[Ifeffit] bond distance resolution and correlation of parameters in MS analyses?

Matt Newville newville at cars.uchicago.edu
Wed Jun 6 18:38:08 CDT 2007


HI Mark,

> Any thoughts and opinions would be greatly appreciated as this relates directly to corrections suggested to be made to my PhD thesis!

This is going to re-iterate most of what Bruce said, but since you asked:

> 1)Does the equation for bond distance resolution (r = pi/2deltak) only apply to SS?

No.  It applies to all EXAFS.

> I have held the opinion that this can be applied to MS analysis

You have been right.

> however I have recently been informed that this equation does not correctly describe distance > resolution in MS analyses. The paper in Coord. Chem. Rev. 2005, 249, 141-160 describes this > and is this concordant with the views of the wider EXAFS community?

You were mis-informed.  This paper is profoundly wrong, and is not
concordant with the views of the wider EXAFS community, as defined by
the standards and criteria documents at
     http://www.i-x-s.org/OLD/subcommittee_reports/sc/err-rep.pdf

This paper states that the number of independent points in an XAFS data set is:
    N_i = [ 2*(rmax - rmin) * (kmax -kmin) / pi ]   + Sum  D*(N-2)   + 1

Here rmin,rmax,kmin, and kmax are the spectral ranges.  I cannot tell
what the sum is over, but
D is the "dimension with a restrained part of the model (ie, three for
a three-dimensional model)"
and N is "the number of independent atoms within the restrained group
of the model".

The (  Sum D*(N-2) )  term asserts that the number of independent
points in the data is dependent on the model.  This is complete
nonsense.

For what it's worth, the standards and criteria report cited above
recommends using
    N_i  =  [ 2*(rmax - rmin) * (kmax -kmin) / pi ]

rounded to the nearest integer.   The report is has a bit more to say,
but note that it is carefully (and deliberately) silent on "+1", "+2",
etc.  This is because N_i is an estimate of the maximum number of
parameters that can be extracted from a signal.  If you're quibbling
whether to add 1 or 2 to this number,  it probably means you should
really subtract 4.


Now, one may apply a variety of modeling approaches (tricks??
assumptions??) to the analysis of multiple scattering in highly
constrained three-dimensional models that are often associated with
organo-metallics.  For example, a histidine ring attached to a metal
will give multiple scattering, and you can usually assert that the
ring is rigid, though you may need to refine its location and
orientation relative to the metal.   That makes the bond distances and
angles (and MS amplitudes) for all the scattering paths from this ring
all dependent on a reduced number of variables.   It does not add
information to the data.

Cheers,

--Matt



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