Henry Rzepa's Blog

MOST is a chemical method of converting photonic or light energy into storable thermal energy which can be released on demand. A recent breakthrough in such methods has been reported[1] in which a pyrimidone molecule is efficiently converted by 310nm light into the isomeric Dewar pyrimidone. This molecule is thermally stable, but when protonated, rapidly releases thermal (enthalpic) energy in converting down to protonated pyrimidone – the energy release is sufficiently rapid that it can boil water and reaching energy storage levels previously inaccessible to MOST systems. The basic chemistry is shown below – treatment with base makes it fully cyclic.

The chemical reactions are interesting. The light catalysed step is a pericyclic electrocyclic reaction, allowed by the Woodward-Hoffmann rules with stereochemical disrotation via suprafacial bond formation. The acid catalysed thermal reaction however, in order to conform to these rules, would nominally need to be an electrocycic ring opening with an antarafacial stereochemical component. This would require the bicyclic ring system to contain a trans rather than the cis bridgehead stereochemistry shown above.This reaction was first studied many years ago[2] when it was shown that the thermal ring opening of a cis Dewar isomer indeed has a high barrier, due to its “forbidden” character. This imparts one of the desirable characteristics of a MOST system, namely the ability to store the high energy compound if necessary for long periods of time. The key step in the above is recognising that protonating the bicyclic nitrogen of the Dewar form should significantly reduce the barrier to ring opening. Here to illustrate these two reactions, I show intrinsic reaction coordinates (IRCs) for both steps.

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References

1. H.P.Q. Nguyen, A.J. Maertens, B.A. Baker, N.M. Wu, Z. Ye, Q. Zhou, Q. Qiu, N. Kaur, D.B. Berkinsky, K.E. Shulenberger, K.N. Houk, and G.G.D. Han, "Molecular solar thermal energy storage in Dewar pyrimidone beyond 1.6 megajoules per kilogram", Science, vol. 392, 2026. https://doi.org/10.1126/science.aec6413
2. M.J.S. Dewar, G.P. Ford, and H.S. Rzepa, "Electrocyclic ring opening of 1α,4α- and 1α,4β-bicyclo[2.2.0]hexa-2,5-dienes (cis and trans Dewar benzenes): MNDO (modified neglect of diatomic overlap) semiempirical molecular orbital calculations", J. Chem. Soc., Chem. Commun., pp. 728-730, 1977. https://doi.org/10.1039/c39770000728

A little while back, I wrote about anomeric-like effects in the sulfur ring S₇.[1] I had started that exploration by retrieving the crystal structure from the ICSD (Inorganic crystal structure database) and then optimising these coordinates using a DFT method (MN15L/Def2-TZVPP to be precise). In demonstrating this effect to a student, I decided to create an initial guess for the molecule coordinates not from the crystal structure but by drawing and then minimising using a simple molecular mechanics force field – and only then subjecting it to DFT re-optimisation.[2] It turns out the result was quite surprising in one respect and so here I tell the rest of the story.

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References

1. H. Rzepa, "Cyclo-Heptasulfur, S₇ – a classic anomeric effect discovered during a pub lunch!", 2025. https://doi.org/10.59350/rzepa.28407
2. H. Rzepa, "Anomeric isomerism in cyclo-heptasulfur.", 2026. https://doi.org/10.14469/hpc/15924

In 2008, the previously elusive hydroxycarbene, H-C-OH was finally reported[1] as having been captured by matrix isolation, accompanied by the observation that “we unexpectedly find that H–C–OH rearranges to formaldehyde with a half-life of only 2h at 11K by pure hydrogen tunnelling through a large energy barrier in excess of 30 kcal mol^–1. A subsequent theoretical study of this tunnelling in 2017[2] reported that “the half-life calculation after monodeuteration is 2.97 × 10¹⁶ hours, which is extremely longer than before monodeuteration that is only 2.5 hours using the same calculation methods“; in other words a kinetic isotope effect k^H/k^D of ~10¹⁶, which is by far the largest ever suggested.[3] In 2011, the original study was extended to methylhydroxycarbene (X=Me)[4], again arguing for “Tunneling Control of a Chemical Reaction.” In this post,^† I explore an alternative mechanism for rearrangement of hydroxycarbene to formaldehyde using a “double hydrogen transfer” via a dimeric transition state (Figure 1).

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References

1. P.R. Schreiner, H.P. Reisenauer, F.C. Pickard IV, A.C. Simmonett, W.D. Allen, E. Mátyus, and A.G. Császár, "Capture of hydroxymethylene and its fast disappearance through tunnelling", Nature, vol. 453, pp. 906-909, 2008. https://doi.org/10.1038/nature07010
2. N.D. Aisyah, R.N. Fadilla, H.K. Dipojono, and F. Rusydi, "A Theoretical Study of Monodeuteriation Effect on the Rearrangement of Trans-HCOH to H 2 CO via Quantum Tunneling with DFT and WKB Approximation", Procedia Engineering, vol. 170, pp. 119-123, 2017. https://doi.org/10.1016/j.proeng.2017.03.024
3. H. Rzepa, "Reinvestigating the reported transition state structure of a concerted triple H-tunneling mechanism.", 2025. https://doi.org/10.59350/qgwfn-rsc92
4. P.R. Schreiner, H.P. Reisenauer, D. Ley, D. Gerbig, C. Wu, and W.D. Allen, "Methylhydroxycarbene: Tunneling Control of a Chemical Reaction", Science, vol. 332, pp. 1300-1303, 2011. https://doi.org/10.1126/science.1203761

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, vol. 392, 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