O·[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 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, an effect more pronounced in complexes with H-bonding. In addition, broken degeneracy of the O-local π orbitals in the complexes results in the stabilization of the low-lying excited O (a Δ)·[polar VOC] state relative to the ground O (X Σ)·[polar VOC] state when compared to bare O. The spectra of the O·[polar VOC] complexes exhibit less pronounced laser photoelectron angular distribution (PADs). The spectrum of O·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 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 π 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.