8.33333%!

      1. John Wayne says:

        Quantized yields … awesome observation 🙂

  3. Porphyrius says:

    Friends, Romans, Chemists: Good to see the wheel well and truly re-invented at last. Just need to coordinate the triple bonds to a nonavalent hub to finish the job off.

    Coordinate bond length and orientation might be a challenge, but Rome wasn’t built in a day. Maybe a pair of C16 wheels with Co as hub a better optionem? See figure a herein for CoB16- representation that’s “reminiscent of a drum” (or better still a dually).

    As we charioteers always used to say – Appearance is temporary, Beauty is Eternal…

    https://chemistry.stackexchange.com/questions/31950/can-an-atom-bond-with-more-than-8-other-atoms

    1. Anon says:

      Uncle Al? Is that you?

      1. Porphyrius says:

        Not Uncle Al, but Uncle Rab. An only child raised to unclehood through pairing off. Uncle Al posts high frequency, Uncle R posts low.

        Uncle R sometimes gets Uncle Al, sometimes not. Very tricky to be sure of getting each other. It’s the frequency range, you see. But that applies not just for uncles. All very Wittgenstein. Or as we charioteers also used to say – What goes round goes round…

  4. tlp says:

    But can it technically be called allotrope if it doesn’t form any appreciable amount of bulk material? What’s IUPAC’s stance on that?

    1. Anonymous says:

      What about atomic carbon as an allotrope of carbon? It’s been known for many decades. Atomic carbon (monocarbon; lambda-zero methane) has been generated in the gas phase by arc discharge. It has been characterized by various means and its lifetime and “decomposition” products described. Of course, it doesn’t exactly decompose. Depending on what is nearby, it often forms polymers.

      I’m thinking back to the work of Shevlin and others on the wet bench-accessible preparations of :C: (atomic carbon; a double carbene), e.g., from diazotetrazole (CN6 -> C + 3 N2; something you MIGHT work with?). I had designed a few syntheses of interesting organic products based on using :C: as a reagent. (I also designed some other sources of :C: that might be safer or easier to handle than the tetrazole.)

      Atomic C – allotrope or not an allotrope?

  5. Scott says:

    Carbon nanotubes are more interesting, and if we could figure out how to make them obscenely long** they’d be ideal for Orbital Elevator tethers. (For those that don’t know, orbital elevators are actually in tension, everything above a break in the tether will go flying out into deep space)

    ** As in, 36,000km for an Earth-based Orbital Elevator. Yes, a single molecule 36,000km long. Talk about carbon sequestration!

    1. Nameless says:

      And making 35kkm long molecules will be cheaper than using existing technology (rockets) to move cargo from one place to another. Especially if we consider that we have to use some form of energy (e.g. rockets) to move the cargo from place A to B.

      1. Scott says:

        Well, cheaper in terms of cost/kg to get into orbit, yes.

        But there is one hyuuuuuuuuuuuge investment in capital that has to happen first, probably on the order of a trillion dollars, to get the ‘beanstalk’ built.

        1. loupgarous says:

          In The Fountains of Paradise Arthur C. Clarke posited very long tethers made of synthetic diamond for the space elevators in his novel, which get around the need for entire molecules 36 thousand km long.

          Or do they? College chemistry was decades ago for me, so I’m ignorant as to how many carbon molecules comprise a diamond. One or very, very many? If they’re repeating units, how are they characterized?

          1. Scott says:

            Diamonds are C20, if I am remembering things correctly. Also, Fountains of Paradise was written about 15 years before the discovery of carbon nanotubes.

          2. Barry says:

            Any diamond is a single molecule; the molecular weight is the diamond’s weight. Within the diamond, Carbons repeat endlessly in a 3-D, each one the center of a perfect tetrahedron equally bonded to the the neighboring Carbons that occupy that tetrahedron’s apices.

          3. jon says:

            the molecular weight depends on what the diamond surface is terminated with… C-H or C-OH bonds

          4. loupgarous says:

            Thanks for the info, folks.

        2. loupgarous says:

          Certainly if you make your long carbon nanotubes close to where tons of free hydrocarbons lie outside of Earth’s gravity well, the rings of Saturn or even directly from the plumes of Enceladus. There, your “tools” could be nanodevices themselves, or complexed batteries of AFMs to strip away carbonyls and other inconvenient bits from your carbon nanostructures. You might even make von Neumann machines to make your nanotools from the clathrates spewing from Enceladus, or scoop them out of Saturn’s rings. It’d cost like sin until you started von Neumann-ing in quantity, then sheer scale of quantity would take over.

      2. tim Rowledge says:

        Bob Forward’s group were working on ways to make suitable tethers with somewhat shorter sections of material quite a few years ago. Last time I spoke with him about it (rather a long time ago) they were learning about lace making from some of the surviving workers of the old Nottingham lace industry. By combining many small threads in very lace-like ways they hoped to make a cable able to handle damage from micro-meteoroids (hah, spellcheck wanted micrometeorologists) and radiation etc. Cool stuff.

    2. Anonymous says:

      Back of the envelope calcs: A (10,0) SWNT is around 7.8 angstroms in diameter. A 36 A length = 3.6 nm length of a (10,0) SWNT contains around 360 C atoms. Or, around 100 C atoms per nm. … A 36,000 km (10,0) SWNT is then:
      (100 C atoms / 10^-9 m) (36000 x 10^3 m ) = around 3.6 x 10^18 C atoms.
      That’s a tiny fraction of one mole (12 g) of carbon to make one 36,000 km (10,0) SWNT.

      Even if I’m off by a couple of orders of magnitude, that’s still not a lot of C for one 36000 km strand of (10,0) SWNT. Other correction factors: (a) Need multiple strands for a space elevator (b) Actually need to tether the far end BEYOND 36,000 km to maintain tension (c) need a lockout mechanism to prevent kids from “pressing all the buttons” on the elevator panel, etc..

  6. Chairman Mao says:

    This compound will soon be listed for sale by Chinese chemical vendors.

    1. ChinaPharm Inc says:

      £1000/g – Synthesis on Demand. Ships in 4-6 weeks.

  7. Barry says:

    Hmmm. 4n+2. If cyclooctadecanonayne is planar, Hueckel expects it to be aromatic. Do the alternating bond-lengths preclude aromaticity? Can you assess ring-current in the AFM?

  8. Chri says:

    Synthesis of polyynes in 2010. (Nature link in name.) They got up to 44 atoms in a chain.

    They’re said to be “not particularly sensitive to light, moisture or oxygen” and suitable for handling under normal lab conditions.

    I remember Eric Drexler talking about carbyne rods in 1988.

    1. thomas ryckmans says:

      could these be used for molecular acupuncture??

  9. Expensive says:

    “This one (so far) has only been seen as twelve individual molecules”.

    Must be the most expensive compound ever made, in terms of cost per gram?

  10. Red Fiona says:

    Does using NaCl as the surface it is attached to have any effect on the structure?

    1. Derek Lowe says:

      I get the impression that it’s more that the compound is unstable on the copper surface, and that the NaCl one is more neutral ground.

      1. Red Fiona says:

        Thanks

  11. li zhi says:

    Seems to me that since the molecule was only found on NaCl substrate, then its independent existence is (still) moot. If we accept that surface bound species are ‘real’, then doesn’t that open up the definition of what is a stable chemical compound enormously? Do we want to go there? I’ve doubts.

    1. Derek Lowe says:

      I can see the point you’re making, but I think it’s more that the compound is unstable on the copper surface and the NaCl one is just neutral. And we count gas-phase molecules as existing (when they decompose on hitting almost any surface). More prosaically, there are compounds that are totally unstable in acid but fine in base, etc., although you can argue that they have other stable situations as well.

      1. Anonymous says:

        Reminds me of Chemical Squonks: compounds that are too unstable to be isolated or “caught.” https://en.wikipedia.org/wiki/Squonk#Scientific_usage (Link in handle.)

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