Henry Rzepa's Blog

This are just a few insights I have got from some of the talks I attended. As usual, this does not represent a report on the WATOC congress itself, but simply some aspects that caught my personal eye. (more…)

The WATOC congresses occur every three years. WATOC25, the 13th in a series which started in 1987  takes places tomorrow in Oslo, Norway, The day before the main event there is something new – a session just for early career researchers or “Young WATOC”. As an “old” WATOCer, I dropped into the opening session and was delighted to find a packed auditorium, with literally standing room only comprising mostly young researchers in their 20s.

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I am in the process of revising my annual lecture to first year university students on the topic of “curly arrows”. I like to start my story in 1924, when Robert Robinson published the very first example[cite]gg9g[/cite] as an illustration of why nitrosobenzene undergoes electrophilic bromination in the para position of the benzene ring. I follow this up by showing how “data mining” can be used to see if this supports his assertion. I have used the very latest version of the CSD crystal structure database to update the version originally posted here in 2020.[cite]10.59350/c6thp-wqe69[/cite]

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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. They responded immediately (thanks Lee!) with 294,778,693 on the date mentioned above. It is not actually possible to answer the first question itself using CAS SciFinder, but again CAS came up with a value: “there are 113,383,649 substances in CAS Registry with only one CAplus citation” equivalent to  “38.5% of the current substances have only 1 reference.” I should add this estimate was qualified by “that can be misleading, since that includes salts, multicomponents, etc. But that’s a first pass.” I am actually impressed that as many as 61.5% are mentioned more than once, since before learning the answer, I had intuitively guessed that percentage as being much lower.

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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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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. Many of the recent calculations done for these blogs have included at least one or two that were done on the Mac. So I was intrigued to find that a real working version of the original Mac is about to be released for sale, but with a twist. Its called the “Pico-mac-nano” and from its name it is truly diminutive, being only 6.2 cm high – half the height of a can of cola – and with a 2″ LCD display. It comes with a connector for a keyboard and mouse, although currently it has no sound.

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