On Thursday 26 June 2008 09:36:16 am Serge Cohen wrote:
> Please some one tells me if I'm wrong ... but I though that indeed one
> is NOT supposed to measure anomalous difference from reflections h and
> h' if those are related by one of the symmetry operator of the point
> group...
This statement is logically equivalent to what Patrick writes below.
You are agreeing with each other.
> That is in monoclinic (P 1 2 1, more precisely) , (h, k, l) and (-h,
> k, -l) should have the same F ... (in a determinist's world)
Yes, but that is not an example of h and h'.
> Though (h, k, l) and (-h, -k, -l) are likely to be different, and
> hence (h, k, l) and (h, -k, l) would show the same difference.
>
> Serge.
>
>
> Le 26 juin 08 à 18:07, Patrick Loll a écrit :
>
> > I've always thought that a Bijvoet pair is any pair for which an
> > anomalous difference could be observed. This includes Friedel pairs
> > (h & h-bar), but it also includes pairs of the form h & h', where h'
> > is symmetry-related to h-bar. Thus Friedel pairs are a subset of all
> > possible Bijvoet pairs.
> >
> > This is what Ed and I say in our book, at least (shameless plug);
> > and you can buy it from Amazon, so it must be right, yes?
> >
> > Pat
> >
> > On 26 Jun 2008, at 11:55 AM, Bernhard Rupp wrote:
> >> Dear All,
> >>
> >> I wonder about the conventions using Friedel vs Bijvoet pair.
> >>
> >> a) there are no differences. As long as h = -h, it's a Friedel
> >> or a Bijvoet pair. They are the same.
> >>
> >> b) A Friedel pair is any reflection h = -h including hR = -h, i.e.
> >> including centric reflections.
> >> A Bijvoet pair is an acentric Friedel pair, it can carry
> >> anomalous amplitude differences, whereas centric Friedel
> >> pairs invariably cannot. Actually, Bijvoet pairs (acentric
> >> Friedel pairs) invariably do carry anomalous amplitude
> >> differences.
> >> There is no such thing as no anomalous scattering.
> >> We may elect to ignore it, only.
> >>
> >> c) of course, this all assumes absence of anisotropic AS.
> >>
> >> def b) seems to be helpful in discussions and make sense given that
> >> absolute
> >>
> >> configuration that needs AS signal is somehow associated with
> >> Bijvoet's
> >> work.
> >>
> >> Are any authoritative answers/conventions/opinions available on
> >> that ?
> >>
> >> Thx, BR
> >>
> >>
>
>
> *******************************************************************
> Dr. Serge COHEN
> GPG Key ID: 0B5CDAEC
>
> N.K.I.
> Department of Molecular Carcinogenesis (B8)
> Plesmanlaan 121
> 1066 CX Amsterdam; NL
>
> E-Mail: [log in to unmask]
> Tel : +31 20 512 2053
> *******************************************************************
>
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>
>
>
--
Ethan A Merritt
Biomolecular Structure Center
University of Washington, Seattle 98195-7742
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