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.[cite]10.1002/anie.201705228[/cite] 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.
Posts Tagged ‘crystal structure search’
Dispersion-induced triplet aromatisation?
Thursday, January 3rd, 2019Intermolecular atom-atom bonds in crystals? The O…O case.
Saturday, July 25th, 2015I recently followed this bloggers trail; link1 → link2 to arrive at this delightful short commentary on atom-atom bonds in crystals[cite]10.1107/S2052252515002006[/cite] 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,[cite]10.1021/ja02261a002[/cite] 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.
Halogen bonds 4: The strongest (?) halogen bond.
Sunday, December 7th, 2014
Continuing my hunt, here is a candidate for a strong(est?) halogen bond, this time between Se and I.[cite]10.1021/ic50038a006[/cite]. 
The features of interest include:
Halogen bonds: Part 1.
Saturday, November 29th, 2014Halogen 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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Tags:crystal structure search, D. Note, frequent commentator, Paul Schleyer
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