This is another of those posts that has morphed from an earlier one noting the death of the great chemist George Olah. The discussion about the norbornyl cation concentrated on whether this species existed in a single minimum symmetric energy well (the non-classical Winstein/Olah proposal) or a double minimum well connected by a symmetric transition state (the classical Brown proposal). In a comment on the post, I added other examples in chemistry of single/double minima, mapped here to non-classical/classical structures. I now expand on the examples related to small aromatic or anti-aromatic rings.
Archive for the ‘Interesting chemistry’ Category
Expanding on the curious connection between the norbornyl cation and small-ring aromatics.
Sunday, March 12th, 2017George Olah and the norbornyl cation.
Friday, March 10th, 2017George Olah passed away on March 8th. He was part of the generation of scientists in the post-war 1950s who had access to chemical instrumentation that truly revolutionised chemistry. In particular he showed how the then newly available NMR spectroscopy illuminated structures of cations in solvents such “Magic acid“. The obituaries will probably mention his famous “feud” with H. C. Brown over the structure of the norbornyl cation (X=CH2+), implicated in the mechanism of many a solvolysis reaction that characterised the golden period of physical organic chemistry just before and after WWII.
More tetrahedral fun. Spherical aromaticity (and other oddities) in N4 and C4 systems?
Thursday, March 2nd, 2017The thread thus far. The post about Na2He introduced the electride anionic counter-ion to Na+ as corresponding topologically to a rare feature known as a non-nuclear attractor. This prompted speculation about other systems with such a feature, and the focus shifted to a tetrahedral arrangement of four hydrogen atoms as a dication, sharing a total of two valence electrons. The story now continues here.
The H4 (2+) dication and its bonding.
Wednesday, February 15th, 2017This post arose from a comment attached to the post on Na2He and relating to peculiar and rare topological features of the electron density in molecules called non-nuclear attractors. This set me thinking about other molecules that might exhibit this and one of these is shown below.
VSEPR Theory: A closer look at bromine trifluoride, BrF3.
Tuesday, February 14th, 2017I analysed the bonding in chlorine trifluoride a few years back in terms of VSEPR theory. I noticed that several searches on this topic which led people to this post also included a query about the differences between it and the bromine analogue. For those who posed this question, here is an equivalent analysis.
Forming a stabilized m-benzyne.
Friday, January 20th, 2017The story so far. Inspired by the report of the most polar neutral compound yet made, I suggested some candidates based on the azulene ring system that if made might be even more polar. This then led to considering a smaller π-analogue of azulene, m-benzyne. Here I ponder how a derivative of this molecule might be made, using computational profiling as one reality check.
Braiding a molecular knot with eight crossings.
Friday, January 20th, 2017This is one of those posts of a molecule whose very structure is interesting enough to merit a picture and a 3D model. The study[1] reports a molecular knot with the remarkable number of eight crossings.
References
- J.J. Danon, A. Krüger, D.A. Leigh, J. Lemonnier, A.J. Stephens, I.J. Vitorica-Yrezabal, and S.L. Woltering, "Braiding a molecular knot with eight crossings", Science, vol. 355, pp. 159-162, 2017. https://doi.org/10.1126/science.aal1619
Ritonavir: a look at a famous example of conformational polymorphism.
Monday, January 2nd, 2017Here is an inside peek at another one of Derek Lowe’s 250 milestones in chemistry, the polymorphism of Ritonavir.[1] The story in a nutshell concerns one of a pharma company’s worst nightmares; a drug which has been successfully brought to market unexpectedly “changes” after a few years on market to a less effective form (or to use the drug term, formulation). This can happen via a phenomenon known as polymorphism, where the crystalline structure of a molecule can have more than one form.[2],[3],[4] In this case, form I was formulated into soluble tablets for oral intake. During later manufacturing, a new less-soluble form appeared and “within weeks this new polymorph began to appear throughout both the bulk drug and formulation areas“[1]
References
- J. Bauer, S. Spanton, R. Henry, J. Quick, W. Dziki, W. Porter, and J. Morris, "Ritonavir: An Extraordinary Example of Conformational Polymorphism", Pharmaceutical Research, vol. 18, pp. 859-866, 2001. https://doi.org/10.1023/a:1011052932607
- J.D. Dunitz, and J. Bernstein, "Disappearing Polymorphs", Accounts of Chemical Research, vol. 28, pp. 193-200, 1995. https://doi.org/10.1021/ar00052a005
- D. Bučar, R.W. Lancaster, and J. Bernstein, "Disappearing Polymorphs Revisited", Angewandte Chemie International Edition, vol. 54, pp. 6972-6993, 2015. https://doi.org/10.1002/anie.201410356
- G.J.O. Beran, I.J. Sugden, C. Greenwell, D.H. Bowskill, C.C. Pantelides, and C.S. Adjiman, "How many more polymorphs of ROY remain undiscovered", Chemical Science, vol. 13, pp. 1288-1297, 2022. https://doi.org/10.1039/d1sc06074k
The dipole moments of highly polar molecules: glycine zwitterion.
Saturday, December 24th, 2016The previous posts produced discussion about the dipole moments of highly polar molecules. Here to produce some reference points for further discussion I look at the dipole moment of glycine, the classic zwitterion (an internal ion-pair).