The Empire Strikes Back

Darth Bader strikes again!

The linked paper describes the application of QTAIM to the anomeric effect for N-C-N functional units.

Say you have a straight-chain molecule with a train of atoms that looks like R-Y-C-Z, where R is an alkyl chain, Y has at least one lone pair, C is carbon, and Z is an element more electronegative than carbon. Looking along the C-Y bond, R and Z actually have a preference for a gauche arrangement, rather than the antiperiplanar arrangement sterics might predict. This is the generalized anomeric effect–some electronic phenomenon is causing R and Z to overcome sterics and “go gauche.” Currently, the most widely accepted explanation for the effect is the “stereoelectronic model,” which attributes gauche stabilization to delocalization of one of Y’s lone pairs into the C-Z antibonding orbital (with which it is aligned when R and Z are gauche). Development of the SM has focused on identifying other gauche-stabilizing orbital overlaps.


Previous QTAIM work on molecules with O-CH2-O units has shown that the stereochemical model doesn’t tell the whole story, however. By calculating the molecular electron density as a function of rotation about the C-O bond, Mosquera discovered that migration of electron density from methylene and alcohol hydrogens to carbon and oxygen was a major contributor to anomeric gauche stabilization. More electron density near the heavier atoms stabilizes the molecule. A similar thing happens in N-C-N structures, although aminic and methylene hydrogens cannot both donate electron density to heavier atoms in the gauche arrangement (because of the lack of a second lone pair), which means QTAIM effects take a backseat to the traditional stereoelectronic model in N-C-N units.

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