Berenbeim Jacob A, Wong Natalie G K, Cockett Martin C R, Berden Giel, Oomens Jos, Rijs Anouk M, Dessent Caroline E H
Department of Chemistry, University of York, Heslington, York, YO10 5DD, U.K.
Institute for Molecules and Materials, FELIX Laboratory, Radboud University, Toernooiveld 7, Nijmegen, 6500 HC, The Netherlands.
J Phys Chem A. 2020 Apr 16;124(15):2919-2930. doi: 10.1021/acs.jpca.0c01295. Epub 2020 Apr 2.
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) 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 / 135 and 161 dominating, and / 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 TD-DFT-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 photoisomerization of the wider class of β-diketone containing molecules.
阿伏苯宗（AB）是一种广泛使用的UVA滤光剂，已知会发生不可逆的光降解。在此，我们通过将紫外激光接口质谱应用于质子化的AB离子，研究AB光降解的详细途径。还展示了利用用于红外实验的自由电子激光FELIX（600 - 1800厘米）获得的气相红外多光子解离（IRMPD）光谱，以确认几何结构。质子化AB的紫外气相吸收光谱（2.5 - 5电子伏特）包含对应于烯醇或二酮形式选择性激发的谱带，这使我们能够探究由此产生的、取决于互变异构体的光化学。首次直接鉴定出许多光碎片（即光降解产物），其中m/z 135和161占主导，m/z 146和177也显著出现。对这些光碎片的产生光谱分析表明，正在发生强烈的烯醇到酮的光异构化，并且质子化显著破坏了烯醇（UVA活性）互变异构体的稳定性。将碎片离子产率与TD - DFT计算的吸收光谱进行密切比较，可提供有关解离激发态表面的位置和身份的详细信息，从而为阿伏苯宗的光降解途径以及更广泛的含β - 二酮分子的光异构化提供新的见解。
