Henry Rzepa's Blog

Here is another of the “large” molecules in the c&e news shortlist for molecule-of-the-year, 2020. This one is testing the Hückel 4n+2 rule out to a value never before seen (n = 40, or 162 π-electrons).[cite]10.1038/s41557-019-0398-3[/cite] The take-home message is that this rule seems to behave well in predicting global aromaticity even at this sort of scale!

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The title derives from an article[cite]10.1038/s41586-020-2614-0[/cite] which was shortlisted for the annual c&en molecule of the year 2020 awards (and which I occasionally cover here). In fact this year’s overall theme is certainly large molecules, the one exception being a smaller molecule with a quadruple bond to boron, a theme I have already covered here.

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Cyclopropenylidene must be the smallest molecule to be aromatic due to π-electrons, with just three carbon atoms and two hydrogen atoms. It has now been detected in the atmosphere of Titan, one of Saturn’s moons[cite]10.3847/1538-3881/abb679[/cite] and joins benzene, another aromatic molecule together with the protonated version of cyclopropenylidene, C₃H₃⁺ also found there.

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Way back in 2010, I was writing about an experience I had just had during an organic chemistry tutorial, which morphed into speculation as to whether a carbon atom might sustain a quadruple bond to nitrogen. A decade on, and possibly approaching 100 articles by many authors on the topic, quadruple bonds to carbon continue to fascinate. Now an article as appeared[cite]10.1039/d0cp03436c[/cite] repeating this speculation for a carbon to iron quadruple bond,^‡ in the very simple species C⩸Fe(CO)₃ (see also a Rh-B equivalent[cite]10.1021/acs.jpclett.9b03484[/cite]). This is particularly exciting because of the very real prospect of synthesising this species and perchance getting a crystal structure (something not possible with most of the other quadruply bonded carbon systems studied to date).

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Here I investigate a recent report[cite]10.1126/sciadv.abc0495[/cite] of a new generation of polyesters with the intrinsic properties of high crystallinity and chemical recyclability. The latter point is key, since many current plastics cannot be easily recycled to a form which can be used to regenerate the original polymer with high yield. Here I show some aspects of this fascinating new type of polymer.

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I occasionally notice that posts that first appeared here many years ago suddenly attract attention. Thus this post, entitled The strongest bond in the universe, from ten years back, has suddently become the most popular, going from an average of 0-2 hits per day to 92 in a single day on May 22nd (most views appear to originate from India). I can only presume that a university there has set some course work on this topic and Google has helped some of the students identify my post. Well, re-reading something you wrote ten years ago can be unsettling. Are the conclusions still sound? Would I establish my claim the same way now? After all, one picks up a little more experience in ten years. So here is my revisitation.

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