Henry Rzepa's Blog Chemistry with a twist

January 3, 2019

Dispersion-induced triplet aromatisation?

There is emerging interest in cyclic conjugated molecules that happen to have triplet spin states and which might be expected to follow a 4n rule for aromaticity.[1] The simplest such system would be the triplet state of cyclobutadiene, for which a non or anti-aromatic singlet state is always found to be lower in energy. Here I explore some crystal structures containing this motif for possible insights.



  1. A. Kostenko, B. Tumanskii, Y. Kobayashi, M. Nakamoto, A. Sekiguchi, and Y. Apeloig, "Spectroscopic Observation of the Triplet Diradical State of a Cyclobutadiene", Angewandte Chemie International Edition, vol. 56, pp. 10183-10187, 2017. http://dx.doi.org/10.1002/anie.201705228

July 25, 2015

Intermolecular atom-atom bonds in crystals? The O…O case.

I recently followed this bloggers trail; link1link2 to arrive at this delightful short commentary on atom-atom bonds in crystals[1] by Jack Dunitz. Here he discusses that age-old question (to chemists), what is a bond? Even almost 100 years after Gilbert Lewis’ famous analysis,[2] we continue to ponder this question. Indeed, quite a debate on this topic broke out in a recent post here. My eye was caught by one example in Jack's article: "The close stacking of planar anions, as occurs in salts of croconic acid …far from producing a lowering of the crystal energy, this stacking interaction in itself leads to an increase by several thousand kJ mol−1 arising from Coulombic repulsion between the doubly negatively charged anions" I thought I might explore this point a bit further in this post.



  1. J.D. Dunitz, "Intermolecular atom–atom bonds in crystals?", IUCrJ, vol. 2, pp. 157-158, 2015. http://dx.doi.org/10.1107/S2052252515002006
  2. G.N. Lewis, "THE ATOM AND THE MOLECULE.", Journal of the American Chemical Society, vol. 38, pp. 762-785, 1916. http://dx.doi.org/10.1021/ja02261a002

December 7, 2014

Halogen bonds 4: The strongest (?) halogen bond.

Filed under: crystal_structure_mining,Interesting chemistry — Tags: , — Henry Rzepa @ 10:28 am

Continuing my hunt, here is a candidate for a strong(est?) halogen bond, this time between Se and I.[1]. OXSELI The features of interest include:



  1. H. Maddox, and J.D. McCullough, "The Crystal and Molecular Structure of the Iodine Complex of 1-Oxa-4-selenacyclohexane, C4H8OSe.I2", Inorganic Chemistry, vol. 5, pp. 522-526, 1966. http://dx.doi.org/10.1021/ic50038a006

November 29, 2014

Halogen bonds: Part 1.

Halogen bonds are less familiar cousins to hydrogen bonds. They are defined as non-covalent interactions (NCI) between a halogen atom (X, acting as a Lewis acid, in accepting electrons) and a Lewis base D donating electrons; D….X-A vs D…H-A. They are superficially surprising, since both D and X look like electron rich species. In fact the electron distribution around X-X (A=X) is highly anisotropic, with the electron rich distribution (the "donor")  being in a torus encircling the bond, and an electron deficient region (the "acceptor") lying along the axis of the bond.


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