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

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 hydrogen bonding to the coadsorbed ketone, is discussed. The reaction was also found to be second order (2.09 ± 0.18) with respect to NO₂, suggesting a possible pre-equilibrium with N₂O₄. Although benzophenone is a well-known photosensitizer, the rate of 4-nitrocatechol formation was not enhanced by UV-A/visible radiation. This observation eliminates this pathway as a possible photoenhanced daytime source of HONO. However, ATR-FTIR detected additional photochemical products resulting from a photoinitiated reaction between benzophenone and 4-nitrocatechol. These results highlight the potential for heterogeneous chemistry involving surface adsorbed organics to form nitroaromatic compounds, which are of interest due to their phytotoxic and UV absorbing properties.