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.
Two examples of the molecule above are known, differing only in the nature of the R group.
CUWWEW is strongly buckled and shows no sign of cyclic conjugation, with the double bonds localised. On the other hand the very similar CUWWIA is essentially planar, being so because it contains ten π-electrons in a planar ring and so is 4n+2 aromatic (n=2) and becomes delocalised. However, neither form is entirely happy; CUWWEW relieves ring string (in a flat 8-ring the internal angles are ~140°, a significant departure from that preferred for sp2 hydridisation) but looses any stabilisation from aromaticity. For CUWWIA the reverse is true.
Clearly these two effects are finely balanced for this system and the result is a pair of molecules on the edge of chaos, where a small change to the R group can tip the molecule over from one state to another., I may return to this particular theme in future posts, whereby two molecules which differ perhaps only in substituents, nevertheless adopt quite different geometries and properties.
- H.S. Rzepa, and N. Sanderson, "Aromaticity on the edge of chaos: An ab initio study of the bimodal balance between aromatic and non-aromatic structures for 10π-diheteroannulenes", Phys. Chem. Chem. Phys., vol. 6, pp. 310-313, 2004. http://dx.doi.org/10.1039/B312724A
- H.S. Rzepa *, and N. Sanderson, "Aromaticity on the edge of chaos: anAb initioCCSD(T) study of the bimodal balance between aromatic and non-aromatic structures for 10-π-diheterocins and heteronins", Molecular Physics, vol. 103, pp. 401-405, 2005. http://dx.doi.org/10.1080/00268970512331317796