January 23rd, 2010
So ingrained is the habit to think of a bond as a simple straight line connecting two atoms, that we rarely ask ourselves if they are bent, and if so, by how much (and indeed, does it matter?). Well Hursthouse, Malik, and Sales, as long ago as 1978, asked just such a question about the unlikeliest of bonds, a quadruple Cr-Cr bond, found in the compound di-μ-trimethylsilylmethyl-bis-[(tri-methylphosphine) (trimethylsilylmethyI)chromium(II)(DOI: 10.1039/dt9780001314[1]). They arrived at this conclusion by looking very carefully at how the overlaps with the Cr d-orbitals might be achieved.
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References
- M.B. Hursthouse, K.M.A. Malik, and K.D. Sales, "Crystal and molecular structure of di-µ-trimethylsilylmethyl-bis[(tri-methylphosphine)(trimethylsilylmethyl)chromium(<scp>II</scp>)](4 Cr–Cr)", J. Chem. Soc., Dalton Trans., pp. 1314-1318, 1978. https://doi.org/10.1039/dt9780001314
Tags: bond, Hypervalency, Interesting chemistry, quadruple
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January 17th, 2010
A Semantic blog is one in which the system at least in part understands about (some of the) concepts and topics that are in the content. The idea is that this content can be more intelligently (is that the correct word?) and importantly, automatically searched, harvested, and connected to the same or similar concepts found elsewhere in other blogs and the Web as whole. I am writing this blog using Firefox, having added a Firefox extension called Zemanta. As I write, the system offers suggestions for similar themes elsewhere that I could choose to link to the blog (and obviously the more one writes, or the more specific the terms one uses, the more sensible the suggestions become. At this precise moment, it is still offering fairly generic suggestions, one of which I have just chosen to add). My purpose in this particular post is to explore how the very process of writing a blog might be affected by such a product. I am also inferring (but cannot add detail at the moment) that all the (semantic) connections or links to other materials will be expressed in this blog using some form of formal declaration, such as e.g. RDF or RDFa.
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Tags: Chemical IT, RDF, semantic
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January 11th, 2010
The scheme below illustrates one of the iconic reactions in organic chemistry. It is a modern representation of Meerwein’s famous experiment from which he inferred a carbocation intermediate, deduced from studying the rate of enantiomerization of isobornyl chloride when treated with the Lewis acid SnCl4.
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Tags: animation, catalytic systems, Chemical intimacy, energy, free energy, gas phase model, Interesting chemistry, Meerwein, solvation free energy terms, watoc11
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January 1st, 2010
In the previous post, the molecule F3S-C≡SF3 was found to exhibit a valence bond isomerism, one of the S-C bonds being single, the other triple, and with a large barrier (~31 kcal/mol, ν 284i cm-1) to interconversion of the two valence-bond forms. So an interesting extension of this phenomenon is shown below:
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Tags: aromaticity, Hypervalency, Interesting chemistry, pericyclic, South Carolina
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December 30th, 2009
A previous post posed the question; during the transformation of one molecule to another, what is the maximum number of electron pairs that can simultaneously move either to or from any one atom-pair bond as part of the reaction? A rather artificial example (atom-swapping between three nitrosonium cations) was used to illustrate the concept, in which three electron pairs would all move from a triple bond to a region not previously containing any electrons to form new triple bonds and destroy the old. Here is a slightly more realistic example of the phenomenon, illustrated by the (narcisistic) reaction below of a bis(sulfur trifluoride) carbene. Close relatives of this molecule are actually known, with either one SF3 of the units replaced by a CF3 group or a SF5 replacing the SF3 (DOI: 10.1021/ja00290a038 ).
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Tags: animation, calculated free energy barrier, Hypervalency, inorganic chemist, Interesting chemistry
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December 9th, 2009
Clar islands are found not so much in an ocean, but in a type of molecule known as polycyclic aromatic hydrocarbons (PAH). One member of this class, graphene, is attracting a lot of attention recently as a potential material for use in computer chips. Clar coined the term in 1972 to explain the properties of PAHs, and the background is covered in a recent article by Fowler and co-workers (DOI: 10.1039/b604769f). The concept is illustrated by the following hydrocarbon:
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Tags: aromaticity, Clar, Clar islands, Cloud Clar islands, computer chips, ELF, Henry Armstrong, Interesting chemistry, non-planar systems, pence, Santos
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December 6th, 2009
In the previous two posts, a strategy for tuning the nature of the CS bond in the molecule HO-S≡C-H was developed, based largely on the lone pair of electrons identified on the carbon atom. By replacing the HO group by one with greater σ-electron withdrawing propensity, the stereo-electronic effect between the O-S bond and the carbon lone pair was enhanced, and in the process, the SC bond was strengthened. It is time to do a control experiment in the other direction. Now, the HO-S group is replaced by a H2B-S group. The B-S bond, boron being very much less electronegative than oxygen, should be a very poor σ-acceptor. In addition, whereas oxygen was a π-electron donor (acting to strengthen the S=C region), boron is a π-acceptor, and will also act in the opposite direction. So now, this group should serve to weaken the S-C bond.
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Tags: Hypervalency, Interesting chemistry
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December 5th, 2009
In my first post on this theme, an ELF (Electron localization function) analysis of the bonding in the molecule HO-S≡C-H (DOI: 10.1002/anie.200903969) was presented. This analysis identified a lone pair of electrons localized on the carbon (integrating in fact to almost exactly 2.0) in addition to electrons in the CC region. This picture seems to indicate that the triple bond splits into two well defined regions of electron density (synaptic basins). In a comment to this post, I also pointed out that an NBO analysis showed a large interaction energy between the carbon lone pair and the S-O σ* orbital, characteristic of anomeric effects (in eg sugars). This latter observation gives us a handle on possibly tweaking the effect. Thus if the S-O σ* orbital can be made a better electron acceptor, then its interaction with the lone pair could be enhanced.
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Tags: Hypervalency, Interesting chemistry, large interaction energy
Posted in Hypervalency, Interesting chemistry | 3 Comments »