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

Abstract

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.