Re: ORGLIST: Circular dichroism problem

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From: Jacob Zabicky (zabicky$##$bgumail.bgu.ac.il)
Date: Thu Aug 09 2001 - 05:32:07 EDT


Hello Bob,

As you address the problem in quite general terms, I may suggest you look
up the term "circular dichroism" in one of the latest editions of March's
"Advanced Organic Chemistry." The book carries a good list of references to
monographs on the subject.

As for the organic ligands being small (say CO or THF) or large (your
helical polymer), the size seems to be irrelevant to this problem, as the
electron density in the ligand regions of the crystal is low in either
case, as compared to that of the heavy elements that determined the space
group. As said before, I may be wrong in this appreciation.

All the best,

Jacob

At 14:10 7/8/1, G. Robert Shelton wrote:
>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
>>
>> x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x
>> 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
>> http://profiler.bgu.ac.il/site/main.cfm
>> Tel. 972-8-6461271/6461062/6472754
>> Fax. 972-8-6472969
>> x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x
>>
>>

x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x
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
http://profiler.bgu.ac.il/site/main.cfm
Tel. 972-8-6461271/6461062/6472754
Fax. 972-8-6472969
x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x





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