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Unravelling the Keto-Enol Tautomer Dependent Photochemistry and Degradation Pathways of the Protonated UVA Filter Avobenzone

preprint
submitted on 14.02.2020 and posted on 17.02.2020 by Jacob A. Berenbeim, Natalie Wong, Martin C. R. Cockett, Giel Berden, Jos Oomens, Anouk Rijs, Caroline E. H. Dessent
Avobenzone (AB) is a widely used UVA filter known to undergo irreversible photodegradation. Here, we investigate the detailed pathways by which AB photodegrades by applying UV laser-interfaced mass spectrometry to protonated AB ions. Gas-phase infrared multiple-photon dissociation (IRMPD) spectra obtained with the free electron laser for infrared experiments, FELIX, (600-1800 cm-1) are also presented to confirm the geometric structures. The UV gas-phase absorption spectrum (2.5-5 eV) of protonated AB contains bands that correspond to selective excitation of either the enol or diketo forms, allowing us to probe the resulting, tautomer-dependent photochemistry. Numerous photofragments (i.e. photodegradants) are directly identified for the first time, with m/z 135 and 161 dominating, and m/z 146 and 177 also appearing prominently. Analysis of the production spectra of these photofragments reveals that that strong enol to keto photoisomerism is occurring, and that protonation significantly disrupts the stability of the enol (UVA active) tautomer. Close comparison of fragment ion yields with the TDDFT-calculated absorption spectra give detailed information on the location and identity of the dissociative excited state surfaces, and thus provide new insight into the photodegradation pathways of avobenzone, and photoisomerisation of the wider class of β-diketone containing molecules.

Funding

Horizon 2020

Leverhulme Trust Project Grant RPG-2017-147

History

Email Address of Submitting Author

caroline.dessent@york.ac.uk

Institution

University of York

Country

United Kingdom

ORCID For Submitting Author

0000-0003-4944-0413

Declaration of Conflict of Interest

No conflict of interest

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