[Nescafe]

Hi,

I always wondered if X-ray crystallography is such a useful technique why is it not used more often. Then I read the limitations which to summarize are

1) Requires solid, if liquid or a solid which does not give crystalline it does not work

2) Lots of time

My question is why does it HAVE to give crystalline? Is it because the x-rays (which are given from the electrons) are the "strongest" or should I say most distinct when emitted from the crystalline?

Anyone have a good explanation for this?

Nescafe.

[discodermolide]

Hi,

I always wondered if X-ray crystallography is such a useful technique why is it not used more often. Then I read the limitations which to summarize are

1) Requires solid, if liquid or a solid which does not give crystalline it does not work

2) Lots of time

My question is why does it HAVE to give crystalline? Is it because the x-rays (which are given from the electrons) are the "strongest" or should I say most distinct when emitted from the crystalline?

Anyone have a good explanation for this?

Nescafe.

You measure the angle of the reflections of the x-rays which are returned when they hit the atoms in a rigid crystal lattice. In a liquid the atoms move around, in a non crystalline solid you do not have a defined crystal lattice so you get a mess of reflections.
It takes time because you need to measure many reflections, it's like making a time lapse film, picture by picture, or in this case reflection by reflection and that in 3D.
Here is a x-ray quality crystal of mine and the X-ray it generated.

[fledarmus]

Nice crystal! and structure!

The X-rays are not being emitted from the crystal - you use an x-ray generator to project the x-rays at the crystal. They are diffracted by the electron density around each of the atoms in the crystal.

The problem is that the signal-to-noise ratio at the detector is very low. Imagine trying to photograph people in a disco. What you would like is the equivalent of a strobe light - if you take a photograph of a disco using a strobelight, you can freeze every single person in place. However, if you use a very low power light, even though you freeze everybody in place, you can't see anything but black in the picture. You can take a much longer exposure so that your film can collect enough light to see the people, but in a disco all that would give you is a blur - you wouldn't be able to tell where any of the people are. If you tell all the people to freeze in place, you can use a very low-power light source and eventually get a long enough exposure to see them all clearly in the picture. If all the people were also identical, you could overlay every single place the person showed up in the picture, and get an even better picture with less exposure time.

That's what you are doing with a single crystal X-ray - you are using a long exposure time over a structure that has identical molecules, all frozen in the same orientation, and overlaying the diffraction patterns to identify every atom - or at least, all of the electron density, which is centered on the atoms. A blast of X-ray massive enough to act as a strobe and freeze all of the normal, disordered motion of molecules in liquids and gasses would ionize the entire lab, and probably the next two or three buildings as well.

[Nescafe]

Thank you both!

[Babcock_Hall]

One other thing to consider, in a liquid or in a completely disordered solid, each molecule would be oriented differently with respect to the x-ray beam.  I am inclined to say that each would produce a different diffraction pattern.  In a crystal, all of the orientations are almost identical.