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: [EMAIL PROTECTED] > Tel : +31 20 512 2053 > ******************************************************************* > > > > > -- Ethan A Merritt Biomolecular Structure Center University of Washington, Seattle 98195-7742
