The left-out spots would be the diffuse spots, which I assume were not
indexed/integrated. The
sharp spots were presumably used to solve the structure.
JPK
==============Original message text===============
On Mon, 27 Aug 2007 11:36:08 am CDT Raji Edayathumangalam wrote:
Very dumb question perhaps:
If there were two interpenetrating lattices of slightly different cell
dimensions, would we not
expect that the indexing program would leave out a lot of the spots as
"unpredicted" or "uncovered"?
Could someone clarify with respect to the diffraction pattern that has just
been posted (diff45..png)?
Raji
---------Included Message----------
>Some small molecule crystallographers have specialized in solving and
>refining structures that, exactly as you describe it, consist of two (or
>more) interpenetrating, non-commensurable lattices. The usual approach is
>to decribe the crystal in up to six dimensional space. The programs SAINT
>and EVALCCD are able to integrate such diffraction patterns and
>SADABS is able to scale them. However the case in point is probably
>commensurate.
>
>George
>
>Prof. George M. Sheldrick FRS
>Dept. Structural Chemistry,
>University of Goettingen,
>Tammannstr. 4,
>D37077 Goettingen, Germany
>Tel. +49-551-39-3021 or -3068
>Fax. +49-551-39-2582
>
>
>On Mon, 27 Aug 2007, Jacob Keller wrote:
>
>> What a beautiful and interesting diffraction pattern!
>>
>> To me, it seems that there is a blurred set of spots with different cell
>> dimensions, although
>> nearly the same, underlying the ordered diffraction pattern. A possible
>> interpretation occurred to
>> me, that the ordered part of the crystal is supported by a less-ordered
>> lattice of slightly
>> different dimensions, which, because the crystal is a like a layer-cake of
>> 2-d crystals, need not
>> be commensurable in the short range with the ordered lattice. The
>> nicely-ordered "cake" part of the
>> crystal you solved, but the "frosting" between is of a different, less
>> ordered nature, giving rise
>> to the diffuse pattern which has slightly different lattice spacing. I would
>> have to see more
>> images to know whether this apparent lattice-spacing phenomenon is
>> consistent, but it at least
>> seems that way to me from the images you put on the web. I would shudder to
>> think of indexing it,
>> however.
>>
>> All the best,
>>
>> Jacob Keller
>>
>> ps I wonder whether a crystal was ever solved which had two
>> interpenetrating, non-commensurable
>> lattices in it. That would be pretty fantastic.
>
>
>Jacob,
>
>Some small molecule crystallographers have specialized in solving and
>refining structures that, exactly as you describe it, consist of two
>interpenetrating, non-commensurate lattices. The usual approach is
>to index the diffraction pattern in multiple dimensional space
>('superspace'). The programs SAINT and EVALCCD are able to integrate
>diffraction patterns in up to six dimensions, SADABS is able to scale
>them and the refinement is almost always performed with Petricek's
>program JANA2000:
>
>http://www-xray.fzu.cz/jana/Jana2000/jana.html >
>However the case in point is probably commensurate.
>
>George
>
>Prof. George M. Sheldrick FRS
>Dept. Structural Chemistry,
>University of Goettingen,
>Tammannstr. 4,
>D37077 Goettingen, Germany
>Tel. +49-551-39-3021 or -3068
>Fax. +49-551-39-2582
>
>
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***********************************
Jacob Keller
Northwestern University
6541 N. Francisco #3
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