Diatomics with eight valence-electrons: formation by radioactive decay.

This is a follow up to my earlier post about C⩸N⁺, itself inspired by this ChemRxiv pre-print[1] which describes a chemical synthesis of singlet biradicaloid C₂ and its proposed identification as such by chemical trapping.

First row diatomics based on the iso-electronic principle of eight valence electrons include both C⩸N⁺ and C⩸C, as well as species such as B⩸N, C⩸O²⁺ and even the unlikely N⩸O³⁺. The diatomic bond is represented here by ⩸ which carries the message of six electrons pairing to form a conventional triple bond and the remaining two valence electrons more weakly spin-pairing to form overall a singlet biradicaloid species with a quadruple bond. The “BDE” (bond dissociation energy) of the 4th pair is around 20 kcal/mol for C⩸C,[2]  which arguably entitles it to be called a weak bond.[3]

Here I am going to explore ^–B⩸N⁺ and isoelectronic C⩸C via formation by radioactive decay of tritium into helium (Table, FAIR data DOI: 10.14469/hpc/5691).

The thermochemistry includes a significant contribution from entropy, which favours the reaction. At its simplest, this involves the replacement of a X-He (X=C,B) bond by the 4th C⩸X bond. The BDEs (bond dissociation energies) of the X-He bond are very small (< 1 kcal/mol) and hence the reaction is driven largely by the enthalpy of forming the final C⩸X bond, together with entropy increase. Contrast this with the reaction reported above involving cleavage of a C–IPh bond,[1] where the C–I BDE is larger (~70-80 kcal/mol; > 20 kcal/mol). This makes the reported trapping of C₂ from this reaction all the more intriguing.

References

1. K. Miyamoto, S. Narita, Y. Masumoto, T. Hashishin, M. Kimura, M. Ochiai, and M. Uchiyama, "Room-Temperature Chemical Synthesis of C2", 2019. http://dx.doi.org/10.26434/chemrxiv.8009633.v1
2. D. Danovich, P.C. Hiberty, W. Wu, H.S. Rzepa, and S. Shaik, "The Nature of the Fourth Bond in the Ground State of C₂: The Quadruple Bond Conundrum", Chemistry – A European Journal, vol. 20, pp. 6220-6232, 2014. http://dx.doi.org/10.1002/chem.201400356
3. S. Shaik, D. Danovich, B. Braida, and P.C. Hiberty, "The Quadruple Bonding in C₂ Reproduces the Properties of the Molecule", Chemistry – A European Journal, vol. 22, pp. 4116-4128, 2016. http://dx.doi.org/10.1002/chem.201600011