High-Resolution Photoelectron Imaging of Cryogenically-Cooled B8- and CB7- Clusters: From Borozene to Carborozene.

The closed-shell B73-, B82-, 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 B82- 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 B82-. 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 B82- borozene to the CB7- carborozene is similar to that from the B12H122- borane to the CB11H12- carborane.