Journal of Physical Chemistry A, Vol.121, No.29, 5459-5467, 2017
O-2(-)center dot[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 pi g orbitals in the complexes results in the stabilization of the low-lying excited O-2 (a (1)Delta(g))center dot[polar VOC] state relative to the ground O-2 (X-3 Sigma(-)(g))center dot[polar VOC] state when compared to bare O-2. The spectra of the O-2(-)center dot[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 pi(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.