X-ray structure analysis question

[Trave11er]

Hello everyone!

I have met such a curious thing: When chemists tried to synthesize CaCl by reducing CaCl2 with H2 they got CaClH instead, but the structure of a compound was determined using X-ray structure analysis which is not suitable for determining the position of light elemnts like hydrogen, thus the presence of hydrogen was missed and CaClH was thought to be a CaCl for a long time. Can anybody explain, why this X-ray technique cannot be applied to light atoms? Thank you.

 

[Rajini]

These days XRD is routinely used by chemists for solving the structure of crystalline compounds and these compounds contains H atoms. So i think it is not difficult to detect H atoms in XRD. But one reason coming to my mind is...H has low electron density around nucleus so less interection of X-ray photons with electrons..so may be difficult to detect.

 

[alxm]

Well, remember what you're measuring with XRD - electronic density.
Hydrogen atoms only have a single electron, (okay, two here) so they're small and not very visible using XRD. It depends on what kind of resolution you have (which in turn is dependent on experimental factors, such as the purity of your crystal)

Now in this specific case, you've got a chlorine atom (very electronegative, lots of electronic density) with a tiny hydrogen next to it, so you're basically just looking for a slight 'bump' on the chlorine density rather than a well-defined sphere, so you'd need pretty good resolution to see it.

Although for an inorganic substance (nice crystals) I'd think the resolution today would usually be good enough. Was this an old experiment? (60s-70s?) I do know that for protein structures (where resolution is much lower) that they often can't really distinguish between OH and Cl, for instance.

 

[Trave11er]

Thank you for such comprhensive explanation - I really appreciate it and learned much new from it. The experiment was carried even earlier - 1953 ^).