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901.
Cvijeta Jakobušić Brala Viktor Pilepić Ivana Sajenko Ana Karković Stanko Uršić 《Helvetica chimica acta》2011,94(9):1718-1731
The effects of charged species on proton‐coupled electron‐transfer (PCET) reaction should be of significance for understanding/application of important chemical and biological PCET systems. Such species can be found in proximity of activated complex in a PCET reaction, although they are not involved in the charge transfer process. Reported here is the first study of the above‐mentioned effects. Here, the effects of Na+, K+, Li+, Ca2+, Mg2+, and Me4N+ observed in PCET reaction of ascorbate monoanions with hexacyanoferrate(III) ions in H2O reveal that, in presence of ions, this over‐the‐barrier reaction entered into tunneling regime. The observations are: a) dependence of the rate constant on the cation concentration, where the rate constant is 71 (at I = 0.0023), and 821 (at 0.5M K+), 847 (at 1.0M Na+), and 438 M ?1 s?1 (at 0.011M Ca2+); b) changes of kinetic isotope effect (KIE) in the presence of ions, where kH/kD=4.6 (at I = 0.0023), and 3.4 (in the presence of 0.5M K+), 3.3 (at 1.0M Na+), 3.9 (at 0.001M Ca2+), and 3.9 (at 0.001M Mg2+), respectively; c) the isotope effects on Arrhenius pre‐factor where AH/AD=0.97 (0.15) in absence of ions, and 2.29 (0.60) (at 0.5M Na+), 1.77 (0.29) (at 1.0M Na+), 1.61 (0.25) (at 0.5M K+), 0.42 (0.16) (at 0.001M Ca2+) and 0.16 (0.19) (at 0.001M Mg2+); d) isotope differences in the enthalpies of activation in H2O and in D2O, where ΔΔH?(D,H)=3.9 (0.4) kJ mol?1 in the absence of cations, 1.3 (0.6) at 0.5M Na+, 1.8 (0.4) at 0.5M K+, 1.5 (0.4) at 1.0M Na+, 5.5 (0.9) (at 0.001M Ca2+), and 7.9 (2.8) (at 0.001M Mg2+) kJ mol?1; e) nonlinear proton inventory in reaction. In the H2O/dioxane 1 : 1, the observed KIE is 7.8 and 4.4 in the absence and in the presence of 0.1M K+, respectively, and AH/AD=0.14 (0.03). The changes when cations are present in the reaction are explained in terms of termolecular encounter complex consisting of redox partners, and the cation where the cation can be found in a near proximity of the reaction‐activated complex thus influencing the proton/electron double tunneling event in the PCET process. A molecule of H2O is involved in the transition state. The resulting ‘configuration’ is more ‘rigid’ and more appropriate for efficient tunneling with Na+ or K+ (extensive tunneling observed), i.e., there is more precise organized H transfer coordinate than in the case of Ca2+ and Mg2+ (moderate tunneling observed) in the reaction. 相似文献
902.
The mechanism of the copper-mediated disproportionation of aromatic imine disulfides to benzothiazoles in the gas phase is investigated by experimental and theoretical methods. Application of infrared multiphoton dissociation and hydrogen/deuterium exchange experiments combined with density functional theory (DFT) calculations of the relevant molecular structures and the associated infrared spectra allows the identification of the observed ionic intermediates. The theoretical investigation of the possible reaction pathways supported by collision-induced dissociation experiments provides a consistent mechanistic picture of the reaction catalyzed by a single copper(I) ion. Activation of the substrate proceeds via homolytic sulfur-sulfur bond cleavage, yielding metal complexes in the formal +3 oxidation state; carbon-sulfur coupling and hydrogen-atom transfer complete the transformation to the products. Exploratory studies demonstrate that in the gas phase, the disproportionation of the imine disulfide can also be mediated by other metal ions via different either homo- or heterolytic mechanisms without involving high-valent intermediates. 相似文献
903.
Staňová A Marák J Rezeli M Páger C Kilár F Kaniansky D 《Journal of chromatography. A》2011,1218(48):8701-8707
The presented study deals with the off-line coupling of preparative isotachophoresis (pITP) with on-line combination of capillary zone electrophoresis with electrospray mass spectrometric detection (CZE-ESI-MS) used for the analysis of therapeutic peptides (anserine, carnosine, and buserelin) in complex matrix (urine). Preparative capillary isotachophoresis, operating in a discontinuous fractionation mode in column-coupling configuration, served as a sample pretreatment technique to separation, and fractionation of mixture of therapeutic peptides present in urine at low concentration level. The fractions isolated by pITP procedure were subsequently analyzed by capillary zone electrophoresis with electrospray mass spectrometric detection. Acetic acid at 200 mmol L(-1) concentration served as background electrolyte in CZE stage and it is compatible with MS detection in positive ionization mode. In pITP fractionation procedure, sodium cation (10 mmol L(-1) concentration) as leading ion and beta-alanine as terminating ion (20 mmol L(-1) concentration) were used. While using CZE-ESI-MS, the limits of detection were 0.18 μg mL(-1) for carnosine, 0.17 μg mL(-1) for anserine and 0.64 μg mL(-1) for buserelin in water and 0.19 μg mL(-1) for carnosine, 0.50 μg mL(-1) for anserine and 0.74 μg mL(-1) for buserelin in 10 times diluted urine, respectively. The cleaning power of pITP sample pretreatment was proved as the peptides provided the higher MS signals at lower concentration levels resulting from the minimized matrix effects. The quality of obtained MS/MS spectra was very good so that they can provide information about the structure of analytes, and they were used for verification of the analytes identities. The pITP pretreatment improved the detection limits of the analyzed therapeutic peptides at least 25 times compared to the CZE-ESI-MS itself. 相似文献
904.
Orentienė A Olšauskaitė V Vičkačkaitė V Padarauskas A 《Journal of chromatography. A》2011,1218(39):6884-6891
The potential of 1.7 μm ethylene bridged hybrid silica phase was investigated for the separation of twelve imidazolium-based ionic liquid cations. U-shaped retention profile was observed for all solutes with an increase in retention at both low and high acetonitrile content. Chromatographic behaviour of imidazolium cations in both hydrophilic interaction chromatography (HILIC) and per aqueous liquid chromatography (PALC) modes was studied by varying key parameters such as buffer concentration and pH, acid additive, organic modifier and column temperature. Experimental data provided some evidences that under PALC conditions cationic solutes are retained predominantly by mixed hydrophobic/ion-exchange interactions. In the HILIC mode, both partitioning and ion-exchange interactions are responsible for the retention of solutes. Compared to PALC, HILIC provided significantly higher efficiencies with less or even no peak tailing, better separation selectivity and greater resistance to overload. In PALC mode gradient elution was required to achieve adequate retentivity of all solutes but selectivity was not sufficient to distinguish between solutes with very similar hydrophobicity. In contrast, under HILIC conditions twelve solutes were almost completely resolved in less than 4 min by using isocratic elution. Summarizing, it could be concluded that ethylene bridged hybrid silica column providing a dual retention mechanism offers the possibility of selecting between the two retention modes with opposite separation selectivity, just by changing the composition of the mobile phase. 相似文献
905.
Zapadlo M Krupčík J Kovalczuk T Májek P Spánik I Armstrong DW Sandra P 《Journal of chromatography. A》2011,1218(5):746-751
A total of 196 out of 209 polychlorobiphenyl (PCB) congeners were resolved using GC×GC-TOFMS with a non-polar/ionic liquid column series consisting of poly(50%-n-octyl-50%-methyl)siloxane and (1,12-di(tripropylphosphonium)dodecane bis(trifluoromethansulfonyl)amide) in the first and second dimension, respectively. It has been found that 13 PCB congeners overlap in five doublets (CB12+CB13, CB62+CB75, CB70+CB76, CB97+CB125 and CB153+CB168) and one triplet (CB90+CB101+CB113). All toxic, "dioxin like" congeners were separated with no interferences from any PCB congener. The 109 PCBs present in Aroclor 1242 and the 82 PCBs present in Aroclor 1260 were resolved GC×GC-TOFMS analysis on this column set. 相似文献
906.
Six derivatives ( 1 , 2 , 3 , 4 , 5 , 6 ) of 2‐phenyl‐1H‐imidazole were tested as catalysts of Henry reaction. Three new ( 4 , 5 , 6 ) 2‐phenyl‐1H‐imidazole derivatives, differently substituted (thio)ureas, were synthesized and determined by 1H NMR and IR spectroscopy and elemental analysis. Two types of catalysis, homogeneous and heterogeneous, were examined and compared. Clay minerals Ca‐MMT and Cu‐MMT were used as solid supports for heterogeneous catalysis. The best results were obtained using compound 2 under conditions of heterogeneous method D from the point of view of yield and reaction time. J. Heterocyclic Chem., (2011) 相似文献
907.
Drahoš B Kotek J Císařová I Hermann P Helm L Lukeš I Tóth É 《Inorganic chemistry》2011,50(24):12785-12801
Mn(2+) complexes represent an alternative to Gd(3+) chelates which are widely used contrast agents in magnetic resonance imaging. In this perspective, we investigated the Mn(2+) complexes of two 12-membered, pyridine-containing macrocyclic ligands bearing one pendant arm with a carboxylic acid (HL(1), 6-carboxymethyl-3,6,9,15-tetraazabicyclo[9.3.1] pentadeca-1(15),11,13-triene) or a phosphonic acid function (H(2)L(2), 6-dihydroxyphosphorylmethyl-3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene). Both ligands were synthesized using nosyl or tosyl amino-protecting groups (starting from diethylenetriamine or tosylaziridine). The X-ray crystal structures confirmed a coordination number of 6 for Mn(2+) in their complexes. In aqueous solution, these pentadentate ligands allow one free coordination site for a water molecule. Potentiometric titration data indicated a higher basicity for H(2)L(2) than that for HL(1), related to the electron-donating effect of the negatively charged phosphonate group. According to the protonation sequence determined by (1)H and (31)P pH-NMR titrations, the first two protons are attached to macrocyclic amino groups whereas the subsequent protonation steps occur on the pendant arm. Both ligands form thermodynamically stable complexes with Mn(2+), with full complexation at physiological pH and 1:1 metal to ligand ratio. The kinetic inertness was studied via reaction with excess of Zn(2+) under various pHs. The dissociation of MnL(2) is instantaneous (at pH 6). For MnL(1), the dissociation is very fast (k(obs) = 1-12 × 10(3) s(-1)), much faster than that for MnDOTA, MnNOTA, or the Mn(2+) complex of the 15-membered analogue. It proceeds exclusively via the dissociation of the monoprotonated complex, without any influence of Zn(2+). In aqueous solution, both complexes are air-sensitive leading to Mn(3+) species, as evidenced by UV-vis and (1)H NMRD measurements and X-ray crystallography. Cyclic voltammetry gave low oxidation peak potentials (E(ox) = 0.73 V for MnL(1) and E(ox) = 0.68 V for MnL(2)), in accordance with air-oxidation. The parameters governing the relaxivity of the Mn(2+) complexes were determined from variable-temperature (17)O NMR and (1)H NMRD data. The water exchange is extremely fast, k(ex) = 3.03 and 1.77 × 10(9) s(-1) for MnL(1) and MnL(2), respectively. Variable-pressure (17)O NMR measurements have been performed to assess the water exchange mechanism on MnL(1) and MnL(2) as well as on other Mn(2+) complexes. The negative activation volumes for both MnL(1) and MnL(2) complexes confirmed an associative mechanism of the water exchange as expected for a hexacoordinated Mn(2+) ion. The hydration number of q = 1 was confirmed for both complexes by (17)O chemical shifts. A relaxometric titration with phosphate, carbonate or citrate excluded the replacement of the coordinated water molecule by these small endogenous anions. 相似文献
908.
Svrčková P Vítek A Karlický F Paidarová I Kalus R 《The Journal of chemical physics》2011,134(22):224310
Temperature dependence of vertical ionization energies is modeled for small argon clusters (N ≤ 13) using classical parallel-tempering Monte Carlo methods and extended interaction models based on the diatomics-in-molecules approach. Quantum effects at the zero temperature are also discussed in terms of zero-point nuclear vibrations, either at the harmonic approximation level or at the fully anharmonic level using the diffusion Monte Carlo calculations. Both approaches lead to a considerable improvement of the theoretical predictions of argon clusters ionization energies and represent a realistic way of modeling of ionization energies for weakly bound and floppy complexes in general. A thorough comparison with a recent electron-impact experiment [O. Echt et al., J. Chem. Phys. 123, 084313 (2005)] is presented and a novel interpretation of the experimental data is proposed. 相似文献
909.
Re(I) carbonyl-diimine complexes [Re(L-AA)(CO)(3)(N,N)](+) (N,N = bpy, phen) containing an aromatic amino acid (AA), phenylalanine (Phe), tyrosine (Tyr), or tryptophan (Trp), linked to Re by a pyridine-amido or imidazole-amido ligand L have been synthesized and their excited-state properties investigated by nanosecond time-resolved IR (TRIR) and emission spectroscopy. Near-UV optical excitation populates a Re(I)(CO)(3)→N,N (3)MLCT excited state *[Re(II)(L-AA)(CO)(3)(N,N(?-))](+). Decay to the ground state (50-300 ns lifetime) is the only excited-state deactivation process observed in the case of Phe and Tyr complexes, whereas the Trp-containing species undergo a Trp(indole)→*Re(II) electron transfer (ET) producing a charge-separated (CS) state, [Re(I)(L-Trp(?+))(CO)(3)(N,N(?-))](+). The ET occurs with a 8-40 ns lifetime depending on L, N,N, and the solvent. The CS state is characterized by ν(CO) IR bands shifted to lower wavenumbers from their respective ground-state positions and two bands at 1278 and 1497 cm(-1) tentatively attributed to Trp(?+). The amido bridge is affected by both the MLCT excitation and the subsequent ET, manifested by the shifts and intensity changes of the amide-I IR band at about 1680 cm(-1). The CS state decays to the ground state by a N,N(?-)→Trp(?+) back-ET the rates of which are comparable to those of the forward ET, 30-60 ns. This study independently demonstrates that Trp can act as an electron-hopping intermediate in photodriven ET systems based on Re-labeled proteins and supramolecules. Photoinduced ET in Trp-containing Re complexes also can be used to generate Trp(?+) and investigate its spectral properties and reactivity. 相似文献
910.
Stanisavljev DR Milenković MC Mojović MD Popović-Bijelić AD 《The journal of physical chemistry. A》2011,115(11):2247-2249
The iodide-peroxide system in an acidic medium was investigated as a potential source of free radicals in iodine-based chemical oscillators. The radicals were detected by EPR spin-trapping using spin-trap 5-(tert-butoxycarbonyl)-5-methyl-1-pyrroline N-oxide (BMPO), which forms stable spin-adducts with oxygen-centered radicals. The iodide-peroxide system is introduced as an easily available laboratory source of free radicals. 相似文献