Archive for August, 2019

Cyclo[18]carbon: The Kekulé vibration calculated and hence a mystery!

Friday, August 30th, 2019

I have discussed the vibration in benzene known as the Kekulé mode in other posts, the first of which was all of ten years ago. It is a stretching mode that lengthens three of the bonds in benzene (a [6]-annulene) and shortens the other three, thus leading to a cyclohexatriene motif (see below). This vibration is real (+ve force constant) in benzene itself, which indicates that distorting the structure from six to three-fold symmetry leads to an increase in energy. Benzene therefore has a symmetrising influence, and it comes as a surprise to most to learn that this is actually due to the σ rather than the π-electrons! But there are good reasons to believe that as the ring size of the annulene increases, the Kekulé vibration will evolve from a real mode into an imaginary (-ve force constant) vibration representing a transition state for mutating the single and double bonds. At some point therefore, the more symmetrical geometry of the annulene in which all the bonds are of equal length will change into one of lower symmetry, in which BLA (bond length alternation) occurs and the symmetrical form becomes a transition state for this process.


A Non-nitrogen Containing Morpholine Isostere; an application of FAIR data principles.

Sunday, August 4th, 2019

In the pipeline reports on an intriguing new ring system acting as an isostere for morpholine. I was interested in how the conformation of this ring system might be rationalised electronically and so I delved into the article.[1] Here I recount what I found.



  1. H. Hobbs, G. Bravi, I. Campbell, M. Convery, H. Davies, G. Inglis, S. Pal, S. Peace, J. Redmond, and D. Summers, "Discovery of 3-Oxabicyclo[4.1.0]heptane, a Non-nitrogen Containing Morpholine Isostere, and Its Application in Novel Inhibitors of the PI3K-AKT-mTOR Pathway", Journal of Medicinal Chemistry, vol. 62, pp. 6972-6984, 2019.