A PID or persistent identifier has been in common use in scientific publishing for around 20 years now. It was introduced as a DOI (Digital Object Identifier), and the digital object in this case was the journal article. From 2000 onwards, DOIs started appearing for most journal articles, journals having obtained them from a registration agency, CrossRef.

In 2001, Shaik and co-workers published the first of several famous review articles on the topic A Different Story of π-Delocalization.

In a welcome move, one of the American chemical society journals has published an encouragement to submit what is called FAIR data to the journal.[cite]10.1021/acs.orglett.0c00383[/cite]. A reminder that FAIR data is data that can be Found (F), Accessed (A), Interoperated(I) and Re-used( R). I thought I might try to explore this new tool here. You start at the ACS Research Data Center

Here is another selection from the Molecules-of-the-Year shortlist published by C&E News, in which hexagonal planar transition metal coordination is identified.

I noted in an earlier blog, a potential (if difficult) experimental test of the properties of the singlet state of dicarbon, C ₂ . Now, just a few days ago, a ChemRxiv article has been published suggesting another (probably much more realistic) test.[cite]10.26434/chemrxiv.11446224.v1[/cite] This looks at the so-called ⁷ Σ open shell state of the molecule where three electrons from one σ and two π orbitals are excited

Having shown that carbon as a carbene centre, C**:** can act as a hydrogen bond acceptor, as seen from a search of crystal structures, I began to wonder if there is any chance that carbon as a radical centre, C• could do so as well. Definitely a subversive thought, since radical centres are supposed to abstract hydrogens rather than to hydrogen bond to them.

In the previous post, I showed that carbon can act as a hydrogen bond acceptor (of a proton) to form strong hydrogen bond complexes. Which brings me to a conceptual connection: can singlet dicarbon form such a hydrogen bond?  Dicarbon can be variously represented as above. The first form shows it as a bis-carbene, with an unbonded lone pair of electrons at each end of a carbon double bond.

A hydrogen bond donor is considered as an electronegative element carrying a hydrogen that is accepted by an atom carrying a lone pair of electrons, as in X:…H-Y where X: is the acceptor and H-Y the donor. Wikipedia asserts that carbon can act as a donor, as we saw in the post on the incredible chloride cage, where six Cl**:**…H-C interactions trapped the chloride ion inside the cage.

All of the molecules in this year’s C&EN list are fascinating in their very different ways. Here I take a look at the twisty tetracene (dodecaphenyltetracene) which is indeed very very twisty.[cite]10.1002/anie.201812418[/cite] Click on image to view 3D model Unfortunately, the authors point that the twisty-ness does not lead to a stable helical configuration at room temperatures and so separate enantiomers cannot be isolated.

My momentum of describing early attempts to use optical rotation to correlate absolute configuration of small molecules such as glyceraldehyde and lactic acid with their optical rotations has carried me to L-Malic acid (below labelled as ( S )-Malic acid). The measured optical rotatory dispersion curve at low wavelengths is shown below (dashed line for Malic acid, solid line for Lactic acid). A sign inversion