The best known example of the gauche effect is 1,2-difluoroethane, which exhibits a relatively small preference of ~0.5 kcal/mol for this conformer over the anti orientation, which is also a minimum. But FSSF, which I discussed in the previous post, beats this hands down! It also, by the way, must surely be the smallest molecule (only four atoms) which could be theoretically resolved into two enantiomers (possibly at say 273K?).‡
Archive for September, 2013
An example of an extreme gauche effect: FSSF.
Saturday, September 21st, 2013Patterns of behaviour: serendipity in action for enantiomerisation of F-S-S-Cl
Thursday, September 19th, 2013Paul Schleyer sent me an email about a pattern he had spotted, between my post on F3SSF and some work he and Michael Mauksch had done 13 years ago with the intriguing title “Demonstration of Chiral Enantiomerization in a Four-Atom Molecule“.[1] Let me explain the connection, but also to follow-up further on what I discovered in that post and how a new connection evolved.
References
A two-publisher model for the scientific article: narrative+shared data.
Sunday, September 15th, 2013I do go on rather a lot about enabling or hyper-activating[1] data. So do others[2]. Why is sharing data important?
References
- O. Casher, G.K. Chandramohan, M.J. Hargreaves, C. Leach, P. Murray-Rust, H.S. Rzepa, R. Sayle, and B.J. Whitaker, "Hyperactive molecules and the World-Wide-Web information system", Journal of the Chemical Society, Perkin Transactions 2, pp. 7, 1995. https://doi.org/10.1039/p29950000007
- R. Van Noorden, "Data-sharing: Everything on display", Nature, vol. 500, pp. 243-245, 2013. https://doi.org/10.1038/nj7461-243a
The dimer of SF2: small is beautiful (and weird).
Thursday, September 12th, 2013Andy 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!
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
Coarctate reactions as a third fundamental organic-mechanistic type.
Wednesday, September 4th, 2013According 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:
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