Chemists love crystals. We don’t do as much recrystallization as we used to, since there are higher-throughput (and less labor-intensive) ways of purifying things these days, but I don’t think I’ve ever met an organic chemist who isn’t happy when a product crystallized out nicely. And we all know what crystals are like – straight-sided, hard, brittle, prone to fracture under stress into smaller shards, etc.
Or not. People who do grow a lot of crystals (typically for X-ray structures) can tell you that while most things fit that description, there are some oddball outliers. You see some compounds that grow long, curved crystals, for one, which makes you think that one face is behaving differently in solution than another. Some of the longer ones are surprisingly twangy, and can take a good deal of bending before shattering, and even the chunky, faceted ones vary quite a bit in how hard and friable they seem to be if you expose them to stress. There’s a lot going on in the crystalline solid state, and not all of it is well understood.
This paper illustrates the point, for sure. It’s looking at what you’d think would be an unremarkable coordination compound, copper (II) acetylacetonate, known to many chemists (as are its kin) as “copper ack-ack”. I would not want to guess how many (acac) metal complexes have been crystallized, alone or with other ligands, but let’s stipulate that it is a large heap, and that it’s a very well known species. But you can grow long needle-like crystals of the plain Cu(II) complex that act very weirdly indeed. They’re long and flexible, and as that photo shows, can actually be threaded into knots and then untied. What’s going on at the atomic and molecular level to allow them to bend this much without breaking?
The paper presents a careful X-ray study that figures it out. The authors (from Queensland) mounted a bent crystal in a synchrotron beam and carefully focused in on different parts of bent shape. They found that a particular crystalline axis was significantly elongated on the outside of the curve, and compressed on the inside, as you’d well imagine. It turns out that while each individual copper-acac molecule is identical as the crystals bends, the arrangement they make with each other certainly changes. The distances between the crystal planes don’t change, but the molecules themselves rotate with respect to each other (here’s a movie from the paper’s SI files illustrating that). The changes are not large at all, but when you add them up across several zillion molecules in a long crystal, it gives you some real wiggle room. A philosophical question comes up: if such a sample is not a regularly arranged array of molecules across its width (or not any more), is it still a crystal, or not? If not, do we have a word for what it is?
This is a nice piece of crystallography, of course, but people into materials science will appreciate that there are a lot of interesting features that might emerge from some changes. The optical and magnetic properties of the different sides of such a crystal could well be different (in fact, might almost have to be), and these changes could well be valuable in real-world applications. These results apparently also go against at least one theory of what sorts of crystals can undergo such deformations and how they do it, which would make you think that there could be several different mechanisms available. There’s a whole world in between crystals and amorphous matter, and there’s a lot being discovered in it.
Note: All opinions, choices of topic, etc. are strictly my own – I don’t in any way speak for my employer