Henry Rzepa's Blog

The recent report[1] of what is termed a “half-Möbius” molecule is generating a lot of excitement. It has its origins in a project to make odd-numbered cyclocarbons on STM (scanning tunnelling microscope) surfaces. I had discussed even-numbered cyclocarbons in another post[2], where I also happened to include several odd-numbered examples, such as C₄₉ and C₅₁. In this study[1] they were focussing on C₁₃ and a precursor to this was to be C₁₃Cl₂. As part of the microscopy, they noticed this latter species was asymmetric (chiral) and so started the story of a “half-Möbius” molecule (molecules with twists in their topology are of course chiral). I should at this stage say that the concept of a half-Möbius is quite new and thought provoking. Perhaps the simplest way of explaining why, is that a conventional Möbius molecule (as with the strip or ribbon) requires two full circuits of the edge of the ribbon to return to the start, whereas this half version requires a full four circuits to achieve the same. More about this later. (more…)

References

1. I. Rončević, F. Paschke, Y. Gao, L. Lieske, L.A. Gödde, S. Barison, S. Piccinelli, A. Baiardi, I. Tavernelli, J. Repp, F. Albrecht, H.L. Anderson, and L. Gross, "A molecule with half-Möbius topology", Science, 2026. https://doi.org/10.1126/science.aea3321
2. H. Rzepa, "Molecules of the year 2025: Cyclo[48]carbon and others – the onset of bond alternation and the Raman Activity Spectrum.", 2025. https://doi.org/10.59350/g4309-gv109

A few posts back, I contemplated the curly arrows appropriate for the formation of nitrosobenzene dimer from nitrosobenzene,[1] and commented on the odd nature of the N=N double bond formed in this process.[2]. Odd, because the valence bond representation of this dimer (1 below[3]) has two formally positive adjacent nitrogen atoms. An energy decomposition analysis (NEDA[4]) of species 1 showed an unusually small negative interaction energy of -27.6 kcal/mol between the two nitrosobenzene fragments (typical ΔE values ~-130 to -180 kcal/mol[5]), commensurate with the facile equilibrium between two monomers and the dimer[6] A little later I went on to speculate upon a similar theme for the more hypothetical nitric oxide dimer, a species 2 which again has two adjacent +ve charges[7] and even a smaller +ve NEDA for the triple bond! You can imagine discussing these results with organic chemists, who would normally shrink from placing two (formal) positive charges on adjacent atoms.

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References

1. H. Rzepa, "Mechanism of the dimerisation of Nitrosobenzene.", 2025. https://doi.org/10.59350/rzepa.28849
2. H. Rzepa, "The mysterious N=N double bond in nitrosobenzene dimer.", 2025. https://doi.org/10.59350/rzepa.29383
3. D.A. Dieterich, I.C. Paul, and D.Y. Curtin, "Structural studies on nitrosobenzene and 2-nitrosobenzoic acid. Crystal and molecular structures of cis-azobenzene dioxide and trans-2,2'-dicarboxyazobenzene dioxide", Journal of the American Chemical Society, vol. 96, pp. 6372-6380, 1974. https://doi.org/10.1021/ja00827a021
4. C.R. Landis, R.P. Hughes, and F. Weinhold, "Bonding Analysis of TM(cAAC)₂ (TM = Cu, Ag, and Au) and the Importance of Reference State", Organometallics, vol. 34, pp. 3442-3449, 2015. https://doi.org/10.1021/acs.organomet.5b00429
5. H. Rzepa, "Energy decomposition analysis of hindered alkenes: Tetra t-butylethene and others.", 2025. https://doi.org/10.59350/rzepa.29410
6. K.G. Orrell, V. Šik, and D. Stephenson, "Study of the monomer‐dimer equilibrium of nitrosobenzene using multinuclear one‐ and two‐dimensional NMR techniques", Magnetic Resonance in Chemistry, vol. 25, pp. 1007-1011, 1987. https://doi.org/10.1002/mrc.1260251118
7. H. Rzepa, "The even more mysterious N≡N triple bond in a nitric oxide dimer.", 2025. https://doi.org/10.59350/rzepa.29429

Sandwich compounds are the colloquial term used for molecules where a metal atom such as an iron dication is “sandwiched” between two carbon-based rings as ligands, most commonly cyclopentadienyl anion (the “bread”) as in e.g. Ferrocene – a molecule first discovered in 1951. An “inverted” sandwich is where the carbon ring is itself sandwiched between two metal ions and one such was reported this year [1] containing benzene in the middle with scandium as the metal. The novelty of the subsequent four-electron reduction of the benzene “filler” and its ring opening to a linear hexadiene unit resulted in this being selected as one “molecule of the year” for 2025.

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References

1. L. Zhang, Z. Jiang, C. Zhang, K. Cheng, S. Li, Y. Gao, X. Wang, and J. Chu, "Room Temperature Ring Opening of Benzene by Four-Electron Reduction and Carbonylation", Journal of the American Chemical Society, vol. 147, pp. 25017-25023, 2025. https://doi.org/10.1021/jacs.5c08414

The annual “Molecules of the Year” selections are available for the year 2025. A theme was elemental allotropes and one such was carbon in the form of C₄₈ stabilised by formation of a catenane C₄₈.M3 (M = red ligand below)[1] – it was not possible however to crystallise C₄₈.M3. When “unmasked” by removal of the M ligand, the true allotrope C₄₈ had a solution half-life of about 1 hour at 20°C. This follows the reports from 2019 onwards of a series of smaller cyclo[n]carbon allotropes, (n=6,10,12,13,14,16,18,20,26)[2],[3] which were only characterised on a solid surface and not in solution.

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References

1. Y. Gao, P. Gupta, I. Rončević, C. Mycroft, P.J. Gates, A.W. Parker, and H.L. Anderson, "Solution-phase stabilization of a cyclocarbon by catenane formation", Science, vol. 389, pp. 708-710, 2025. https://doi.org/10.1126/science.ady6054
2. K. Kaiser, L.M. Scriven, F. Schulz, P. Gawel, L. Gross, and H.L. Anderson, "An sp-hybridized molecular carbon allotrope, cyclo[18]carbon", Science, vol. 365, pp. 1299-1301, 2019. https://doi.org/10.1126/science.aay1914
3. H. Rzepa, "Cyclo[18]carbon: The Kekulé vibration calculated and hence a mystery!", 2019. https://doi.org/10.59350/jdy16-7rv58

In the previous post,[1] I was commenting that the transition state for borohydride reduction of a ketone contained some close contacts between the hydrogen of the borohydride and the hydrogen of water. A systematic search of the CSD reveals a modest number of such contacts have been observed in crystal structures (Table).  Since it is always good to have a reality check for constructed transition states, here I take a look at some of compounds showing the closest H…H contacts in the experimental database of structures.

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References

1. H. Rzepa, "The mechanism of borohydride reductions. Part 2: 4-t-butyl-cyclohexanone – Dispersion induced stereochemistry.", 2025. https://doi.org/10.59350/x5k75-t2m40

In the previous post[1] I mooted the possibility that a high energy form of the dimer of nitric oxide 1 might nonetheless be able to be detected using suitable traps (such as hydrogenation or cycloaddition). However, an interesting alternative is that this species could be trapped by nitric oxide itself. According to [2] in an article entitled “Decomposition of nitric oxide at elevated pressures” the rate of this termolecular reaction 3NO → N₂O + NO₂ are said to obey third order kinetics. One plausible mechanism for this process is shown below.

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References

1. H. Rzepa, "Hydrogenating the even more mysterious N≡N triple bond in a nitric oxide dimer.", 2025. https://doi.org/10.59350/rzepa.29626
2. T. Melia, "Decomposition of nitric oxide at elevated pressures", Journal of Inorganic and Nuclear Chemistry, vol. 27, pp. 95-98, 1965. https://doi.org/10.1016/0022-1902(65)80196-8

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).

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References

1. H. Rzepa, "The even more mysterious N≡N triple bond in a nitric oxide dimer.", 2025. https://doi.org/10.59350/rzepa.29429
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. https://doi.org/10.14469/hpc/15468
3. H. Rzepa, "Nitrosobenzene dimer NEDA=2, 0,1 0,1 0,1 Total Interaction (E) : -27.564", 2025. https://doi.org/10.14469/hpc/15444

I started adding citations to my blog posts around 2012 using the Kcite plugin.^‡ Rogue Scholar is a service that monitors registered blog sources and adds all sorts of value to the original post, including identifying such citations and creating a list of them.

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Previously, I pondered about the strange N=N double bond in nitrosobenzene dimer[1] as a follow up to commenting on the curly arrow mechanism of the dimerisation.[2] By the same curly arrow method, one can produce the below, showing how the simpler nitric oxide radical could potentially dimerise to a species with a N≡N triple bond!^† This involves a total of six electrons entering the N-N region, and hence raises the question of whether these all move in a single concerted/synchronous bond forming reaction, or whether they might go in (asynchronous) stages. Here are some calculations[3]) which might shed some light on this aspect.

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References

1. H. Rzepa, "The mysterious N=N double bond in nitrosobenzene dimer.", 2025. https://doi.org/10.59350/rzepa.29383
2. H. Rzepa, "Mechanism of the dimerisation of Nitrosobenzene.", 2025. https://doi.org/10.59350/rzepa.28849
3. H. Rzepa, "N2O2 as strong dimer TS as biradical cis, G = -259.785500", 2025. https://doi.org/10.14469/hpc/15483