Photochemistry of 4‑Chlorophenol in Liquid and Frozen Aqueous Media Studied by Chemical, Compound-Specific Isotope, and DFT Analyses Anna Zelmer, † Ning Zhang, § Kater ̌ ina Komínkova ́ , † Dana Nachtigallova ́ , ∥ Hans Hermann Richnow, § and Petr Kla ́ n* ,†,‡ † RECETOX, Faculty of Science and ‡ Department of Chemistry, Faculty of Science, Masaryk University, Kamenice 5, 625 00 Brno, Czech Republic § Department of Isotope Biogeochemistry, Helmholtz Centre for Environmental Research-UFZ, Permoserstrasse 15, 04318 Leipzig, Germany ∥ Institute of Organic Chemistry and Biochemistry, Flemingovo nam. 2, 166 10 Prague, Czech Republic * S Supporting Information ABSTRACT: The photochemistry of 4-chlorophenol in liquid and frozen aqueous solutions and on the surface of ice grains yields substantially different photoproducts. Several complementary experimental and theoretical methods, such as trace analyses of the photoproducts, trapping experiments, compound- specific isotope analyses, and quantum chemical calculations, were used to study the reaction mechanism differences. A similar carbon kinetic isotope effect determined for the photolysis of 4-chlorophenol samples in the temperature range of 20 to −40 °C and the results of trapping experiments suggest that heterogeneous cleavage of the C−Cl bond in the excited state is probably a common key step that leads to the formation of carbene and hydroxyphenyl cation intermediates. We conclude that the subsequent specific reactions of these species under various conditions are responsible for the formation of different final photoproducts. ■ INTRODUCTION A large part of the Earth’s surface is permanently or temporarily covered by snow and ice. 1 Many (semi)volatile natural and anthropogenic organic compounds have been identified in cold environments. 2−4 The complex physicochemical properties of snow and ice at different ambient conditions play an important role in the air−snow/ice exchange processes of these trace impurities. 2,5,6 Despite low temperatures, many compounds can undergo various photochemical as well as dark reactions at the snow/ice−air interface. 2,5,7 A number of laboratory studies have demonstrated that the photodegradation products formed in ice may differ from those formed in the liquid aqueous phase. 2,5,8,9 The resulting new products can pose a toxicological risk 10 when they enter the environment by evaporation or during snowmelt. 2,6 Moreover, photodegradation on ice may be an important sink of contaminants in the environment. 2 Halophenols are semivolatile organic compounds that can be found in various environmental compartments, such as surface and ground waters, soils, or air. 11 They have been produced as herbicides, insecticides, fungicides, antiseptics, or wood preservatives or as synthetic intermediates in the chemical industry 12 and can be transformed, for example, to toxic dioxins. 13 The photochemistry of 4-chlorophenol (1), as a typical representative of halophenols, has received recurrent attention aimed at the development of techniques for the photodegradation of organic pollutants. 14 The 4-chlorophenol photoreaction mechanism has been investigated using steady-state and time-resolved techniques. In neutral aqueous solutions, 1 absorbs significantly in the region of 250−295 nm with an absorption maximum at 280 nm. 9 Upon excitation, 1 undergoes efficient intersystem crossing (Φ isc ∼ 1) to its triplet state in both aprotic and protic solvents (Scheme 1). 15 Subsequently, dehalogenation via homolytic cleavage of the C−Cl bond is the major reaction pathway in nonpolar solvents. 15,16 The photoreaction is less efficient in polar aprotic media, such as acetonitrile or ethyl acetate, because the triplet state is short-lived, but in aqueous and alcoholic solutions, heterolytic C−Cl bond scission proceeds with a quantum yield of 0.75. 15 Grabner and co-workers studied the photochemistry of 1 in aqueous solutions using nanosecond laser flash photolysis (LFP). 15 It was shown that a triplet carbene, 4-oxocyclohexa-2,5-dienylidene (3), 15,17,18 formed by HCl elimination from the triplet excited chlorophenol (1 3 *) or via a (still undetected) triplet hydroxyphenyl cation (2), is the major intermediate for the production of the photoproducts, 15 such as phenol (4) produced in the presence of hydrogen donors, hydroquinone (5) and benzoquinone (6) formed by reaction with water and in the presence of molecular oxygen, respectively, and a coupling product, 5-chlorobiphenyl-2,4′-diol Received: August 11, 2015 Revised: September 15, 2015 Article pubs.acs.org/Langmuir © XXXX American Chemical Society A DOI: 10.1021/acs.langmuir.5b02990 Langmuir XXXX, XXX, XXX−XXX