Probing structural trends in neutral boron oxide clusters by potassium-tagged infrared photodissociation spectroscopy.

Neutral BxOy clusters are important model systems for understanding key structural motifs relevant to borate materials and inorganic boron oxides. Here, we report infrared photodissociation (IRPD) spectroscopy of mass-selected gas-phase potassium-tagged boron oxide clusters, in terms of [B2O2-4K]+, [B3O4,5K]+ and [B4O6K]+, by monitoring the K+ photofragment yield. Comparison of IRPD spectra with quantum chemistry calculations allows us to determine their structures and bonding nature. [B2O2K]+ is identified as having a quasi-linear K+-bound BO-OB motif. [B2O3K]+ is assigned to a K+-tagged structure derived from neutral C2v B2O3, whereas [B2O4K]+ is assigned to an open-chain isomer containing a significantly elongated O-O bond. The lowest-energy structures of [B3O4K]+, [B3O5K]+ and [B4O6K]+ clusters contain rhombic B2O2 rings with terminal OB-O units. These assigned lowest-energy [BxOyK]+ structures reproduce the observed IRPD spectra and retain a close structural framework to that of the corresponding neutral oxide clusters, suggesting that K+ acts as a mildly perturbing spectroscopic tag. Chemical bonding analyses reveal unusual delocalized bonding patterns, including partially delocalized 3c-2e π O-B-O bonding within the B-O skeleton and delocalized 4c-4e o-bonding in the rhombic B2O2 ring. These results demonstrate that K+-tagged IRPD spectroscopy is a powerful approach for probing structural trends in neutral oxygen-rich boron oxide clusters.