Heterogeneous and Photochemical Reactions of Solid Benzophenone−Catechol Films with NO2

Abstract

Solid−air interfaces are ubiquitous to the atmosphere, and heterogeneous reactions between gaseous oxidants and surface adsorbed organics on these interfaces can impact tropospheric chemistry. Solid benzophenone−catechol films serve as model photosensitizer−polyphenol compounds that we reacted with NO2 in the parts-per-billion range under dark and light conditions at 300 K and 20% relative humidity. Attenuated total reflectance infrared spectroscopy (ATR-FTIR) monitored chemical changes in the organic film during these reactions to directly identify condensed-phase products. Catechol, when mixed with benzophenone or dicyclohexyl ketone, reacted with NO2 under dark conditions, forming 4-nitrocatechol as the exclusive condensed-phase product; pure catechol films did not react. Kinetic isotope experiments found rate[C6H4(OH)2]/rate[C6H4(OD)2] = 3.3 ± 0.5, indicating that breaking an O−H bond was critical to the rate-determining step. A mechanism involving the ortho-semiquinone radical, possibly stabilized by...