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

The COH 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 ex- perimental results. The bond dissociation enthalpies of all of the CH bonds of nicotine were determined using DFT calculations, coupled with the isodesmic reaction. From time-dependent density functional response theory, esti- mates were obtained of the location of the dominant transi- ent absorption bands (lmax), their intensities (electronic os- cillator strength, f), and the electronic composition of these transitions. OH radicals as well as other potent oxidants re- acted with free nicotine through separated, concerted elec- tronproton transfer, leading mostly to the formation of an alpha-aminoalkyl radical located on the C2' carbon of the ali- phatic ring (A2'). Protonated nicotine underwent hydrogen atom abstraction at the C2' and N1' positions, resulting in the formation of the conjugate acid of A2' (A2'H + ) and the al- kylamine radical cation (N + ), respectively. Doubly protonat- ed nicotine underwent the same reaction pathways, leading to two corresponding conjugate acid species, protonated at the aromatic nitrogen position: PyrH + A2'H + and PyrH + N + . All these radicals interconverted between each other through hydrolytic reactions. The radical A2' and its conju- gate acid PyrH + A2' absorbed 10 times stronger than the N + species, based on calculations of f. From the growth of the transient absorption of A2' (lmax = 330 nm, e = 8080 m � 1 cm � 1 ), second-order rate constants were deter- mined: 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 dispro- portionation to form iminium cations 1-5, which contribut- ed to an increase in the specific conductivity of the basic solutions of nicotine following electron pulse irradiation.