Archive for the ‘crystal_structure_mining’ Category

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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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

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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CH⋅⋅⋅π Interactions between methyl and carbonyl groups in proteins: a small molecule check.

Monday, May 29th, 2017
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Derek Lowe highlights a recent article[1] postulating CH⋅⋅⋅π interactions in proteins. Here I report a quick check using the small molecule crystal structure database (CSD).

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References

  1. F.A. Perras, D. Marion, J. Boisbouvier, D.L. Bryce, and M.J. Plevin, "Observation of CH⋅⋅⋅π Interactions between Methyl and Carbonyl Groups in Proteins", Angewandte Chemie International Edition, vol. 56, pp. 7564-7567, 2017. http://dx.doi.org/10.1002/anie.201702626

How does carbon dioxide coordinate to a metal?

Saturday, May 6th, 2017
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Mention carbon dioxide (CO2) to most chemists and its properties as a metal ligand are not the first aspect that springs to mind. Here thought I might take a look at how it might act as such.

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The challenges in curating research data: one case study.

Friday, April 28th, 2017
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Research data (and its management) is rapidly emerging as a focal point for the development of research dissemination practices. An important aspect of ensuring that such data remains fit for purpose is identifying what curation activities need to be associated with it. Here I revisit one particular case study associated with the molecular structure of a product identified from a photolysis reaction[1] and the curation of the crystallographic data associated with this study.

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References

  1. Y. Legrand, A. van der Lee, and M. Barboiu, "Single-Crystal X-ray Structure of 1,3-Dimethylcyclobutadiene by Confinement in a Crystalline Matrix", Science, vol. 329, pp. 299-302, 2010. http://dx.doi.org/10.1126/science.1188002

π-Facial hydrogen bonds to alkynes (revisited): how close can an acidic hydrogen approach?

Monday, April 17th, 2017
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Following on from my re-investigation of close hydrogen bonding contacts to the π-face of alkenes, here now is an updated scan for H-bonds to alkynes. The search query (dataDOI: 10.14469/hpc/2478) is similar to the previous one:

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π-Facial hydrogen bonds to alkenes (revisited): how close can an acidic hydrogen approach?

Saturday, April 15th, 2017
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Back in the early 1990s, we first discovered the delights of searching crystal structures for unusual bonding features.[1] One of the first cases was a search for hydrogen bonds formed to the π-faces of alkenes and alkynes. In those days the CSD database of crystal structures was a lot smaller (<80,000 structures; it’s now ten times larger) and the search software less powerful. So here is an update. 

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References

  1. H.S. Rzepa, M.H. Smith, and M.L. Webb, "A crystallographic AM1 and PM3 SCF-MO investigation of strong OH ⋯π-alkene and alkyne hydrogen bonding interactions", J. Chem. Soc., Perkin Trans. 2, pp. 703-707, 1994. http://dx.doi.org/10.1039/P29940000703

The π-π stacking of aromatic rings: what is their closest parallel approach?

Thursday, April 13th, 2017
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Layer stacking in structures such as graphite is well-studied. The separation between the π-π planes is ~3.35Å, which is close to twice the estimated van der Waals (vdW) radius of carbon (1.7Å). But how much closer could such layers get, given that many other types of relatively weak interaction such as hydrogen bonding can contract the vdW distance sum by up to ~0.8Å or even more? This question was prompted by the separation calculated for the ion-pair cyclopropenium cyclopentadienide (~2.6-2.8Å).

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The conformation of carboxylic acids revealed.

Tuesday, April 11th, 2017
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Following my conformational exploration of enols, here is one about a much more common molecule, a carboxylic acid.

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