The topic of this post originates from a recent article which is attracting much attention.[cite]10.1038/s41586-019-1059-9[/cite] The technique uses confined light to both increase the spatial resolution by around three orders of magnitude and also to amplify the signal from individual molecules to the point it can be recorded. To me, Figure 3 in this article summarises it nicely (caption: visualization of vibrational normal modes ).
There is a predilection amongst chemists for collecting records; one common theme is the length of particular bonds, either the shortest or the longest.
A bond index (BI) approximately measures the totals of the bond orders at any given atom in a molecule. Here I ponder what the maximum values might be for elements with filled valence shells.
Another post inspired by a comment on an earlier one; I had been discussing compounds of the type I.I n (n=4,6) as possible candidates for hypervalency.
Last year, this article[cite]10.1038/nchem.2716[/cite] attracted a lot of attention as the first example of molecular helium in the form of Na 2 He. In fact, the helium in this species has a calculated ‡ bond index of only 0.15 and it is better classified as a sodium electride with the ionisation induced by pressure and the presence of helium atoms.
I discussed the molecule the molecule CH3F2- a while back.
Recollect the suggestion that diazomethane has hypervalent character[cite]10.1039/C5SC02076J[/cite]. When I looked into this, I came to the conclusion that it probably was mildly hypervalent, but on carbon and not nitrogen. Here I try some variations with substituents to see what light if any this casts.
In the previous post, I referred to a recently published review on hypervalency[cite]10.1039/C5SC02076J[/cite] which introduced a very simple way (the valence electron equivalent γ) of quantifying the effect. Diazomethane was cited as one example of a small molecule exhibiting hypervalency (on nitrogen) by this measure.
I started this story by looking at octet expansion and hypervalence in non-polar hypercoordinate species such as S(-CH 3 ) 6 , then moved on to S(=CH 2 ) 3 . Finally now its the turn of S(≡CH) 2 . ‡ As the triple bonds imply, this seems to represent twelve shared valence electrons surround the sulfur, six from S itself and three from each carbon.
Previously: “Non-polar” species such as SeMe6, SMe6, ClMe3, ClMe5 all revealed interesting properties for the Se-C, S-C or Cl-C “single” bonds. The latter two examples in particular hinted at internal structures for these single bonds, as manifested by two ELF basins for some of the bonds.