Posts Tagged ‘resonance’

The Many Guises of Aromaticity » American Scientist

Friday, January 23rd, 2015

The Many Guises of Aromaticity #Resonance is hyped; hence, many proposals for compounds w/ it that aren’t benchstable

Today, an inflation of hype threatens this beautiful concept. Molecules constructed in silico are extolled as possessing surfeits of aromaticity—“doubly aromatic” is a favorite descriptor. Yet the molecules so dubbed have precious little chance of being made in bulk in the laboratory. One can smile at the hype, a gas of sorts, were it not for its volume. A century and a half after the remarkable suggestion of the cyclic structure of benzene, the conceptual value of aromaticity—so useful, so chemical—is in a way dissolving in that hype. Or so it seems to me.

Bench-Stable, Bottleable

Computers made the determination of the structure of molecules in crystals easy—what took half a year in 1960 takes less than an hour today. They also made computations of the stability of molecules facile.

Whoa! What do you mean by stability? Usually what’s computed is stability with respect to decomposition to atoms. But that is pretty meaningless; for instance, of the four homonuclear diatomic molecules (composed of identical atoms) that are most stable with respect to atomization, N2,C2, O2, and P2, two (C2 and P2) are not persistent. You will never see a bottle of them. Nor the tiniest crystal. They are reactive, very much so. In chemistry it’s the barriers to reaction that provide the opportunity to isolate a macroscopic amount of a compound. Ergo the neologism, “bench-stable.” “Bottleable” is another word for the idea. A lifetime of a day at room temperature allows a competent graduate student at the proverbial bench to do a crystal structure and take an NMR scan of a newly made compound. Or put it into a bottle and keep it there for a day, not worrying that it will turn into brown gunk.