Isotope Fractionation Associated with the Direct Photolysis of 4‑Chloroaniline

Compound-specific isotope analysis is a useful approach to track transformations of many organic soil and water pollutants. Applications of CSIA to characterize photochemical processes, however, have hardly been explored. In this work, we systematically studied C and N isotope fractionation associated with the direct photolysis of 4-Cl-aniline used as a model compound for organic micropollutants that are known to degrade via photochemical processes. Laboratory experiments were carried out at an irradiation wavelength of 254 nm over the pH range 2.0 to 9.0 as well as in the presence of Cs⁺ as a quencher of excited singlet 4-Cl-aniline at pH 7.0 and 9.0. We observed considerable variation of C and N isotope enrichment factors, ϵ_C and ϵ_N, between −1.2 ± 0.2‰ to −2.7 ± 0.2‰ for C and −0.6 ± 0.2‰ to −9.1 ± 1.6‰ for N, respectively, which could not be explained by the speciation of 4-Cl-aniline alone. In the presence of 1 M Cs⁺, we found a marked increase of apparent ¹³C-kinetic isotope effects (¹³C-AKIE) and decrease of 4-Cl-aniline fluorescence lifetimes. Our data suggest that variations of C and N isotope fractionation originate from heterolytic dechlorination of excited triplet and singlet states of 4-Cl-aniline. Linear correlations of ¹³C-AKIE vs ¹⁵N-AKIE were distinctly different for these two reaction pathways and may be explored further for the identification of photolytic aromatic dechlorination reactions.