FIGURE 1. Electron delocalization in amides.

a, Standard amide bonds (where R is a hydrogen atom, or any general hydrocarbon chain) are a hybrid of two forms, known as resonance structures. The double-headed arrow indicates that the true electronic structure of the bonding lies somewhere between the extremes of an apolar and a dipolar form. The electrons are actually spread out between the nitrogen, carbon and oxygen atoms. This electron delocalization, which strengthens the amide bond, is only maximized if the atoms around the carbon–nitrogen double bond in the dipolar form are coplanar. b, For 2-quinuclidone, the geometry of the molecule prevents coplanarity of the relevant bonds (marked in red) in the dipolar form. Electron delocalization is inhibited, so the carbon–nitrogen bond is weak compared with untwisted amides. c, In the dipolar form of penicillin, the bonds in red cannot all be coplanar, again inhibiting electron delocalization through resonance, and destabilizing the amide bond.