Spectroscopy and Bonding Analysis of Ar_nBO⁺ (n = 1-3) Cations That Possess Argon-Boron Multiple Bonds

Ar_nBO⁺ (n = 1-3) complexes have been prepared and subjected to spectroscopic characterization in the gas phase. Mass-selected infrared photodissociation spectroscopy, in combination with theoretical calculations, reveals the coexistence of two nearly isoenergetic structural isomers in Ar₂BO⁺. One isomer entails two equivalent Ar atoms chemically bound to BO⁺, while the other features an ArBO⁺ core ion accompanied by a weakly tagging argon atom. However, only the structure with an ArBO⁺ core ion was observed for the Ar₃BO⁺ complex. Quantum chemical calculations using density functional theory and ab initio methods complement the experimental work. The calculations help to identify the spectroscopically observed cations whose equilibrium structures and bond dissociation energies are given. The electronic structure and bonding situation are analyzed with a variety of theoretical methods. The ArBO⁺ core ion is characterized as having an exceptionally strong Ar-B covalent bond with some multiple bonding character.