Transcript of an interview with
Prof. Leslie Leiserowitz (Weizmann
Institute of Science)
at the Stockholm Town Hall
Saturday, September 14, 2002
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J.M.McBride :
Well, Leslie, as we sit here in front of the
Stockholm Town Hall, the site of the Nobel Prizes, I think about your
work on the tetraphenylbutatriene, which I talk about each year in my
course. We just talked about it last
Friday.
I’d like you to say a few words about
how you came to study that compound and what you were looking for.
Well, I’m not sure whether I recollect
it all exactly, but at that time Meir [1]
was very interested in solid state reactions involving C-C double
bonds, and we at the time, on the other side,[2]
were also interested in electron densities, but also electron
densities from the point of view that we wanted to see lone pair
electrons, etc.
Now, Phil Coppens [3]
some years prior to that had studied the triazine and he had been
able by low-temperature x-ray diffraction to establish the lone-pair
density of the nitrogen atom.
So we thought perhaps in order to convince
the stereochemists and others that people measuring deformation
electron density do yield meaningful results, it would be nice to
show the electron density of neighboring C-C double bonds in the
diene or triene, because according to theory, the electron pi lobes
should be perpendicular to one another.
Was this a sophisticated theory at the time,
or just sort of a qualitative theory that suggested that that the
double bonds should be perpendicular when they are successive to one
another?
LL:
Well, we thought it was a well-established theory, because it is
certainly known that in dienes the planes of the systems are
perpendicular, you know. You have the carbon atoms
just apart and you can see that the system is perpendicular to one
another. And so it was well
established as far as we were concerned.
So we went ahead with the low-temperature
study on tetraphenylbutatriene.
But I think I should say one more thing. We had an internal
check.[4] There were several aromatic
bonds which were chemically identical but crystallographically
different. So we had an internal check as to how good the data was.
So actually the purpose of this experiment
was to confirm theory, but also to establish the technique and its
ability to show what bonds really were.
Yes, that’s exactly correct. That’s
quite so. It was also to establish a technique and to show other
people that it was a technique that was worthwhile applying.
But moreover we had other ideas then. It was in 1971. I had come here in fact, to
Uppsala, to do an experiment to prove the existence of a C-H with
triple bond interaction. We did that with propiolic acid - the cis
structure.
We wanted to get also hold of the electron
density distributions of the C-C single bond, the C-C double bond,
C-C triple bond. And in
butatriene, as I recall, the central bond does have some triple bond
character, and so we tried to kill two birds with one stone
Great.
Thank you very much.
So you see, Stockholm is a beautiful place,
but we’re still working. Say
hello to the students in Chemistry 125.
Hi folks. I hope it wasn’t as clear as mud, but it’s for you to decide. Thank you very much.
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[1] Prof. Meir Lahav, a physical-organic chemist, is a long term collaborator of Prof. Leiserowitz, with whom he shared the Aminoff Prize - which was the occasion of our being together in Stockholm. Lahav had been particularly interested in reactions where two double bonds, side by side, react to form a four-membered "cyclobutane" ring. Reaction between the central bonds of two butatriene molecules would have given a cyclobutane in which each carbon was also a member of a double bond directed outward from the ring. The reaction did not work.
[2] By "on the other side" Leiserowitz is indicating that his main research focus has been on determining structure by x-ray diffraction, not studying organic solid-state reactions for their own sake. A special strength of the Lahav/Leiserowitz collaboration has been the way their individual strengths complement one another.
[3] Prof. Philip Coppens of the University at Buffalo, was a graduate student together with Leiserowitz under Professor Gerhardt Schmidt at the Weizmann Institute. He has carried out a number of important high-precision studies of electron density in crystals.
[4] It was important that any differences in the shape of the bond cross section should be clearly real and not due to an error in the measurements. How similar the cross sections were in four independent phenyl rings would help evaluate how large experimental error in the experiment might be.
