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).

Entry system ΔΔG ΔΔH
1 [Li-C≡C-T →] Li-C≡C-He+ + e → Li+ + C⩸C + He  -44.9 -27.6
2 [(-)C≡C-T →] (-)C≡C-He+ + e → C⩸C + He  -42.2 -31.9
3 [Li-N≡B-T →] Li-N≡B-He+ + e → Li+ + B⩸N+ + He  -9.0 +2.9

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 (< 20 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 CIPh bond,[1] where the CI 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

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