Gas phase structures and charge localization in small aluminum oxide anions: Infrared photodissociation spectroscopy and electronic structure calculations

We use cryogenic ion trap vibrational spectroscopy in combination with quantum chemical calculations to study the structure of mono- and dialuminum oxide anions. The infrared photodissociation spectra of D₂-tagged AlO_1-4⁻ and Al₂O_3-6⁻ are measured in the region from 400 to 1200 cm⁻¹. Structures are assigned based on a comparison to simulated harmonic and anharmonic IR spectra derived from electronic structure calculations. The monoaluminum anions contain an even number of electrons and exhibit an electronic closed-shell ground state. The Al₂O_3-6⁻ anions are oxygen-centered radicals. As a result of a delicate balance between localization and delocalization of the unpaired electron, only the BHLYP functional is able to qualitatively describe the observed IR spectra of all species with the exception of AlO₃⁻. Terminal Al–O stretching modes are found between 1140 and 960 cm⁻¹. Superoxo and peroxo stretching modes are found at higher (1120-1010 cm⁻¹) and lower energies (850-570 cm⁻¹), respectively. Four modes in-between 910 and 530 cm⁻¹ represent the IR fingerprint of the common structural motif of dialuminum oxide anions, an asymmetric four-member Al–(O)₂–Al ring.