O₂^–·[Polar VOC] Complexes: H-Bonding versus Charge–Dipole Interactions, and the Noninnocence of Formaldehyde

Abstract

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.

Singlet Oxygen 1O2 in Photocatalysis on TiO2. Where Does It Come from?

“…At the same time, this process was detected for free superoxide anion O 2 – and its van der Waals complexes with water and other molecules. One-photon PD of electron from superoxide anion O 2 – by laser radiation at 355 nm gives rise to singlet oxygen O 2 (a 1 Δ g ) together with the ground state ( X 3 Σ g – ) as it was shown by photoelectron spectroscopy. − The photon energy h ν = 3.495 eV is much higher than the threshold for electron detachment from O 2 – giving rise to O 2 (a 1 Δ g ), which is equal to 1.43 eV. The spectral maximum of this PD efficiency corresponds to the “vertical” electron PD with photon of about 2 eV energy (wavelength of about 620 nm) .…”

“…The polar environment provides the solvation of superoxide anion, which shifts the threshold of PD to the higher energy value. However, the energy of the photon of radiation at 355 nm is still sufficient for one-photon PD of electron providing 1 O 2 from the complexes of O 2 – with polar organic molecules and from complex with one water molecule O 2 – (H 2 O). − The further increase in the number of water molecules in the hydration shell O 2 – (H 2 O) 2 and O 2 – (H 2 O) 3 makes the energy threshold for 1 O 2 production equal or close to the energy of photon at 355 nm . Thus, we expect that 1 O 2 production by PD of electron from a highly hydrated superoxide anion should be a two-photon process at 355 nm.…”

Reply to “Comment on ‘Singlet Oxygen 1O2 in Photocatalysis on TiO2. Where Does It Come from?’”

“…This claim is not correct. The fact of the one-photon electron photodetachment from • O 2 – by radiation at 355 nm is well established in many studies carried out with the superoxide anion and its van der Waals complexes in molecular beams − (refs 35–40 in ref ). Probably the authors of the Comment mean that only a minor part of the pulse energy in experiments of ref could be used for the photodetachment.…”

Leonurine ameliorates D-galactose-induced aging in mice through activation of the Nrf2 signalling pathway

“…The superoxide anion radical assay was performed according to a previously modified method [56]. Briefly, 1 ml of the 30-200 μg/ml leonurine solutions were first diluted to 3 ml with 0.05 mM Tris-HCl buffer (pH 8.2), and then 100 μl pyrogallol was added and mixed well.…”