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Re: [sdpd] Re: Are restraints as good as observations ?
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<p>>These numbers suggest there is still room for improvement. Without <br>
>looking at the structural connectivity: are the differences between C-OÂ <br>
>bonds significant?<br>
>What about the inter-molecular distances holding the structure together? <br>
>How do the esd's on those change when restraints are added?<br>
<br>
The room for improvement always exists and these unrestrained values allow estimating the necessary degree of the improvement. Since the uncertainties here are REAL, one may definitely say that the differences between C-O bonds are insignificant. After imposing restraints one can say nothing definite as the restrained distances and uncertainties are ARTIFICIAL.<br>
<br>
>Restraints allow you to get the (unknown and much harder to guess)<br>
>intermolecular bonds far more accurately from your limited data.<br>
<br>
Hypothetically, yes, but for this I prefer rigid constraints as they limit the room for speculations.<br>
<br>
>To see why restraints are useful (if someone genuinely doesn't get it <br>
>and it not just trolling) try fitting a low symmetry zeolite structure <br>
>with and without the Si-O distances and tetrahedral angles and look at <br>
>the resulting pore geometries.<br>
<br>
The pore geometry is a complex subject to look at. In my recent (April 1) Rietveld-list message I showed an example of an ALPO zeolite for which I got NARROWER distributions of T-O distances and O-T-O angles from an unrestrained DDM refinement compared to the original Rietveld refinement with restrained geometry. It looked like a joke, but the results are absurdly real. The CIFs, data and ddm-file are here:<br>
<a href="http://sites.google.com/site/ddmsuite/EMM-3-DDM.zip";>http://sites.google.com/site/ddmsuite/EMM-3-DDM.zip</a><br>
The unrestrained refinement was done on the same dataset from the supplementary materials of Chem. Mater. (2006) 1697, applying anisotropic broadening, preferred orientation, other corrections and DDM. This is just a single example demonstrating that the powder data capacity is strongly underestimated, while the necessity of restraints is inadequately overestimated.<br>
 <br>
<br>
*******************************************************<br>
Leonid A. Solovyov<br>
Institute of Chemistry and Chemical Technology<br>
660049, K. Marx 42, Krasnoyarsk, Russia<br>
<a href="http://sites.google.com/site/solovyovleonid";>http://sites.google.com/site/solovyovleonid</a><br>
*******************************************************<br>
<br>
________________________________<br>
From: Jon Wright <<a href="mailto:wright%40esrf.fr";>wright...@esrf.fr</a>><br>
To: <a href="mailto:sdpd%40yahoogroups.com";>sdpd...@yahoogroups.com</a> <br>
Cc: L_Solovyov <<a href="mailto:l_solovyov%40yahoo.com";>l_solovyov...@yahoo.com</a>>; <a href="mailto:rietveld_l%40ill.fr";>rietveld_l...@ill.fr</a> <br>
Sent: Tuesday, July 30, 2013 12:59 PM<br>
Subject: Re: [sdpd] Re: Are restraints as good as observations ?<br>
<br>
<br>
On 30/07/2013 07:06, L_Solovyov wrote:<br>
> C2-O1 1.231(39)<br>
> C2-O2 1.296(45)<br>
> C4-O3 1.284(51)<br>
> C4-O4 1.277(42)<br>
> What's the necessity for restraints in this instance ? I ccan't<br>
> understand. If you see it, please, let me know ? I'm curioous.<br>
<br>
I make that:<br>
<br>
C2-O1 1.23(4)<br>
C2-O2 1.30(5)<br>
C4-O3 1.28(5)<br>
C4-O4 1.28(5)<br>
<br>
These numbers suggest there is still room for improvement. Without <br>
looking at the structural connectivity: are the differences between C-O <br>
bonds significant?<br>
<br>
What about the inter-molecular distances holding the structure together? <br>
How do the esd's on those change when restraints are added?<br>
<br>
By adding restraints you admit you already know those distances from a <br>
quick look at the connectivity and an undergraduate course in chemistry. <br>
If you didn't do that course or follow the last ~100 years of <br>
crystallography then that could seem a bit overwhelming. Restraints <br>
allow you to get the (unknown and much harder to guess) intermolecular <br>
bonds far more accurately from your limited data.<br>
<br>
To see why restraints are useful (if someone genuinely doesn't get it <br>
and it not just trolling) try fitting a low symmetry zeolite structure <br>
with and without the Si-O distances and tetrahedral angles and look at <br>
the resulting pore geometries.<br>
<br>
The fact that some theoretician is able to compute the intramolecular <br>
bond lengths does not make them interesting experimental data from <br>
powders. Send that person off to look at data from a high quality charge <br>
density study!<br>
<br>
Cheers,<br>
<br>
Jon<br>
<br>
PS: Original message below, added a cc to the Rietveld list to see if it <br>
works for me.<br>
<br>
[Non-text portions of this message have been removed]<br>
<br>
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