[Ifeffit] why ss_2 is negative?
hw26 at njit.edu
hw26 at njit.edu
Fri Feb 23 21:41:20 CST 2007
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.
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
> > an ionic liquid, 1-methoxyethyl-3-methyl imidazolium
> > (MOEMIMBF4). The Fitting seems ok except that the ss_2 is negative. I
> > know how can I fix this problem. Can you get a chance to take a look?
> > fitting is simple, only three single scattering pathes which were
> > by Six Pack. The fitting file is attached. I really appreciate it if
> > 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.
> 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
> Argonne IL 60439, USA fax: (1) 630 252
> My homepage: http://cars9.uchicago.edu/~ravel
> EXAFS software: http://cars9.uchicago.edu/~ravel/software/
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