A short note here mainly to call to the reader’s attention a fascinating “trialogue” on the C2 molecule.1 Shaik, Danovich, Wu, Su, Rzepa, and Hiberty2 recently presented a full CI study of C2 and concluded that the molecule contains a quadruple bond (see my previous post on this paper). This work was inspired in part by a blog post by Henry Rzepa.
The trialogue1 is a conversation between Sason Shaik, Henry Rzepa and Roald Hoffmann about the nature of C2, its 4th bond, its diradical character, and some historical detours to see how some of our theoretical chemistry ancestors came close to proposing a quadruple bond. The discussion weaves together simple MO pictures, simple VB models, and the need for much more sophisticated analysis to ultimately approach the truth. Very much worth pointing out is the careful analysis of trying to tease out bond dissociation energies, especially analyzing the assumptions made here – including the possibility of errors in the experiments and not just errors in the computations! This is a very enjoyable read, following these three theoreticians as they traipse about the complex C2 landscape!
References
(1) Shaik, S.; Rzepa, H. S.; Hoffmann, R. "One Molecule, Two Atoms, Three Views, Four Bonds?," Angew. Chem. Int. Ed. 2013, 52, 3020-3033, DOI: 10.1002/anie.201208206.
(2) Shaik, S.; Danovich, D.; Wu, W.; Su, P.; Rzepa, H. S.; Hiberty, P. C. "Quadruple bonding in C2 and analogous eight-valence electron species," Nat. Chem. 2012, 4, 195-200, DOI: 10.1038/nchem.1263.
Henry Rzepa responded on 27 Apr 2013 at 10:38 am #
This tiniest and apparently simplest of molecules seems to be generating both a lot of discussion, and indeed new insights and methodologies into bonding. I mention here an interesting article that takes as its start point the two articles noted above and applies natural orbital functional theory to the problem. It concurs with valence bond theory in asserting that the (integer) bond order of C2 is FOUR. This article contains a succinct summary of the various ways at looking what chemists mean by a chemical bond.
Computational Organic Chemistry » Three interesting recent Angew. Chem. papers responded on 24 Jun 2013 at 11:01 am #
[…] the nature of C2, especially the notion that this molecules may possess a quadruple bond (see this post for a previous post on this article.) Frenking argues that the force constant associated with the […]