A time-resolved study of the multiphase chemistry of excited carbonyls: Imidazole-2-carboxaldehyde and halides

Imidazole-2-carboxaldehyde (IC) reactivity in the presence of halide anions (Cl^–, Br^–, I^–) has been studied by laser flash photolysis in aqueous solution at room temperature. The absorption spectrum of the triplet state of IC has been measured with a maximum absorption at 330 nm and a weaker absorption band around 650 nm. Iodide anions proved to be efficient quenchers of the triplet state IC, with a rate coefficient k_q of (5.33 ± 0.25) × 10⁹ M^?1 s^?1. Quenching by bromide and chloride anions was less efficient, with k_q values of (6.27 ± 0.53) × 10⁶ M^?1 s^?1 and (1.31 ± 0.16) × 10⁵ M^?1 s^?1, respectively. The halide (X^–) quenches the triplet state; the resulting transient absorption feature matches that of the corresponding radical anion (X₂^–). We suggest that this type of quenching reactions is a driving force of oxidation reactions in the oceanic surface microlayer (SML) and a source of halogen atoms in the atmosphere.