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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...
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)
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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