September 12th, 2013
Andy Extance at the Chemistry World blog has picked up on a fascinating article[1] on the dimer of SF2. This molecule has three F atoms on one S, and only one on the other; FSSF3. But all four S-F bonds are of different length. Lindquist and Dunning claim that the minimum energy pathway to dissociation to two SF2 molecules does not involve breaking either the longest or the weakest SF bond. This was too much for me to resist investigating further!
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References
- B.A. Lindquist, and T.H. Dunning, "Bonding in FSSF<sub>3</sub>: Breakdown in Bond Length-Strength Correlations and Implications for SF<sub>2</sub> Dimerization", The Journal of Physical Chemistry Letters, vol. 4, pp. 3139-3143, 2013. https://doi.org/10.1021/jz401578h
Tags: Andy Extance, minimum energy pathway
Posted in Interesting chemistry, reaction mechanism | 6 Comments »
September 4th, 2013
According to Herges[1],[2] the mechanism of single-step (concerted) reactions can be divided into three basic types; linear (e.g. substitution, elimination etc), pericyclic (e.g. Diels Alder) and a third much rarer, and hence very often overlooked type that was named coarctate. This is based on the topology of bond redistribution patterns, an explicit real example[3] illustrating:
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References
- R. Herges, "Coarctate transition states: the discovery of a reaction principle", Journal of Chemical Information and Computer Sciences, vol. 34, pp. 91-102, 1994. https://doi.org/10.1021/ci00017a011
- B.S. Young, R. Herges, and M.M. Haley, "Coarctate cyclization reactions: a primer", Chemical Communications, vol. 48, pp. 9441, 2012. https://doi.org/10.1039/c2cc34026g
- C. Berger, C. Bresler, U. Dilger, D. Geuenich, R. Herges, H. Röttele, and G. Schröder, "A Spontaneous Fragmentation: From the Criegee Zwitterion to Coarctate Möbius Aromaticity", Angewandte Chemie International Edition, vol. 37, pp. 1850-1853, 1998. https://doi.org/10.1002/(sici)1521-3773(19980803)37:13/14<1850::aid-anie1850>3.0.co;2-b
Tags: Ozonolysis
Posted in reaction mechanism | 2 Comments »
August 25th, 2013
The concept of a “hidden intermediate” in a reaction pathway has been promoted by Dieter Cremer[1] and much invoked on this blog. When I used this term in a recent article of ours[2], a referee tried to object, saying it was not in common use in chemistry. The term clearly has an image problem. A colleague recently sent me an article to read (thanks Chris!) about isotope effects in the epoxidation of ethene[3] and there I discovered a nice example of hidden intermediates which I share with you now.
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References
- E. Kraka, and D. Cremer, "Computational Analysis of the Mechanism of Chemical Reactions in Terms of Reaction Phases: Hidden Intermediates and Hidden Transition States", Accounts of Chemical Research, vol. 43, pp. 591-601, 2010. https://doi.org/10.1021/ar900013p
- H.S. Rzepa, and C. Wentrup, "Mechanistic Diversity in Thermal Fragmentation Reactions: A Computational Exploration of CO and CO<sub>2</sub> Extrusions from Five-Membered Rings", The Journal of Organic Chemistry, vol. 78, pp. 7565-7574, 2013. https://doi.org/10.1021/jo401146k
- T. Koerner, H. Slebocka-Tilk, and R.S. Brown, "Experimental Investigation of the Primary and Secondary Deuterium Kinetic Isotope Effects for Epoxidation of Alkenes and Ethylene with <i>m</i>-Chloroperoxybenzoic Acid", The Journal of Organic Chemistry, vol. 64, pp. 196-201, 1998. https://doi.org/10.1021/jo981652x
Tags: activation free energy, arrow pushing, curly arrow, dichloromethane solution, Dieter Cremer, hidden intermediate, model, using per-ethanoic acid
Posted in Curly arrows, reaction mechanism | 3 Comments »
August 18th, 2013
We have been experimenting with full-colour 3D printing of molecular objects. I thought I might here share some of our observations. Firstly, I list the software used:
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Tags: 3D colour printing, 3D printing, Bob Hanson, chemical models, EUR
Posted in Interesting chemistry | 21 Comments »
August 11th, 2013
The ultimate reduction in size for an engineer is to a single molecule. It’s been done for a car; now it has been reported for the pixel (picture-element).[1]
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References
- J.E. Kwon, S. Park, and S.Y. Park, "Realizing Molecular Pixel System for Full-Color Fluorescence Reproduction: RGB-Emitting Molecular Mixture Free from Energy Transfer Crosstalk", Journal of the American Chemical Society, vol. 135, pp. 11239-11246, 2013. https://doi.org/10.1021/ja404256s
Tags: above energy diagram, energy, energy transfer, energy transfer occurring, energy transfers, engineer, free energy
Posted in Interesting chemistry | 1 Comment »
July 27th, 2013
Valence shell electron pair repulsion theory is a simple way of rationalising the shapes of many compounds in which a main group element is surrounded by ligands. ClF3 is a good illustration of this theory.
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Tags: 2-electron-3-centre
Posted in Hypervalency, Interesting chemistry | 5 Comments »
July 18th, 2013
This potential example of a molecule on the edge of chaos was suggested to me by a student (thanks Stephen!), originating from an inorganic tutorial. It represents a class of Mo-complex ligated by two dithiocarbamate ligands and two aryl nitrene ligands (Ar-N:).
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Posted in Interesting chemistry | 2 Comments »
July 14th, 2013
I noted previously that some 8-ring cyclic compounds could exist in either a planar-aromatic or a non-planar-non-aromatic mode, the mode being determined by apparently quite small changes in a ring substituent. Hunting for other examples of such chemistry on the edge, I did a search of the Cambridge crystal database for metal sulfides.
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Tags: Cambridge, chemical reasons, metal, metal sulfides, neutral systems, transition metal series
Posted in Interesting chemistry | No Comments »
July 11th, 2013
The butterfly effect summarises how a small change to a system may result in very large and often unpredictable (chaotic) consequences. If the system is merely on the edge of chaos, the consequences are predictable, but nevertheless finely poised between e.g. two possible outcomes. Here I ask how a molecule might manifest such behaviour.
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Posted in Interesting chemistry | 5 Comments »