Photo- and Radiation-Chemical Generation and Thermodynamic Properties of the Aminium and Aminyl Radicals Derived fromN-Phenylglycine and (N-Chloro,N-phenyl)glycine in Aqueous Solution: Evidence for a New Photoionization Mechanism for Aromatic Amines

Abstract

The generation of N-centered radicals (aminium and aminyl radicals) by photoionization and by pulse-radiolytic one-electron oxidation of N-phenylglycine (1), or by laser flash photolysis or reductive dechlorination of (Nchloro,N-phenyl)glycine (2) has been studied in order to model photodamage to amino acids and proteins, as well as degradation of nitrogen-containing drinking-water pollutants. The aminium radical .‡(H-N-phenyl)glycine (1.‡), a gdistonic radical zwitterion, was found as a product of photoionization (FPI(1)ˆ 0.34, 0.22, 0.30, and 0.01 at 193, 248, 266, and 308 nm) or chemical one-electron oxidation of 1. The aminyl radical .(N-phenyl)glycine (1(NyH) .), a g-distonic radical anion, is a product of reductive dehalogenation of 2, or of Nchloro photohomolysis, for which the upper limits of FNCl(2) at 193, 248, 266, or 308 nm excitation are 0.61, 0.43, 0.60, and 0.38, or of deprotonation of 1.‡ (pKa(1 .‡)ˆ 7.35 0.15). The reduction potential E8(1.‡/1) at pH 6 is 0.89 0.02 V versus a normal hydrogen electrode (NHE). The quantum yields of 193, 248, 266, or 308 nm photoionization of 2 are FPI(2)ˆ 0.29, 0.10, 0.08, and 0.01. For 1, an additional process leading to 1.‡ and H . takes place that is identified in terms of water-assisted ionization for which the quantum yields FWA(1) at 193, 248, 266, or 308 nm are 0.37, 0.26, 0.50, and 0.01, respectively. A full thermodynamic description of the system is given, including the different possible acid ± base forms, their redox potentials, and bond dissociation energies.