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Topic: X-Ray Crystallography, what's wrong with this procedure?  (Read 1879 times)

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Offline AlexDart10

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X-Ray Crystallography, what's wrong with this procedure?
« on: May 20, 2016, 06:04:21 AM »
A poorly trained crystallographer undertakes the experiment described below. Describe how the research should have been better planned and the experiment better conducted.

A needle-shaped crystal was obtained from a reaction mixture left in a fridge overnight. The reaction was designed to produce a complex of platinum. The crystallographer mounted the crystal on a diffractometer equipped with a copper X-ray tube and determined a unit cell, then collected the data. The low temperature equipment had run out of liquid nitrogen so the experiment was run at ambient temperature. No absorption correction was performed. An analysis of the data revealed a weak dataset, with few observations at high Bragg angle. During the refinement of the structure of the platinum complex, the crystallographer finds that many atoms go non-positive-definite when Uaniso refinement is attempted. A colleague looks over the crystallographer's shoulder at the computer screen and says "we published that one last year!".

So the key points  I pick out from that are
•   Needle shaped crystal- More prone to cracking perhaps?
•   Left in fridge overnight- I think this is okay
•   Copper xray tube- seems reasonable
•   Run at ambient temperature- Bad, use lower temperature
•   No absorption correction-Bad but struggling to elaborate
•   Weak dataset, few observations-‘’’’’’’’’’’’’’’’’’’’’’’’
•   Many non-positive definite when Uaniso refinement attempted-‘’’’’’’’’’’’’’’’’
•   Already published- Check work is new as process very long and time consuming

This was a 25 marker and I'm unsure about most of it.

Thanks in advance

Offline Babcock_Hall

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Re: X-Ray Crystallography, what's wrong with this procedure?
« Reply #1 on: May 20, 2016, 08:48:31 AM »
What about the high Bragg angle issue?

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