Henry Rzepa's Blog

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? 

Dicarbon can be variously represented as above. The first form shows it as a bis-carbene, with an unbonded lone pair of electrons at each end of a carbon double bond. The middle form has emerged in the last ten years or so as a serious alternative to describing the singlet state structure. It contains a so-called triple endo–bond and one further much weaker exo-bond (indicated separately by the symbol above the bond), referred to for simplicity as quadruple-bonded dicarbon. The third form would be a triplet biradical with triple bonded carbon. The species is known to be a singlet ground state with a significant excitation energy to the triplet.^‡ One can then ask the question: would either of these singlet state species be capable of being a hydrogen bond acceptor?

Time for calculations, at the CCSD(T)/Def2-TZVPP level using HF as the hydrogen bond donor (to enable advantage to be taken of the axial symmetry), data DOI: 10.14469/hpc/6554.

1. The singlet quadruple bonded form emerges as 32 kcal/mol higher in total energy than the singlet dicarbene.
2. The quadruple bonded form shows no sign of forming a hydrogen bond. The geometry optimisation curve is shown below followed by the final geometry (Å).

    

   

3. The bis-carbene form  (calculated by a double electron excitation, orbitals 10 to 12 and 11 to 15) DOES form such a complex. The hydrogen bond length (2.04Å) is exactly that found from the crystal structures of the shortest such bonds.

   

4. Two  of the normal vibrational modes of this species are shown below, being respectively the H…C and C=C stretches (153 and 1394 cm^-1).   

So dicarbon CAN form a short hydrogen bond to a donor such as HF, but only in its excited singlet state, which is some 32 kcal/mol above the quadruple-bonded form. Perhaps because of that fourth bond, the hydrogen bonding ability of this species is entirely inhibited. We have gotten to the point I wanted to reach; an experimental prediction that if singlet dicarbon can ever be trapped in a very inert matrix at very low temperatures in the presence of a hydrogen bond donor, it will not form a hydrogen bond to that donor. That is going to be a difficult experiment, but at least the prediction is out there as a challenge!

^‡At the same level of theory, the triplet ³Σ_u state is calculated to be 27.0 kcal/mol higher than the singlet ¹Σ_g.