The question was: which is F+ and which F-.
I think the answer is that the hkl are in a right-handed axis system and
-h-k-l are in a left-handed one. So it's the guy who first decides which
are a,b,and c. I think his name is denzo.
BS
On Fri, 27 Jun 2008, James Holton wrote:
Ahh. The history of science. I've always wondered how these naming
conventions get decided. Who is the authority on what gets named after who?
Historically, it seems to vary a lot.
- When Patterson published his incredibly useful map he called it the
"F-square synthesis". Does anyone NOT call it a Patterson map?
- When Rontgen discovered a new kind of light, he called it "x-rays". Now
only the Germans call them Rontgen rays.
- When the largest protein ever was discovered, it was called "connectin",
but a subsequent paper called it "titin" and the second name has stuck. I
actually can't remember who the "connectin" guy was ...
- When Joseph Fourier discovered that heat radiated from the earth could be
reflected back by gasses in the atmosphere, he simply named it by describing
it (in French). Now this is (incorrectly) called the "greenhouse effect".
Why not the Fourier effect? Fortunately for Fourier, a mathematical series
was named after him, although he neither discovered it (Budan did that), nor
implemented it (Navier did that). All Fourier did was present a theorem
based on a flawed premise that turned out to be right anyway.
So, I decided to look up Friedel and Bijvoet in the Undisputed Source of All
Human Knowledge (wikipedia) and found that Friedel's Law "... is a property
of Fourier transforms <http://en.wikipedia.org/wiki/Fourier_transform> of
real functions."
I am willing to believe that. And considering this origin I would think it
appropriate to call (hkl) and (-h-k-l) a "Friedel pair" (or "Friedel's pair"
as it is described in the USAHK). G. Friedel was indeed a crystallographer,
but I doubt he considered more than this simple centrosymmetric property.
Who would care in 1913 which is F+ and F-? The atomic scattering factors had
not yet been worked out at that time. Ewald may have predicted it, but
anomalous scattering was not shown to exist until the classic work of Koster,
Knol and Prins (1930). I guess that goes to show that if you want something
named after you... keep it at one or two authors.
Perhaps it has to do with the original paper getting old enough that it gets
too hard to find. I'm sure in G. Friedel's paper in 1913 he cited Joseph
Fourier's Paper from 1822. Or did he? I wonder if they were already calling
it a Fourier Transform at that time?
Okay, so what, exactly did Bijvoet do? Everyone cites his Nature paper
(1951), but one thing that I was NOT KIDDING about in my April Fool's joke
was that this paper (like so many other high-profile papers) contains almost
no information about how to reproduce the results. I was also not kidding
that boring little details like the reasoning behind the conclusion (the hand
of the microworld) were relegated to a more obscure journal (the one in the
Proc. Royal. Soc. Amsterdam). I WAS kidding about having found and read that
paper. I have never seen it. Still, Bijvoet did the first experiment to
elucidate the absolute configuration, and he definitely deserves credit for
that.
So, particularly in that light, I would agree that any pair of reflections
that would be equivalent if not for anomalous scattering effects could be
called a "Bijvoet pair". This is because they contain the information needed
to apply Bijvoet's technique.
Something that has always eluded me is who decided which is F+ and F-? After
all, the reciprocal lattice is very very nearly centrosymmetric. You cannot
tell by looking at a single diffraction image whether that spot at a given
X,Y pixel coordinate is F+ or F-, you need to know the axis convention of the
camera. At some point in writing the CCP4 libraries with their asymmetric
unit definitions, someone must have established a convention. What is it?
To me, the reasoning behind these assignments is, in fact, they key to
assigning the absolute configuration, not the anomalous scattering effect
itself. So, who worked this out? Should we really be calling them Dodson
pairs?
-James Holton
MAD Scientist
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