Minds (and memories) can work in wonderful ways. In 1987[cite]10.1021/jo00389a050[/cite] we were looking at the properties of “stable” tetrahedral intermediates formed in carbonyl group reactions. The reaction involved adding phenylhydroxylamine to acetyl cyanide.
Previously, I explored the unusual structure of a molecule with a hydrogen bonded interaction between a phenol and a pyridine. The crystal structure name was RAKQOJ and it had been reported as having almost symmetrical N…H…O hydrogen bonds. This feature had been determined using neutron diffraction crystallography, which is thought very reliable at determining proton positions.
Proton transfers are amongst the most common of all chemical reactions.
The previous examples of four atom systems displaying two layers of aromaticity illustrated how 4 (B 4 ), 8 (C 4 ) and 12 (N 4 ) valence electrons were partitioned into 4n+2 manifolds (respectively 2+2, 6+2 and 6+6). The triplet state molecule B 2 C 2 with 6 electrons partitioned into 2π and 4σ electrons, with the latter following Baird’s aromaticity
In my previous post on the topic, I introduced the concept that data can come in several forms, most commonly as “raw” or primary data and as a “processed” version of this data that has added value. In crystallography, the chemist is interested in this processed version, carried by a CIF file.
The meeting covered the scientific life of Professor Sir Geoffrey Wilkinson from the perspective of collaborators, friends and family and celebrated three anniversaries, the centenary of his birth (2021), the half-century anniversary of the Nobel prize (2023) and 70 years almost to the day (1 April) since the publication of the seminal article on Ferrocene (2022).[cite]10.1021/ja01128a527[/cite] The meeting was organised as “inverse hybrid”
Normally, aromaticity is qualitatively assessed using an electron counting rule for cyclic conjugated rings. The best known is the Hückel 4n+2 rule (n=0,1, etc) for inferring diatropic aromatic ring currents in singlet-state π-conjugated cyclic molecules ‡ and a counter 4n rule which infers an antiaromatic paratropic ring current for the system.
I discussed in the previous post the small molecule C 4 and how of the sixteen valence electrons, eight were left over after forming C-C σ-bonds which partitioned into six σ and two π. So now to consider B 4 . This has four electrons less, and now the partitioning is two σ and two π (CCSD(T)/Def2-TZVPPD calculation, FAIR DOI: 10.14469/hpc/10157). Again both these sets fit the Hückel 4n+2 rule (n=0). Since B 4 has
When you talk π-aromaticity, benzene is the first molecule that springs to mind. But there are smaller molecules that can carry this property; cyclopropenylidene (five atoms) is the smallest in terms of atom count I could think of until now, apart that is from H 3 + which is the smallest possible molecule that carries σ-aromaticity.
Scientific data in chemistry has come a long way in the last few decades. Originally entangled into scientific articles in the form of tables of numbers or diagrams, it was (partially) disentangled into supporting information when journals became electronic in the late 1990s.[cite]10.1021/acs.orglett.5b01700[/cite] The next phase was the introduction of data repositories in the early naughties.
Ever since the concept of a shared two-electron bond was conjured by Gilbert N. Lewis in 1916,[cite]10.1021/ja02261a002[/cite] chemists have been fascinated by the related concept of a bond order (the number of such bonds that two atoms can participate in, however a bond is defined) and pushing it ever higher for pairs of like-atoms.
