High-ResolutionPhotoelectron Imaging of Cryogenically-CooledB₈^– and CB₇^– Clusters: From Borozene to Carborozene

The closed-shell B₇^3–, B₈^2–, and B₉^– species are recognized recently to be electron-precise molecular wheels with three delocalized π-bonds reminiscent of benzene, giving rise to the concept of “borozene”. The B₈^2– borozene is especially stable because the B₇ ring has the right size to host a central boron atom. Replacing a B atom by C yields a highly stable closed-shell CB₇^–, which is isoelectronic to B₈^2–. Here we use high-resolution cryogenic photoelectron imaging to probe B₈^– and CB₇^–, revealing rich vibrational information about B₈ and CB₇ and the vibrational modes responsible for the structure changes from the anions to the neutrals. Surprisingly, a minor isomer is also observed for B₈^– and found to be due to Jahn–Teller splitting, analogous to the Jahn–Teller effect in the C₆H₆⁺ benzene cation. The transformation from the B₈^2– borozene to the CB₇^– carborozene is similar to that from the B₁₂H₁₂^2– borane to the CB₁₁H₁₂^– carborane.