Henry Rzepa's Blog

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

In the previous post,[1] I introduced the N=N double bond in nitrosobenzene dimer, arguing that even though it was a formal double bond, its bond dissociation energy made it nonetheless a very weak double bond! This was backed up by a technique known as energy decomposition analysis or EDA. Here I use a variant of this method  known as  NEDA to look at some other strained alkenes, including the famously non-existent tetra t-Butyl ethene.

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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

In an earlier blog, I discussed[1] the curly arrows associated with the known dimerisation of nitrosobenzene, and how the N=N double bond (shown in red below) forms in a single concerted process.

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References

1. H. Rzepa, "Mechanism of the dimerisation of Nitrosobenzene.", 2025. https://doi.org/10.59350/rzepa.28849

I thought I was done with exploring anomeric effects in small sulfur rings. However, I then realised that all the systems that I had described had an odd number of atoms and that I had not looked at any even numbered rings. Thus hexasulfur is a smaller (known) ring version of S₈, the latter by far the best known allotrope of this element of course.

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