[Ifeffit] why ss_2 is negative?

[hw26 at njit.edu]

Dear all,

Thanks for reply promptly! First I have to say sorry since I am a new guy in 
this filed I may ask some stupid questions. But I am really glad there is such 
a great platform where I can get advice and learn from experts and scientists.

Yes, I am modeling the uranyl ligand and two equatorial ligands, one of which 
is hypothesized as U-F (from BF4 anion), and the other as U-O (from 
hydration),because in the uranyl nitrate standard sloution(without MOEMIMBF4), 
there is only one peak that represents the U-O equatorial ligand, but with the 
MOEMIMBF4, the equatorial peak is split to two peaks. So I guess the other 
equatorial U-F formed.
In the literature, the R for mondentate U-F is 2.24, bidentate is about 2.35; 
the R for mondentate U-O is 2.41, bidentate is about 2.54. So I choose 2.24 
for U-F and 2.42 for U-O in the path. In the first run, I only set the number 
of axial oxygen as 2 (n0), and guess all other parameters. After obtaining 
parameters from the first run, I fixed some parameters(especially the 
parameters for axial ligand) , and modify others. However, the sigma^2 is 
negative, and the enot_2 is too large. In addition, the dr_1 and dr_2 also 
seem too large compared with the R from literature. But if I force sigma^2 to 
be positive and e_2 to be below 10 ev, the R-factor will increase and the 
fitting doesn't seem good. 
As for the 2.59 Angstrom for equatorial U-O, it is even a little bit larger 
than the bidentate U-O. I don't think it resonable either,  since there is no 
ligand here that can form the bidentate U-O. Also, the 2.44 Angstrom for U-F 
is quite larger than 2.24 from literature, even larger the 2.35 Angstrom for 
the bidentate. I don't know if this is due to wrong modeling or because of 
some unique characteristcs of ionic liquid. 

Best regards!
Hao

Quoting Bruce Ravel <bravel at anl.gov>:

> On Friday 23 February 2007 16:24, hw26 at njit.edu wrote:
> > I am doing model fitting using Artemis. My sample is uranium solution
> with
> > an ionic liquid,  1-methoxyethyl-3-methyl imidazolium
> tetrafluoroborate
> > (MOEMIMBF4). The Fitting seems ok except that the ss_2 is negative. I
> don't
> > know how can I fix this problem. Can you get a chance to take a look?
> My
> > fitting is simple, only three single scattering pathes which were
> produced
> > by Six Pack. The fitting file is attached. I really appreciate it if
> you
> > could take a look and give some advice!
> 
> Let me ask you a couple questions that I think are closely related to
> yours.  You seem to be modeling an uranyl ligand and two equatorial
> ligands.  Focussing on the second path, the one with a negative
> sigma^2 and the one that (I am guessing) represents either a hydration
> shell or a bidentate ligand:
> 
>   1.  Is 35.996 a reasonable value for e0?
>   2.  Is 2.59 Angstrom a reasonable value for an equatorial oxygen
>       ligand in a uranyl moiety?
> 
> It would seem that your fit has quite a few problems beyond the
> sigma^2 value.  I question whether your parameterization of these
> three paths is physically defensible.
> 
> B
> 
> 
> -- 
>  Bruce Ravel  ----------------------------------------------
> bravel at anl.gov
> 
>  Molecular Environmental Science Group, Building 203, Room E-165
>  MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007
> 
>  Argonne National Laboratory         phone and voice mail: (1) 630 252
> 5033
>  Argonne IL 60439, USA                                fax: (1) 630 252
> 9793
> 
>  My homepage:    http://cars9.uchicago.edu/~ravel 
>  EXAFS software: http://cars9.uchicago.edu/~ravel/software/
> 
> 
> 
> 
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