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• W2065206795 endingPage "11926" @default.
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• W2065206795 abstract "This work has examined the photoreactivity of benzophenone (3), 2-benzoylthiophene (4), 4-methoxybenzophenone (5), 4,4′-dimethoxybenzophenone (6), and 4-carboxybenzophenone (7) with 2-aminobenzimidazole (1). Laser flash photolysis (LFP) revealed quenching of the aromatic ketone triplets by 1, leading to formation of ketyl radicals plus aminyl radical 1-H•. The quenching rate constants obtained for 3 (nπ* triplet) and 4 (ππ* triplet) were 6.2 × 109 and 3.9 × 109 M−1 s−1, respectively. The similarity between the two values suggests that the process is not a pure hydrogen abstraction but rather a charge transfer followed by proton transfer. This is in agreement with thermodynamic calculations, using the Rehm−Weller equation. In the case of 5 and 6, the transient absorption spectra showed distinct bands corresponding to both types of triplets (nπ* and ππ*); their ratio was found to depend on solvent polarity. In the presence of 1, spectral changes were also consistent with formation of the aminyl/ketyl radical pairs. The rate constants for quenching of both types of triplets were very high, in the range 109−1010 M−1 s−1. When an electron acceptor substituent was attached to the aromatic ring, as in 7 (nπ* triplet), the quenching rate constant was higher (8 × 109 M−1 s−1), close to diffusion control. The reaction mechanism for hydrogen abstraction from 1 by triplet excited 3 or 4 was theoretically studied using density functional theory (DFT) methods. The results suggest formation of ground state molecular complexes, where one electron is transferred from the 2-aminobenzimidazole to the benzophenone or benzoylthiophene moieties upon excitation, giving radical ion pairs; subsequent proton transfer from the amino group to the carbonyl oxygen atoms leads to the neutral biradicals. A comparison between the relative energies and geometries of the species involved in the photochemical reactions indicates that all ketones follow a similar mechanism." @default.
• W2065206795 created "2016-06-24" @default.
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• W2065206795 date "2010-08-24" @default.
• W2065206795 modified "2023-09-23" @default.
• W2065206795 title "Experimental and Theoretical Studies on the Mechanism of Photochemical Hydrogen Transfer from 2-Aminobenzimidazole to nπ* and ππ*Aromatic Ketones" @default.
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• W2065206795 doi "https://doi.org/10.1021/jp1053327" @default.
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