Bond-stretch isomerism refers to isomers that differ simply in their bond lengths. Seppelt and coworkers suggests that the hexafluorobenzne radical cation C6F6.+ exhibits bond-stretch isomerism.1

The oxidized C6F6 with O2+SbF11- and obtained C6F6.+Sb­2F11- as a crystalline solid. X-ray diffraction identified 2 structures. B3LYP/TZPP computations confirmed the identity of two isomers, a “quinoid” form 1 and a “bisallyl” form 2, shown in Figure 1. The two structures are nearly degenerate, with 1 predicted to be 0.09 kcal mol-1 more stable than 2. The computed two unique C-C bond lengths are 1.371 and 1.427 Å in 1 and 1.449 and 1.389 Å in 2, and these distance agree well with the X-ray experimental values.

1 – quinoid

2 – bisallyl

Figure 1. UB3LYP/6-311+G(d) optimized structures of 1 and 2. Note once again the article and supporting materials lacked the full description of these structures!)

The potential energy surface in the neighborhood of these two isomers is like that of a sombrero. The two isomers lie in the circular trough and movement around this trough is nearly flat. The peak of the sombrero is the D6h structure, which is a transition state interconverting 1 and 2, with a barrier of 3 kcal mol-1.

1 and 2 are clear examples of bond-stretch isomerism, though it is likely that the complexation with the counter ion is what freezes out the rapid interconversion of the two.

References

(1) Shorafa, H.; Mollenhauer, D.; Paulus, B.; Seppelt, K., "The Two Structures of the Hexafluorobenzene Radical Cation C6F6.+," Angew. Chem. Int. Ed. 2009, 48, 5845-5847, DOI: 10.1002/anie.200900666