The strongest bond in the universe: revisited ten years on.

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.

The hypothesis was that mono and then diprotonating dinitrogen strengthened the N-N bond, in the sequence N≡N → H-N≡N+ → H-N≡N-H2+ to the point that the latter was now a candidate for the strongest known bond between two (non-hydrogen) atoms. One of my original criteria was to calculate the N-N stretching wavenumber. To get this, I had to project out the coupling between the N-N stretching mode and the H-N modes in the latter two species. In the original discussion, Igor suggested another candidate O22+ in the comments. I speculated that the heavy atom diatomic force constant might be a better way of estimating how strong the bond was rather than the stretching wavenumber, but I never followed this up! So time to do so now.

The calculations are now at the CCSD(T)/Def2-TZVPP level (FAIR DOI: 10.14469/hpc/7214).

Species Heavy atom Force constant, mDyne/Å Projected heavy atom stretch, cm-1 Bond length, Å
N≡N 45.5177 2349 1.1029
HN≡N+ 50.2662 2469 1.0983
HN≡NH2+ 56.4903 2617 1.0859
O≡O2+ 44.9542 2184 1.0510
N≡O1+ 49.0119 2365 1.0678
HN≡O2+ 50.8061 2411 1.065
HN≡C 38.4408 2234 1.175

I did learn one new trick. To project out mode mixing between the hydrogen stretch and the heavy atom stretch, the hydrogen atom masses are now set at 10,000! This has the effect of suppressing any mode mixing, but it also results in exactly the same reduced mass for the NN stretch (14.0) for the three species N≡N, H-N≡N(+) and H-N≡N-H(2+), thus facilitating a like for like comparison. Ten years ago I had also tried the other direction, setting the mass of H to 0.001. Although this latter also eliminates the mode mixing, it does not result in the same reduced masses for the NN mode. So we will stick to the heavy hydrogen projection for comparisons.

The force constant increases almost linearly on mono and then diprotonation of dinitrogen, reaching 56.5 mDyne/Å. In comparison, both O22+ and NO1+ are a little lower. Does monoprotonating NO1+ strengthen its bond? Yes, but not quite surpassing HN≡NH2+. And the neutral HN≡C, which is isoelectronic with HN≡N+, also shows a weaker bond.

So my revisitation ten years on still shows that diprotonated nitrogen has the strongest bond (presumably in the universe), as now judged by the diatomic force constant. The hunt is still on for a species where the force constant between two non-hydrogen atoms is higher. Maybe I will return in another ten years to see the state of this challenge!


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