Steric Clashes, Doughnuts and Exploding Cigars: Some Comments on Non‐Covalent Interactions

Abstract

AbstractIsosurfaces appearing in three‐dimensional Non‐Covalent Interactions (NCI) representations of molecules are attributed to bonding interactions (sign(λ2)*ρ < 0) or to non‐bonding interactions, often referred to as “steric clashes” and typical of ring systems (sign(λ2)*ρ > 0). Since the second eigenvalue of the Hessian of the electron density, λ2, is positive for a ring or a cage critical point and negative for a bond critical point, in many cases NCI isosurfaces simply surround QTAIM critical points, which means that the attribution of positive‐λ2 isosurfaces to steric clashes in rings or cages is not justified. Examples are given for sterically congested hydrocarbons, where isosurfaces associated with C–H…H–C interactions enclose bond and ring critical points. In the same way, their appearance in the absence of critical points can be attributed to a variation of λ2. It is shown that for small, very strained rings, positive‐λ2 isosurfaces are not found when the recommended plot parameters are used.