- Phototransformation of 5-nitro-2-furaldehyde in aqueous solution. A laser flash photolysis and product analysis study
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Laser flash photolysis of 5-nitro-2-furaldehyde (NFA) in solution shows a short-lived transient absorption with λmax = 475 ± 5 nm, which is relatively insensitive to solvent polarity and is assigned to the lowest triplet state of NFA (3NFA?). In water, the 3NFA? absorption decays to a long-lived absorption, the study of which, at different times after the end of the laser pulse, reveals it to be due to a furyloxyl radical (λmax ≈ 375 nm) and to the radical anion NFA- (λmax ≈ 400 nm). These radicals were produced independently to confirm the assignment. The lifetime of 3NFA? depends both on the solvent and the ground-state concentration of NFA. An (n, π?) nature is attributed to 3NFA? on the basis of the propensity of 3NFA? to abstract a hydrogen-atom from the solvent. Kinetic evidence for triplet excimer formation was obtained from the self-quenching of 3NFA? in solvents where the triplet decay is slower. The effect of acidity on the triplet lifetime is discussed with respect to an electron-transfer self-quenching mechanism, assisted by the triplet excimer which is proposed to dissociate into radical ions. Chromatographic and spectroscopic analysis of the photolysed aqueous solution of NFA enabled the identification of 5-hydroxymethylene-2(5H)-furanone, nitrite ion and an unknown substance as the major photoproducts. Conclusive evidence is presented that the observed 5-hydroxymethylene-2(5H)-furanone is formed from the furyloxyl radical. It is shown that the unknown substance can also be obtained from both the photoreduction of NFA in propan-2-ol and chemical reduction of NFA by Fe(s) in water (along with 5-amino-2-furaldehyde). Based on 1H- and 13C-NMR (with 2-D HMQC) and vibrational absorption spectroscopy, a tentative structure is proposed for the substance of tR 3.69 minutes obtained as a photoreduction product of NFA in water. Inorganic anions are shown to be one-electron oxidised by 3NFA? (as indicated by the observation of both the radical anion of NFA and the inorganic radical) with second-order rate constants being dependent on E17 of the inorganic radical. The implications of the results from complete quenching of 3NFA? by inorganic anions, and subsequent product analysis, for the phototransformation mechanism of NFA in water are discussed.
- Martins, Luis J.A.,Ferreira, Jo?o M. M.
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p. 721 - 735
(2017/07/07)
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