O 2 - ·[Polar VOC] Complexes: H-Bonding versus Charge-Dipole Interactions, and the Noninnocence of Formaldehyde.

Anion photoelectron imaging was used to measure the photodetachment spectra of molecular complexes formed between O ₂ ^- and a range of atmospherically relevant polar molecules, including species with a carbonyl group (acetone, formaldehyde) and alcohols (ethanol, propenol, butenol). Experimental spectra are analyzed using a combination of Franck-Condon simulations and electronic structure calculations. Strong charge-dipole interactions and H-bonding stabilize the complex anions relative to the neutrals, resulting in a ca. 1 eV increase in electron binding energy relative to bare O ₂ ^- , an effect more pronounced in complexes with H-bonding. In addition, broken degeneracy of the O ₂ -local π _g orbitals in the complexes results in the stabilization of the low-lying excited O ₂ (a ¹ Δ _g )·[polar VOC] state relative to the ground O ₂ (X ³ Σ _g ^- )·[polar VOC] state when compared to bare O ₂ . The spectra of the O ₂ ^- ·[polar VOC] complexes exhibit less pronounced laser photoelectron angular distribution (PADs). The spectrum of O ₂ ^- ·formaldehyde is unique in terms of both spectral profile and PAD. On the basis of these experimental results in addition to computational results, the complex anion cannot be described as a distinct O ₂ ^- anion partnered with an innocent solvent molecule; the molecules are more strongly coupled through charge delocalization. Overall, the results underscore how the symmetry of the O ₂ π _g orbitals is broken by different polar partners, which may have implications for atmospheric photochemistry and models of solar radiation absorption that include collision-induced absorption.