Inverse Kinetic Solvent Isotope Effect in TiO2 Photocatalytic Dehalogenation of Non‐adsorbable Aromatic Halides: A Proton‐Induced Pathway

An efficient redox reaction between organic substrates in solution and photoinduced h⁺_vb/e^?_cb on the surface of photocatalysts requires the substrates or solvent to be adsorbed onto the surface, and is consequentially marked by a normal kinetic solvent isotope effect (KSIE≥1). Reported herein is a universal inverse KSIE (0.6–0.8 at 298 K) for the reductive dehalogenation of aromatic halides which cannot adsorb onto TiO₂ in a D₀]methanol/D₄]methanol solution. Combined with in situ ATR‐FTIR spectroscopy investigations, a previously unknown pathway for the transformation of these aromatic halides in TiO₂ photocatalysis was identified: a proton adduct intermediate, induced by released H⁺/D⁺ from solvent oxidation, accompanies a change in hybridization from sp² to sp³ at a carbon atom of the aromatic halides. The protonation event leads these aromatic halides to adsorb onto the TiO₂ surface and an ET reaction to form dehalogenated products follows.