RE: [ccp4bb]: magic angle

[Harry Powell <harry@mrc-lmb.cam.ac.uk>]

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On Wed, 12 Dec 2001, Rizkallah, PJ (Pierre) wrote:

> I must say here that a Chi axis is equally capable of achieving the same
> thing (for single crystals not powders). The historical reasons for choosing
> chi or kappa escape me, but my impression was that some engineers thought
> kappa geometry allows more easy access to the crystal for better alignment.
> I suppose it is a matter of taste, or perhaps a unique selling point.
> Whatever, most of our work these days can be done with a single axis
> diffractometer, the plain phi axis camera. We are not restricted anymore to
> measuring each reflection separately, with a single counter which could only
> move in the 2-theta direction.

and Marius wrote;
> 
> Can someone explain to me (in simple words) the pratical
> and theoretical usefullness of a diffractometer with kappa 
> geometry with magic angle (54.? deg) using pre-oriented 
> and randomly oriented crystals. 

The kappa angle used in single crystal diffractometers has nothing at all
to do with the magic angle used in solid state nmr (except an apparent
(and very approximate) coincidence of numeric value).

The kappa angle used on a CAD4 is 50o. The magic angle for nmr is ~54.7o.

Why 50o? Well, if it's 45o, you can get all reflections onto the Ewald
sphere - just. 50o gives you a bit more leeway.

To answer Pierre's question, the principal advantage of a kappa block
over a chi circle is that it is easier to avoid mechanical clashes with
the collimator or detector with a kappa block. There are other advantages, 
and also disadvantages as well. I think the two main reasons why many
companies produced diffractometers with chi circles rather than kappa
blocks were (a) to avoid clashes with Enraf-Nonius patents/licences/etc,
and (b) conservatism. KUMA seem to have avoided both of these recently...

And yes, it is also a matter of taste!

Harry 
-- 
Dr Harry Powell, MRC Laboratory of Molecular Biology, MRC Centre, Hills
Road, Cambridge, CB2 2QH