Henry Rzepa's Blog

I occasionally notice that posts that first appeared here many years ago suddenly attract attention. Thus this post, entitled The strongest bond in the universe, from ten years back, has suddently become the most popular, going from an average of 0-2 hits per day to 92 in a single day on May 22nd (most views appear to originate from India). I can only presume that a university there has set some course work on this topic and Google has helped some of the students identify my post. Well, re-reading something you wrote ten years ago can be unsettling. Are the conclusions still sound? Would I establish my claim the same way now? After all, one picks up a little more experience in ten years. So here is my revisitation.

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In 2001, Shaik and co-workers published the first of several famous review articles on the topic A Different Story of π-Delocalization. The Distortivity of π-Electrons and Its Chemical Manifestations[cite]10.1021/cr990363l[/cite]. The main premise was that the delocalized π-electronic component of benzene is unstable toward a localizing distortion and is at the same time stabilized by resonance relative to a localized reference structure.  Put more simply, the specific case of benzene has six-fold symmetry because of the twelve C-C σ-electrons and not the six π-electrons. In 2009, I commented here on this concept, via a calculation of the quintet state of benzene in which two of the six π-electrons are excited from bonding into anti-bonding π-orbitals, thus reducing the total formal π-bond orders around the ring from three to one. I focused on a particular vibrational normal mode, which is usefully referred to as the Kekulé mode, since it lengthens three bonds in benzene whilst shortening the other three. In this case the stretching wavenumber increased by ~207 cm-1 when the total π-bond order of benzene was reduced from three to one by spin excitation. In other words, each C-C bond gets longer when the π-electrons are excited, but the C-C bond itself gets stronger (in terms at least of the Kekulé mode).^† This behaviour is called a violation of Badger’s rule[cite]10.1063/1.1749433[/cite] for the relationship between the length of a bond and its stretching force constant. 

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In a welcome move, one of the American chemical society journals has published an encouragement to submit what is called FAIR data to the journal.[cite]10.1021/acs.orglett.0c00383[/cite]. A reminder that FAIR data is data that can be Found (F), Accessed (A), Interoperated(I) and Re-used( R). I thought I might try to explore this new tool here.

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I noted in an earlier blog, a potential (if difficult) experimental test of the properties of the singlet state of dicarbon, C₂. Now, just a few days ago, a ChemRxiv article has been published suggesting another (probably much more realistic) test.[cite]10.26434/chemrxiv.11446224.v1[/cite] This looks at the so-called ⁷Σ open shell state of the molecule where three electrons from one σ and two π orbitals are excited into the corresponding σ^* and π^* unoccupied orbitals. The argument is presented that these states are not dissociative, showing a deep minimum and hence a latent quadruple bonding nature. They also note that the isoelectronic BN molecule IS dissociative.^† Thus to quote: “Hence, the proof of existence of a minimum in the ⁷Σ_u+ for C₂ and the absence of such a minimum in the equivalent case for BN is likely to corroborate our findings on quadruple bonding in these two cases.“

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In the previous post, I showed that carbon can act as a hydrogen bond acceptor (of a proton) to form strong hydrogen bond complexes. Which brings me to a conceptual connection: can singlet dicarbon form such a hydrogen bond? 

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