Calculating Carbyne

A group of researchers at Rice University have performed some nice calculations on carbyne, or linear acetylenic carbon, determining its mechanical properties and lots of other fun stuff. Carbyne is a carbon chain with alternating single and triple bonds, and therefore there's plenty of reason to expect that it'd have some interesting mechanical properties like being really strong. Of course, being basically a whole lot of acetylene molecules spaced apart by ethane, it's supposedly be really reactive, and it's strongly suspected that it'd cross-link to death if left to its own devices. (Alternatively, it could be double bonds all the way down; more on that later.)

The group did periodic DFT calculations in VASP to calculate the material's strength - when it breaks -  and Young's modulus, a measure of its resistance to stretching. If you're not familiar with periodic boundary conditions, imagine a Pac-Man or Asteroids game board. Everything that goes out one side wraps around to the other. So a snippet of the molecule sees an image of itself continuing at each of its own ends, and those ends in turn see their own duplicates, and so on. This is an excellent model of an infinitely long chain that doesn't change anywhere along its length. This model of an infinitely long chain is, itself, is a good approximation of a really long, but finite carbyne molecule - and by stretching that chain they could measure its Young's modulus.

They also did some molecular calculations on some rings of the material to estimate its resistance to bending, and on finite-length carbyne molecules (capped at the end with different functional groups) to measure its resistance to twisting. To finish it off they determined the energy barrier to cross-linking when two molecules come together, how well it conducts electricity, and whether the single-triple or double-double bond structures are preferred.

Thorough work!

It transpires that it's not only really rigid but spectacularly strong. Its Young's modulus is double that of the next stiffest material, graphene, and three times that of diamond. It's also comfortably stronger than either. That's right, if you're looking for something to build a space elevator cable out of, miles of nanotubes are no longer the cool hypothetical material to go for. Its resistance to bending and twisting are on a similar order of magnitude to double stranded DNA, a whopping great hydrogen- and covalent-bonded monster of a material. (Interestingly, carbyne's properties are strongly dependent upon what capping groups are used, which could make for some interesting fine-tuning.)

Fun stuff. The real question is, will it hang around when you make it?

Well by these calculations, when bringing two chains together there's an energy barrier of 0.6 eV to them cross-linking. That's pretty substantial. On the other hand, it's very unlikely that two carbyne molecules would just benignly wander up to each other like this. I suspect that when you put the molecule into a real-world soup of radicals and ions, it's not going to have a hard time finding a way around that wall. They also determined that two carbyne chains won't cross-link together down their whole length; the difficulty of pulling the two chains alongside means that there are alternating stretches of untouched and cross-linked carbyne. Again, I'm not sure that carbyne would behave itself quite so well in the messy real world, but it's promising.

You can read the paper at Arxiv, "Carbyne from first principles: Chain of C atoms, a nanorod or a nanorope?" by Mingjie Liu, Vasilii I. Artyukhov, Hoonkyung Lee, Fangbo Xu, and Boris I. Yakobson; a quick Google News search for Carbyne should give you plenty of news coverage.

As always, all errors here are my own, and I would be really grateful for any corrections you might want to send my way.