Hydrogenating the even more mysterious N≡N triple bond in a nitric oxide dimer.

Previously[1] I looked at some of the properties of the mysterious dimer of nitric oxide  1 – not the known weak dimer but a higher energy form with a “triple” N≡N bond. This valence bond isomer of the weak dimer was some 24 kcal/mol higher in free energy than the two nitric oxide molecules it would be formed from. An energy decomposition analysis (NEDA) of 1 revealed an interaction energy[2] of +4.5 kcal/mol for the two radical fragments, compared to eg -27 kcal/mol for the equivalent analysis of the N=N double bond in nitrosobenzene dimer[3] So here I take a look at another property of N≡N bonds via their hydrogenation energy (Scheme), mindful that the dinitrogen molecule requires forcing conditions to hydrogenate, in part because of the unfavourable entropy terms (See Wiki and also here^‡ for a calculation of ΔG₂₉₈).

Calculations at the ωB97XD/Def2-TZVPP/SCRF=water level[4] that whilst hydrogenation of the triple bond in N₂ is strongly endo-energic, the same process for molecule 1 is exo-energic (ΔΔG -26.32 kcal/mol). The direct product is a zwitterion, but presumed rapid proton transfer to a neutral form 2 increases exo-energicity. Whilst the second hydrogenation step  of N₂ is  exo-energic, the equivalent second step for 1 to  give 3 is now mildly endo-energic. Overall however, the thermodynamic energies of these two types of triple bond hydrogenation could not be more different.

So forming a N≡N triple bond by forcing two nitric oxide molecules to dimerise (using high pressure) in water produces a system where hydrogenation of that “difficult” N≡N bond is made very much easier thermodynamically. Time for an experiment?^♥

^‡This site reports a gas phase experimental value for ΔG -8.1 kcal/mol at 298K for this equilibrium, although the pressure is not given. The calculated value shown in the scheme above (-20.1 kcal/mol)  is for 298K and 1 atm for a model using water as solvent – which might be expected to differentially solvate the product ammonia and hence promote the reaction. In the limit of low pressure (0.0001M)[5] this reduces to -13.0 kcal/mol, increases to -26.6 kcal/mol at 10M and becomes -14.3 kcal/mol at 10M/800K, illustrating how higher pressures make the reaction more exo-energic and higher temperatures less exo-energic. This was of course the problem solved in the Haber process of finding the sweet spot between pressure and temperature.

^♥Perhaps not, given the report that at high pressures, nitric oxide can become explosive.[6]

References

1. H. Rzepa, “The even more mysterious N≡N triple bond in a nitric oxide dimer.”, 2025.
2. H. Rzepa, “N2O2 as strong dimer? bent NEDA 0 1 0 2 0 -2 Total Interaction (E) : 4.520 Wiberg NN bond index 1.0072 NN stretch 2604 cm-1”, 2025.
3. H. Rzepa, “Nitrosobenzene dimer NEDA=2, 0,1 0,1 0,1 Total Interaction (E) : -27.564”, 2025.
4. H. Rzepa, “[Embargoed]”, 2025.
5. G. Luchini, J.V. Alegre-Requena, I. Funes-Ardoiz, and R.S. Paton, “GoodVibes: automated thermochemistry for heterogeneous computational chemistry data”, F1000Research, vol. 9, pp. 291, 2020.
6. T. Melia, “Decomposition of nitric oxide at elevated pressures”, Journal of Inorganic and Nuclear Chemistry, vol. 27, pp. 95-98, 1965.