If you’ve ever been around an X-ray crystallography setup, one of the constants is a tube directing a blast of chilly vapor at the crystal that’s mounted for analysis. It’s usually a stream of cold nitrogen gas, often set up as a blast of the cold stuff surrounded by a second concentric layer of dry room-temperature nitrogen, so that you don’t immediately start growing frost all over your sample. Cooling the crystal is nearly universal in the X-ray diffraction business, for several good reasons. It preserves the crystal from deterioration, for one thing, both from being exposed to ambient conditions and from being blasted by an X-ray beam. The X-ray beam will slowly demolish your crystal – or if you’re using a synchrotron source, they will quickly demolish it – and the key is to extract good reflection data before the damage gets too severe.
And even more importantly, cooling slows down the molecular/atomic motions as much as possible, giving you tighter data. It’s not like you can’t get a decent X-ray at room temperature (you certainly can) but keeping all the thermally wiggling parts (getting technical here) from wiggling quite so much can be a real help. Those drawings of your molecule that software will give you from X-ray data are “thermal ellipsoids” for each atom, and they look a lot better as peas than they do as watermelons or serving platters.
But the benefits of such cooling are not universal. Here’s a good example from the recent metal-organic framework literature, and it describes an effect that I’ve encountered a bit myself. (I’ve actually had occasion to synthesize some MOFs, and I can tell you that from a bench chemistry perspective that although it can be a frustrating field, making all those brilliant multicolored crystals is a great time). The thing about MOFs, though, is that they are, by definition, frameworks of organic ligands held together by linkages to metal atoms, and the spaces in between are wide open – and full of solvent.
That’s a problem, because that solvent is pretty much disordered, except in some special cases where you can see (for example) benzene molecules lined up in there, which sends you toward the very interesting and still-evolving world of getting small molecule guests in their for crystallographic studies themselves. But in all MOF crystal studies, you have to deal with the X-ray diffraction noise from all that disordered solvent. There are software tools that can help, but they have to be applied judiciously, because you can throw away or obscure useful information while you’re trying to deal with the slop. And crystallographers, a famously rigorous bunch, would rather not massage the data any more than absolutely necessary, and good for them. (A nice thing about a full crystallographic report is that you can see exactly what had to be done to get the structure, and decide if you agree with those decisions or not).
The new paper referenced above points out in detail what some crystallographers had been noticing empirically: some MOF crystals can actually look better at room temperature than they do under cryo conditions. The Yaghi group (very well-known in the field) reports some zirconium MOFs, of which they have prepared a good many over the years, with typically large unit cells and high porosity. Collecting reflections at 100K gave a notably worse data set than a room-temperature run, with weaker high-angle data and worse signal/noise overall (you really need those high-angle spots for good resolution). Ask a crystallographer which of those two data sets shown they’d be more enthusiastic about working with, and you’ll get a definite and possibly profane answer.
What seems to be going on is interaction between the solvent molecules and the framework. As you cool things down, these become more fixed and less fluid/exchangeable, and they deform the framework at random places throughout the crystal lattice. That pretty much destroys your chances of getting clean reflection data. Interestingly, taking a cooled crystal and warming it back to RT gives improved data, but not back to what it was initially – some of those solvent interactions apparently stick, which means that you in turn are stuck.
This is not going to be the case every time, because there are several factors at work simultaneously. The amount of solvent in the pores (and the basic layout of the MOF lattice) is one variable, and the degree of solvent interaction with the framework and metal atoms is another big one. That in turn will vary by what solvent you have in there, and the degree to which it coordinates with the framework (and with itself). But it’s definitely something to consider – the Yaghi paper shows this effect in several different MOFs, to varying degrees depending on framework and solvent. If you’ve made a new one and it looks crappy at 100K then, try fishing out another crystal and just mount the thing and collect at room temperature like it was 1952 again. You might be surprised!