Volume 225, Issue 8 -

The oxidation of ascorbate with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical in water and in water-acetonitrile (1:1 v/v) mixed solvent has been demonstrated to be a proton-coupled electron transfer (PCET) process, involving hydrogen tunnelling at room temperature. The observations of significant changes in isotope effect on the Arrhenius pre-factor, A H / A D caused by the presence of tetraethylammonium ions in the reaction clearly suggest changes in tunnelling regime in the reaction. The evidence comprises: a) the spectroscopic and kinetic evidence for the interaction of ascorbate and TEMPO; b) the observation of KIEs k H / k D of 25.4(0.3) in water-acetonitrile (1:1 v/v) without tetraethylammonium ions and 23.3(0.1) in presence of teraethylammonium ions; c) the observation of k H / k D of 21.9(0.2) in water in presence of tetraethylammonium ions; d) the observation of isotope effect on the Arrhenius pre-factor, A H / A D of 0.82(0.10) in the reaction in water-acetonitrile (1:1 v/v) without tetraethylammonium ions; e) the observation of isotope effect on the Arrhenius pre-factor, A H / A D of 0.22(0.03) in the reaction in water-acetonitrile (1:1 v/v) in the presence of tetraethylammonium ions; f) the observation of isotope effect on the Arrhenius pre-factor, A H / A D of 1.34(0.15) in the reaction in water in the presence of tetraethylammonium ions; g) the observation of isotope differences in the enthalpies of activation in water and D 2 O, in presence of tetraethylammonium ions Δ r H ‡ (in H 2 O) = 33.7(0.4) kJ/mol, Δ r H ‡ (in D 2 O) = 40.7(0.1) kJ/mol; h) the observation of isotope differences in the enthalpies of activation in water-acetonitrile (1:1 v/v) and D 2 O-acetonitrile (1:1 v/v) in absence of tetraethylammonium ions, Δ r H ‡ (in H 2 O-acetonitrile) = 31.1(0.1) kJ/mol, Δ r H ‡ (in D 2 O-acetonitrile) = 39.5(0.4) kJ/mol; i) the observation of isotope differences in the enthalpies of activation in water-acetonitrile (1:1 v/v) and D 2 O-acetonitrile (1:1 v/v) in presence of tetraethylammonium ions, Δ r H ‡ (in H 2 O-acetonitrile) = 29.4(0.3) kJ/mol, Δ r H ‡ (in D 2 O-acetonitrile) = 41.0(0.4) kJ/mol. These results were discussed following a framework of Marcus-like tunnelling model, taking into account dynamical features of the systems.

Variations of the kinetic isotope effects and tunnelling regime in two small molecule tunnelling systems, the reaction of ascorbate monoanion with hexacyanoferrate(III) ions in water-acetonitrile (1:1 v/v), water-dioxane (1:1 v/v) and the oxidation of ascorbate with ferricinium ions in water were investigated. The kinetic isotope effects and tunnelling regime undergo to changes due to presence of cations in the reactions. The evidence comprise: 1.) the spectroscopic and kinetic evidences for the interaction of ascorbate monoanion with hexacyanoferrate(III) ions in water-acetonitrile (1:1 v/v), water-dioxane (1:1 v/v) and for the oxidation of ascorbate with ferricinium ions in water; 2.) for the interaction of ascorbate monoanion with hexacyanoferrate(III) ions: a) the observation of change of KIE from k H / k D =8.25(0.09) in water-acetonitrile (1:1 v/v) to 5.86(0.17) in presence of 0.1 M K + in the same solvent; b) the observation of change of KIE from k H / k D =8.37(0.16) in water-dioxane (1:1 v/v) to 5.75(0.1) in presence of 0.3 M Na + in the same solvent; c) the observation of change of isotope effect on the Arrhenius pre-factors, from A H / A D =0.49(0.09) in the reaction in water-acetonitrile (1:1 v/v) to A H / A D =0.14(0.02) in presence of 0.1 M K + in the same solvent; d) the observation of change of isotope effect on the Arrhenius pre-factors, from A H / A D =0.046(0.008) in the reaction in water-dioxane (1:1 v/v) to A H / A D =0.07(0.01) in presence of 0.3 M Na + in the same solvent; 3.) for the oxidation of ascorbate with ferricinium ions: a) the observation of change of KIE from k H / k D =1.91(0.02) in water to k H / k D =2.01(0.03) in presence of 0.5 M Na + ; b) the observation of change of isotope effect on the Arrhenius pre-factors, from A H / A D =0.80(0.09) in the reaction in water to A H / A D =0.52(0.10) in presence of 0.5 M Na + ; c) the observation of change of KIE from k H / k D =1.91(0.02) in water to k H / k D =1.72(0.02) in presence of 0.1 M of acetate ion; d) the observation of catalysis by acetate ions in the reaction. These results were discussed tentatively following a framework of Marcus-like tunnelling model, taking into account differences between the transition structures in the reactions and dynamical features of the systems.

The photochemical properties of 4'-N-acetyl derivative of norfloxacin (ANFX) were investigated in different solutions. Both UV-Vis absorption and the quantum yields of excited states are pH-dependent, and p K a value of ground state for the protonation of 3-carboxylic group was determined to be 6.5 ± 0.2. Pulse radiolysis and laser flash photolysis experiments were carried out to characterize transient species of ANFX and to investigate reactions with tryptophan (TrpH) and 2'-seoxyguanosine-5'-monophosphoric acid disodium salt (dGMP). The ANFX undergoes the photoejection of electron by a mixed mechanism of one-photon and two-photon processes. Under moderate laser energy conditions, two-photon process is predominantly. The ANFX radical dianion (ANFX(–H) ·2− ) formed in reaction with e aq − is characterized by the absorption around 370 nm, and the rate constant was determined to be 1.2 × 10 10 dm 3 mol −1 s −1 . The 3 ANFX(–H) −∗ is able to oxidize TrpH and dGMP with bimolecular rate constants of 5.4 × 10 8 and 9.5 × 10 6 dm 3 mol −1 s −1 , respectively. The ANFX(–H) ·2− and the oxidized radicals of TrpH and dGMP were observed directly. Under aerobic conditions, the photo-oxidations involve both type I and type II mechanisms.

In this investigation, a novel surfactant (N, N, N-trimethyl-4-(5-(octyloxy) naphthalene-1-yloxy) butan-1-aminium bromide, TNBAB) with a naphthalene chromophore moiety was designed and synthesized. The UV, fluorescence and 1 H NMR measurements and molecular modeling have been carried out at 298.2 K for TNBAB + H 2 O, and TNBAB + β -CD + H 2 O systems. The stoichoimetry, stability constant and potential energy surface for the inclusion complex of TNBAB and β -CD have been determined. The results of UV and fluorescence suggested that β -CD formed strong complex with TNBAB, the stoichoimetry was found to be almost 1:1 , and the inclusion constants obtained are larger than 2.1×10 4 L mol -1 . 1 H NMR and molecular modeling studies indicated β -CD and TNBAB could form a two different inclusion complexes (Fig. 9), the two part of the aliphatic chain (one including partly naphthalene nucleus) were encapsulated in the cavity of the β -CD through hydrophobic interaction, respectively.

From extraction experiments and γ -activity measurements, the extraction constants corresponding to the general equilibrium C + (aq) + I − (aq) ⇔ C + (org) + I − (org) taking place in the two-phase water–phenyltrifluoromethyl sulfone (abbrev. FS 13) system (C + = organic cation; aq = aqueous phase, org = FS 13 phase) were evaluated. Furthermore, the individual extraction constants of 8 univalent organic cations in the mentioned two-phase system were calculated.

Alumina nano powders have been prepared by a chemical method named combustion synthesis using new fuels. The effect of key factors on the synthesis of nanoparticles was studied and optimized using Taguchi L 16 ( 4 5 ) array design. The products were characterized by XRD, TGA, EDX, FESEM, and TEM analyses. Results show that aluminum oxide powders had crystal sizes between 7.2 nm and 13 nm and specific surface areas between 21 m 2 g -1 and 70 m 2 g -1 . It was found that there is a good linear correlation ( R 2 =0.953 ) between crystallite size and specific surface area of the synthesized γ -alumina. The surface area of the sample prepared under optimum condition was 72 m 2 g -1 and its average particle size measured by DLLS was 81 nm. The synthesis of γ -alumina was modified in order to achieve higher specific surface area (123 m 2 g -1 ). A network of nano-fibers was successfully obtained by the modified route. The length and diameter of fibers were about 160 nm and 10 nm respectively.