The Role of pH in the Mechanism of^.OH Radical Induced Oxidation of Nicotine

Abstract

Abstract The . OH radical induced oxidation of nicotine was studied using pulse radiolysis techniques from pH 1 to 13.6. Theoretical calculations were used to help interpret the experimental results. The bond dissociation enthalpies of all of the CH bonds of nicotine were determined using DFT calculations, coupled with the isodesmic reaction. From time‐dependent density functional response theory, estimates were obtained of the location of the dominant transient absorption bands ( λ max ), their intensities (electronic oscillator strength, f ), and the electronic composition of these transitions. OH radicals as well as other potent oxidants reacted with free nicotine through separated, concerted electronproton transfer, leading mostly to the formation of an alpha‐aminoalkyl radical located on the C 2′ carbon of the aliphatic ring ( A 2′ ). Protonated nicotine underwent hydrogen atom abstraction at the C 2′ and N 1′ positions, resulting in the formation of the conjugate acid of A 2′ ( A 2′ H + ) and the alkylamine radical cation ( N + ), respectively. Doubly protonated nicotine underwent the same reaction pathways, leading to two corresponding conjugate acid species, protonated at the aromatic nitrogen position: PyrH + A 2′ H + and PyrH + N + . All these radicals interconverted between each other through hydrolytic reactions. The radical A 2′ and its conjugate acid PyrH + A 2′ absorbed 10 times stronger than the N + species, based on calculations of f . From the growth of the transient absorption of A 2′ ( λ max =330 nm, ε =8080 M −1 cm −1 ), second‐order rate constants were determined: k (OH+Nic) =6.7×10 9 M −1 s −1 , k (OH+NicH) =1.0×10 9 M −1 s −1 . The alpha‐aminoalkyl radicals decayed by disproportionation to form iminium cations 1 – 5 , which contributed to an increase in the specific conductivity of the basic solutions of nicotine following electron pulse irradiation.