Structure and Fluxionality of B Probed by Infrared Photodissociation Spectroscopy.

We use cryogenic ion vibrational spectroscopy to characterize the structure and fluxionality of the magic number boron cluster B . The infrared photodissociation (IRPD) spectrum of the D -tagged all- B isotopologue of B is reported in the spectral range from 435 to 1790 cm and unambiguously assigned to a planar boron double wheel structure based on a comparison to simulated IR spectra of low energy isomers from density-functional-theory (DFT) computations. Born-Oppenheimer DFT molecular dynamics simulations show that B exhibits internal quasi-rotation already at 100 K. Vibrational spectra derived from these simulations allow extracting the first spectroscopic evidence from the IRPD spectrum for the exceptional fluxionality of B .