Photoelectron spectroscopy of arylboronic acids

Highlights: • Electronic structure of phenylboronic acids. • Valence ionization energies. • Nature of interactions between boron atom and aromatic ring. We report the gas-phase UV photoelectron spectra (UPS) of some arylboronic acids for the first time. The electronic structures of 2-methyl-, 3-methyl- and 3-chloro-phenylboronic acids and 3-thienyl-boronic acid have been studied in the gas phase by HeI/HeII photoelectron spectroscopy and EOM-CCSD quantum chemical calculations. The comparison of the spectra of arylboronic acids with the spectra of toluene, chlorobenzene and thiophene shows remarkably small changes in the ionization energies of the highest occupied π-orbitals upon the attachment of the boronic acid group B(OH)₂ to the aryl core. The appearance of additional spectral bands with ionization energies of 10.5–11 eV corresponds to ionization of CB σ-bonding orbitals.