Archive for the ‘Interesting chemistry’ Category

Sign inversions in optical rotation as a function of wavelength (ORD spectra)

Monday, December 9th, 2019

I have been discussing some historical aspects of the absolute configuration of molecules and how it was connected to their optical rotations. The nomenclature for certain types of molecules such as sugars and less commonly amino acids includes the notation (+) to indicate that the specific optical rotation of the molecule has a positive (rather than a negative) value. What is rarely mentioned is the implicit wavelength at which the rotation is measured. Historically polarimeters operated at the so-called sodium Fraunhofer D-line (588.995nm, hence the name [α]D) and only much more recently at the mercury e-line (546.073nm). The former was used for uncoloured organic molecules, since it was realised early on that colour and optical rotation did not mix well. Here I take a closer look at this aspect by constructing the hypothetical molecule shown below.

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What effect do explicit solvent molecules have on calculated optical rotation: D-(“+”)-Glyceraldehyde.

Saturday, December 7th, 2019

In this series of posts on optical rotations, I firstly noted Kirkwood’s 1937 attempt to correlate the optical rotation of butan-2-ol with its absolute configuration. He had identified as essential knowing the relative orientation (the term conformation was not yet in common use) of the two methyl groups (the modern terms are gauche vs anti) and also that of the hydroxyl group, noting that anisotropy from this group could influence his result (he had assumed it was linear, or axially symmetric). I then looked at D-(+)-glyceraldehyde, noting that this species itself has a strongly negative rotation and that it is the hydrated diol that results in a positive rotation and hence the (+) designation. Here I take another look at this latter system to see what effect adding explicit water molecules to the unhydrated form of glyceraldehyde might have on its computed rotation, on the premise that strong hydrogen bonds can also contribute anisotropy to the system.

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The (+) in D-(+)-glyceraldehyde means it has a positive optical rotation? Wrong!

Friday, December 6th, 2019

Text books often show the following diagram, famously consolidated over many years by Emil Fischer from 1891 onwards. At the top sits D-(+)-glyceraldehyde, to which all the monosaccharides below are connected by painstaking chemical transformations.

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Prediction preceding experiment in chemistry – how unlucky was John Kirkwood?

Saturday, November 30th, 2019

Some areas of science progressed via very famous predictions that were subsequently verified by experiments. Think of Einstein and gravitational waves or of Dirac and the positron. There are fewer well-known examples in chemistry; perhaps Watson and Crick’s prediction of the structure of DNA, albeit based on the interpretation of an existing experimental result. Here I take a look at a what if, that of John Kirkwood’s prediction of the absolute configuration of a small molecule based entirely on matching up the sign of a measured optical rotation with that predicted by (his) theory.

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The Structure of Tetrodotoxin as a free base – with a better solvation model.

Tuesday, November 26th, 2019

In the previous post, I discussed the structure of the free base form of tetrodotoxin, often represented as originally suggested by Woodward[1] below in an ionic form:

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References

  1. R.B. Woodward, "The structure of tetrodotoxin", Pure and Applied Chemistry, vol. 9, pp. 49-74, 1964. http://dx.doi.org/10.1351/pac196409010049

The Structure of Tetrodotoxin as a free base.

Saturday, November 9th, 2019

The notorious neurotoxin Tetrodotoxin is often chemically represented as a zwitterion, shown below as 1. This idea seems to originate from a famous article written in 1964 by the legendary organic chemist, Robert Burns Woodward.[1] This structure has propagated on to Wikipedia and is found in many other sources.
With the elegance and the unique style that is typical Woodward, his article is a tour de force because of the way in which he deploys a large armoury of spectroscopic (X-ray crystal, NMR, IR) as well as physicochemical (pKa) tools to infer this structure; an approach that has been subsequently widely emulated. The article a well worth a read for the elegant logic that slowly builds to a climax on page 73 (sic!) of the article, when he unveils his final structure (XXXVIII, or 38). The lecture(s) from which the article is apparently derived must have been one hell of an occasion.

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References

  1. R.B. Woodward, "The structure of tetrodotoxin", Pure and Applied Chemistry, vol. 9, pp. 49-74, 1964. http://dx.doi.org/10.1351/pac196409010049

Does Kekulene have Kekulé vibrational modes? Yes!

Saturday, October 19th, 2019

Increasingly, individual small molecules are having their structures imaged using STM, including cyclo[18]carbon that I recently discussed. The latest one receiving such treatment is Kekulene.[1]

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References

  1. I. Pozo, Z. Majzik, N. Pavliček, M. Melle-Franco, E. Guitián, D. Peña, L. Gross, and D. Pérez, "Revisiting Kekulene: Synthesis and Single-Molecule Imaging", Journal of the American Chemical Society, vol. 141, pp. 15488-15493, 2019. http://dx.doi.org/10.1021/jacs.9b07926

Bond length alternation (BLA) in large conjugated rings: an (anti-aromatic) update.

Thursday, October 3rd, 2019

In the previous post, I looked at a class of molecule known as hexaphyrins, inspecting bond length alternation (BLA) at the so-called meso position, the carbon atom joining two pyrrole rings. A search of the difference in bond lengths at this position had shown two significant clusters of crystal structures.
Molecules in the bottom left of this diagram shows little or no bond length alternation. The right middle shows another cluster with more extreme (and unequal) bond length alternation. I have selected one molecule from this cluster, EGIJEK and it differs from EGIHUY in having four NH units in the ring, whereas the latter has only two.

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Bond length alternation (BLA) in large aromatic rings: an experimental reality check.

Monday, September 30th, 2019

The theme of the last three posts derives from the recently reported claimed experimental observation of bond length alternation (BLA) in cyclo[18]carbon, a ring of just 18 carbon atoms.[1] Having found that different forms of quantum calculation seem to find this property particularly difficult to agree upon, not only for cyclocarbon but for twisted lemniscular annulenes (which contain CH rather than just C units), I thought it might be time to look at some more experimental data and my chosen system is a class called the hexaphyrins, of which there are a number of experimental crystal structures.

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References

  1. K. Kaiser, L.M. Scriven, F. Schulz, P. Gawel, L. Gross, and H.L. Anderson, "An sp-hybridized molecular carbon allotrope, cyclo[18]carbon", Science, vol. 365, pp. 1299-1301, 2019. http://dx.doi.org/10.1126/science.aay1914

The Kekulé vibration as a function of aromatic ring size. A different perspective using lemniscular rings.

Friday, September 27th, 2019

In the previous posts, I tried to track down the onset of bond length alternation (BLA) as a function of ring size in aromatic cyclocarbons, finding the answer varied dramatically depending on the type of method used to calculate it. So here I change the system to an unusual kind of aromatic ring, the leminiscular or figure-eight annulene series. I explore the Kekulé vibration for such species for which a 4n+2 π electron count means they are cyclically Möbius aromatic.[1]

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References

  1. P.L. Ayers, R.J. Boyd, P. Bultinck, M. Caffarel, R. Carbó-Dorca, M. Causá, J. Cioslowski, J. Contreras-Garcia, D.L. Cooper, P. Coppens, C. Gatti, S. Grabowsky, P. Lazzeretti, P. Macchi, . Martín Pendás, P.L. Popelier, K. Ruedenberg, H. Rzepa, A. Savin, A. Sax, W.E. Schwarz, S. Shahbazian, B. Silvi, M. Solà, and V. Tsirelson, "Six questions on topology in theoretical chemistry", Computational and Theoretical Chemistry, vol. 1053, pp. 2-16, 2015. http://dx.doi.org/10.1016/j.comptc.2014.09.028