The rather presumptious title assumes the laws and fundamental constants of physics are the same everywhere (they may not be). With this constraint (and without yet defining what is meant by strongest), consider the three molecules:
|NN length, Å||1.0967||1.0915||1.0795|
|NN stretch, cm-1||2418.8||2356.4
|ELF NN basin
|aValue for hydrogen mass of 10,000 bValue for hydrogen mass of 0.001.|
The series explores the effect of protonating dinitrogen (generally considered as strong as a diatomic bond gets).
- Firstly, one notes that the N-N distance decreases with mono and then diprotonation, the second protonation having the greater effect. Is shorter stronger?
- What about the NN stretching vibration? Here one encounters an annoying feature of vibrations; the modes are not always pure. Thus whilst in N2 itself, there is only one normal mode, and it is as pure as they get, by the time we have di-protonated, we have three stretching modes, two involving H-N and one N-N. They mix and none can now be considered a pure N-N stretch. Thus in H2N2, the highest wavenumber mode of 3024 is a mixture of H-N and N-N, and likewise the 2226 mode, albeit in different proportions. So a trick has to be played. If the mass of each hydrogen is increased to 10,000, modes involving these super-heavy atoms no longer mix with any other mode. Now, the N-N mode becomes pure, and its value is 2688, a significant increase on nitrogen itself. The monoprotonated form also shows a lesser increase.
- The ELF disynaptic basins for the three molecules also steadily increase their populations. Electrons that were previously in the terminal nitrogen lone pairs now creep into the N-N region instead, and hence make the bond stronger. The population does not reach six (the nominal value for a triple bond), since the H-N regions still contain more than 2 electrons. But ELF matches the previous two results.
- QTAIM measures the electron density ρ(r) at the bond critical point. Here different behaviour is seen, with ρ(r) lower for the monprotonated, and the diprotonated form intermediate between the other two. Perhaps absolute electron densities measured at a single point do not measure bnd strengths after all. The Laplacian, ∇2ρ(r) steadily decreases along the series.
So is the NN bond in HNNH2+ the strongest bond in the universe? Almost certainly. OK, so bonds with higher formal bond orders (Cr2 for example) exist, but they come nowhere near HN≡NH2+, which is crowned champion.
Oh, by the way, another article (DOI: 10.1063/1.1576756) claimed the title in 2003, but I make the claim for a stronger bond here!
Tags: Interesting chemistry