Henry Rzepa's Blog Chemistry with a twist

March 10, 2017

George Olah and the norbornyl cation.

George 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. 


April 15, 2016

Azane oxide, a tautomer of hydroxylamine.

In the previous post I described how hydronium hydroxide or H3O+…HO, an intermolecular tautomer of water, has recently been observed captured inside an organic cage[1] and how the free-standing species in water can be captured computationally with the help of solvating water bridges. Here I explore azane oxide or H3N+-O, a tautomer of the better known hydroxylamine (H2N-OH).



  1. M. Stapf, W. Seichter, and M. Mazik, "Unique Hydrogen-Bonded Complex of Hydronium and Hydroxide Ions", Chemistry - A European Journal, vol. 21, pp. 6350-6354, 2015. http://dx.doi.org/10.1002/chem.201406383

April 9, 2011

Why are α-helices in proteins mostly right handed?

Understanding why and how proteins fold continues to be a grand challenge in science. I have described how Wrinch in 1936 made a bold proposal for the mechanism, which however flew in the face of much of then known chemistry. Linus Pauling took most of the credit (and a Nobel prize) when in a famous paper[1] in 1951 he suggested a mechanism that involved (inter alia) the formation of what he termed α-helices. Jack Dunitz in 2001[2] wrote a must-read article[3] on the topic of “Pauling’s Left-handed α-helix” (it is now known to be right handed). I thought I would revisit this famous example with a calculation of my own and here I have used the ωB97XD/6-311G(d,p) DFT procedure[4] to calculate some of the energy components of a small helix comprising (ala)6 in both left and right handed form.



  1. L. Pauling, R.B. Corey, and H.R. Branson, "The structure of proteins: Two hydrogen-bonded helical configurations of the polypeptide chain", Proceedings of the National Academy of Sciences, vol. 37, pp. 205-211, 1951. http://dx.doi.org/10.1073/pnas.37.4.205
  2. J.D. Dunitz, "Pauling's Left-Handed α-Helix", Angewandte Chemie International Edition, vol. 40, pp. 4167-4173, 2001. http://dx.doi.org/10.1002/1521-3773(20011119)40:22<4167::AID-ANIE4167>3.0.CO;2-Q
  3. K.S. Thanthiriwatte, E.G. Hohenstein, L.A. Burns, and C.D. Sherrill, "Assessment of the Performance of DFT and DFT-D Methods for Describing Distance Dependence of Hydrogen-Bonded Interactions", Journal of Chemical Theory and Computation, vol. 7, pp. 88-96, 2010. http://dx.doi.org/10.1021/ct100469b

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