Ring strain relaxation

Strain of small ring molecules cannot be fully understood in terms of the deviation of the bond angles from the ideal ones of the hybrid orbitals. The mechaitism of the relaxation of the strain has been proposed. Here, we briefly review a relaxation by the a—kj interactions between the geminal a bonds in the rings and % relaxation by the Ji—kj interaction between the endocyclic n bond with the vicinal a bonds on the ring atoms. A more detailed review is made by Naruse and Inagaki elsewhere in this volume. 

Keywords tr-Relaxation o-Relaxation Geminal interaction Inverted bond Lone pair effect Orbital phase continuity Ring strain... 

Cremer proposed that the higher electron density inside the ring would relax the strain [9]. The surface delocalization stabilizes the lowest orbital of the ring system, which might relax the ring strain. 

The solid-state structures of orthorhombic S864 and monoclinic Se866 are shown in Figure 15. As may be seen from the average E-E-E bond and EE EE torsion angles, this structure is almost completely relaxed and consequently lone pair repulsion and ring strain are lowest in this arrangement. Other S8 structures were assessed by quantum chemical calculations.69... 

From this state, ring strain facilitated predissociation to a "biradical-like" transition state [135] or vibrational relaxation (k ) to S may occur. It is also conceivable that transition state [135] could be produced directly from S °. Alternatively, molecules in the S ° state could intersystem cross (kST) to the triplet manifold (T ). For 2-alkylidenecyclobutanones, reactivity is manifested in isomerization about the exocyclic carbon-carbon double bond, while for the saturated cyclobutanone derivatives studied, definitive evidence for solution-phase reactivity is not available. If analogy is again made to the vapor-phase photochemistry of cyclobutanone [21], reactivity could conceivably result in decarbonylated products. Indeed, preliminary evidence has been obtained from sensitization experiments employing m-xylene as triplet sensitizer that decarbonylation of a saturated cyclobutanone is enhanced by selective population of its state (35). ... 

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