ChemInform Abstract: Relaxation of Ring Strains

Relaxation of strain in small ring molecules is reviewed in terms of two mechanisms – σ-relaxation and π-relaxation. The σ-relaxation occurs in in-plane interaction of the ring σ bonds. Previously proposed σ-aromaticity and surface delocalization are briefly discussed. The geminal interaction theory for the σ-relaxation of ring strains is discussed and applied. The geminal interaction is less antibonding or more bonding with decrease in the bond angle, when the hybrid orbital has low s-character for the ring bonds or has an s-rich lone pair (lone pair effect). π-Relaxation of unsaturated ring molecules results from the cyclic delocalization of π electrons through the σ bonds on the saturated ring atoms. The two mechanisms of the σ- and π-relaxation are shown to be necessary for better understanding of ring strains.