Re: ORGLIST: Circular dichroism problem

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From: G. Robert Shelton (rshelton$##$chem.ufl.edu)
Date: Tue Aug 07 2001 - 14:10:24 EDT


Jacob,

In your letter you address problems that might have arrised from the
structure determination. If for instance you have a large metal with a
couple of small organic ligands, such as tungsten with a couple CO
ligands, or 12 or 13 heavy metals with a THF or two around the complex,
then yes your point is valid. But I would hope that whoever determined
the structure would have taken such things into account when they reported
the P21/C.

The question that remains is the CD experiment. My limited knowledge of CD
tells me that racemic compounds absorb similarly, but a very slight
difference does exist. Its this difference in ciruclar polarized light
that chemists use to determine the right or left hand of the compound. SO
knowing this both would show CD activity. But I am not familiar with CD
outside of carpenters' (or was it borden's??) work with it on the
pyrolysis of cyclopropane questions.

Either way what can the group tell me about CD to help answer this
question??

-Bob


G. Robert Shelton
Graduate Student
Department of Chemistry
University of Florida

Graduate Assistant
The Center for Xray Crystallography
Department of Chemistry
University of Floida

"While stuck on this planet, take nothing but photographs, leave nothing
but footprints, kill nothing but time."

"The world isn't run by weapons anymore, or energy, or money. It's run by
little ones and zeroes, little bits of data. It's all just electrons."


On Tue, 7 Aug 2001, Jacob Zabicky wrote:

> Hello Ashis
>
> This is only tentative, and I have to confirm it with more knowledgable
> colleagues who are vacationing at present. It is possible to imagine a
> crystalline complex where heavy element ions form a monoclinic lattice of
> the type you mention, while the organic ligands occupy more general
> positions in the crystal. Due to the lower electron density in the organic
> regions, it is probably the heavy atoms that effectively determine the
> space group of the crystal, despite the lack of strict symmetry of the
> organic part.
>
> All the best,
>
> Jacob
>
> At 14:15 6/8/1, J.Aires de Sousa wrote:
> >-------- Original Message --------
> >From: A De <ocad$##$mahendra.iacs.res.in>
> >To: "Dr J.a.deSousa" <jas$##$mail.fct.unl.pt>
> >
> >
> >
> >Can any one suggest?
> >
> >Recently, I synthesized a water soluble
> >polymeric coordination compound. X-ray
> >crystallography has established this molecule
> >to have a single stranded helical structure
> >with centrosymmetric space group, P21/c
> >(No. 14), Z = 4. In solution (water or
> >methanol as solvent) the same compound
> >shows circular dichroism (CD active).
> > My question is whether a molecule
> >with centrosymmetric space group in the
> >crystalline state can be CD active in solution?
> >It is expected that a centrosymmetric crystal
> >will produce the right-handed and left-handed
> >helicates in identical number and the solution
> >of such crystal will be a recimic mixture.
> > Thank you for your atention in advance.
> >
> > Sincerely,
> > Ashis De
>
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> Prof. Jacob Zabicky
> Institutes for Applied Research
> Ben-Gurion University of the Negev Private: POB 12366, Beer-Sheva 84863
> POB 653, Beer-Sheva 84105, ISRAEL Tel. 972-8-6496792
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