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