Archive for June, 2017

Dispersion “bonds” not involving hydrogen. A Cl…Cl candidate?

Thursday, June 29th, 2017
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In the previous post, I noted the crystallographic detection of an unusually short non-bonded H…H contact of ~1.5Å, some 0.9Å shorter than twice the van der Waals radius of hydrogen (1.2Å, although some sources quote 1.1Å which would make the contraction ~0.7Å). This was attributed to dispersion attractions accumulating in the rest of the molecule. I asked myself what the potential might be for other elements to reveal significantly contracted non-bonded distances as a result of dispersive attractions.

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Dispersion “bonds”: a new example with an ultra-short H…H distance.

Monday, June 26th, 2017
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About 18 months ago, there was much discussion on this blog about a system reported by Bob Pascal and co-workers containing a short H…H contact of ~1.5Å[1]. In this system, the hydrogens were both attached to Si as Si-H…H-Si and compressed together by rings. Now a new report[2] and commented upon by Steve Bachrach, claims a similar distance for hydrogens attached to carbon, i.e. C-H…H-C, but without the ring compression.

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References

  1. J. Zong, J.T. Mague, and R.A. Pascal, "Exceptional Steric Congestion in anin,in-Bis(hydrosilane)", Journal of the American Chemical Society, vol. 135, pp. 13235-13237, 2013. http://dx.doi.org/10.1021/ja407398w
  2. S. Rösel, H. Quanz, C. Logemann, J. Becker, E. Mossou, L. Cañadillas-Delgado, E. Caldeweyher, S. Grimme, and P.R. Schreiner, "London Dispersion Enables the Shortest Intermolecular Hydrocarbon H···H Contact", Journal of the American Chemical Society, vol. 139, pp. 7428-7431, 2017. http://dx.doi.org/10.1021/jacs.7b01879

Chemistry rich diagrams: do crystal structures carry spin information? Iron-di-imine complexes.

Sunday, June 18th, 2017
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The iron complex shown below forms the basis for many catalysts.[1] With iron, the catalytic behaviour very much depends on the spin-state of the molecule, which for the below can be either high (hextet) or medium (quartet) spin, with a possibility also of a low spin (doublet) state. Here I explore whether structural information in crystal structures can reflect such spin states.

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References

  1. M.P. Shaver, L.E.N. Allan, H.S. Rzepa, and V.C. Gibson, "Correlation of Metal Spin State with Catalytic Reactivity: Polymerizations Mediated by α-Diimine–Iron Complexes", Angewandte Chemie International Edition, vol. 45, pp. 1241-1244, 2006. http://dx.doi.org/10.1002/anie.200502985

Twenty one years of chemistry-related Java apps: RIP Java?

Saturday, June 10th, 2017
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In an earlier post, I lamented the modern difficulties in running old instances of Jmol, an example of an application program written in the Java programming language. When I wrote that, I had quite forgotten a treasure trove of links to old Java that I had collected in 1996-7 and then abandoned. Here I browse through a few of the things I found.

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How to search data repositories for FAIR chemical content and data: SubjectScheme

Thursday, June 8th, 2017
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As data repositories start to flourish, it is reasonable to ask questions such as what sort of chemistry can be found there and how can I find it? Here I give an updated[1] worked example of a digital repository search for chemical content and also pose an important issue for the chemistry domain.

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References

  1. H.S. Rzepa, A. Mclean, and M.J. Harvey, "InChI As a Research Data Management Tool", Chemistry International, vol. 38, 2016. http://dx.doi.org/10.1515/ci-2016-3-408

Research data: Gravitational waves as an example from the astrophysics community.

Friday, June 2nd, 2017
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In 2016, the world heard that gravitational waves had been detected and now a third instance is reported. Given that the data associated with these detections are perhaps amongst the most important instances in recent times, I thought I might take a peek at how it was managed.

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Tautomeric polymorphism.

Thursday, June 1st, 2017
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Conformational polymorphism occurs when a compound crystallises in two polymorphs differing only in the relative orientations of flexible groups (e.g. Ritonavir). At the Beilstein conference, Ian Bruno mentioned another type;  tautomeric polymorphism, where a compound can crystallise in two forms differing in the position of acidic protons. Here I explore three such examples.

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