Tom recently emailed me this question: Do you know how to find out how many of the compounds that appear in the chemical literature are mentioned just once? Intrigued, I first set out to find out how many substances , as Chemical Abstracts refers to the them, there were as of 5 June, 2025 . There is a static estimate here (219 million), but to get the most up to date information, I asked CAS directly.

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 S ₈ , the latter by far the best known allotrope of this element of course.

Last year I reminisced on the occasion of the 40th Anniversary of the Macintosh computer.[cite]10.59350/f11dr-93t29[/cite] Four decades of advances in technology now mean I can do a fair amount of computational quantum modelling on a recent Mac (one from 2022 with M1 processor), and since then they have only got even (~2 or 3 times) faster with the M4 processor.

In this series of posts about the electronic effects in small sulfur rings[cite]10.59350/rzepa.28615[/cite] I have explored increasingly large induced geometric effects. Here is the largest so far, for the compound S ₇ I ¹⁺ [cite]10.1021/ic50225a048[/cite] The calculated geometry[cite]10.14469/hpc/15236[/cite] is shown below, with the crystallographic values in parentheses – the two matching very well.

The two previous  posts[cite]10.59350/rzepa.28515[/cite],[cite]10.59350/rzepa.28407[/cite] on the topic of anomeric effects in 7-membered sulfur rings illustrated how orbital interactions between the lone pairs in the molecules and S-S bonds produced widely varying S-S bond lengths in the molecules, some are shorter than normal (which is ~2.05Å for e.g. the S ₈ ring) by ~ 0.1Å and some are longer by ~0.24Å.

The monosulfoxide of cyclo-heptasulfur was reported along with cycloheptasulfur itself in 1977,[cite]10.1002/anie.197707161[/cite] along with the remarks that “ The δ modification of S ₇ contains bonds of widely differing length: this has never been observed before in an unsubstituted molecule. and “the same effect having also been observed in other sulfur rings (S ₈ O, S ₇ I ¹⁺ and S ₇ O).”

Way back in 1977, the crystal structure of the sulfur ring S ₇ was reported.[cite]10.1002/anie.197707151[/cite] The authors noted that “ The δ modification of S ₇ contains bonds of widely differing length: this has never been observed before in an unsubstituted molecule. ” No explanation was offered, although they note that similar effects have been observed in S ₈ O, S ₇ I ⁺ and

Back in early 2012, I pondered about the relationships between a science-based blog post and a science-based journal article[cite]10.59350/3pbz1-vcd67[/cite]. This was in part induced by my discovering a blog plugin called Kcite , which allow a journal articles to be appended to the blog in the form of a numbered reference list.

X-ray crystallography is the technique of using the diffraction of x-rays by the electrons in a molecule to determine the positions of all the atoms in that molecule. Quantum theory teaches us that the electrons are to be found in shells around the atomic nuclei. There are two broad types, the outermost shell (also called the valence shell) and all the inner or core shells.

Starting around 2016, journal publishers started including mandatory “Data Availability” statements as part of research articles; a typical (dated) example is linked here, including guidelines for how to cite the data itself. I wrote about these aspects last year in a blog post for the RSC journal Digital Discovery[cite]10.26434/chemrxiv-2024-dz2dv[/cite] and here I follow up with more news.

Derek Lowe tells the story of “carbyne”, a potential further allotrope of carbon, comprising linear chains of carbon atoms, C-C≡C-C≡C-C. Whether such a molecule can exist on its own has long been the the topic of speculation.