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71.
The protonation constants () of 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene-3,6,9-triacetic acid (PCTA) and stability constants of complexes formed between this pyridine-containing macrocycle and several different metal ions have been determined in 1.0 M KCl at 25 degrees C and compared to previous literature values. The first protonation constant was found to be 0.5-0.6 log units higher than the value reported previously, and a total of five protonation steps were detected (log = 11.36, 7.35, 3.83, 2.12, and 1.29). The stability constants of complexes formed between PCTA and Mg2+, Ca2+, Cu2+, and Zn2+ were also somewhat higher than those previously reported, but this difference could be largely attributed to the higher first protonation constant of the ligand. Stability constants of complexes formed between PCTA and the Ln3+ series of ions and Y3+ were determined by using an "out-of-cell" potentiometric method. These values ranged from log K = 18.15 for Ce(PCTA) to log K = 20.63 for Yb(PCTA), increasing along the Ln series in proportion to decreasing Ln3+ cation size. The rates of complex formation for Ce(PCTA), Eu(PCTA), Y(PCTA), and Yb(PCTA) were followed by conventional UV-vis spectroscopy in the pH range 3.5-4.4. First-order rate constants (saturation kinetics) obtained for different ligand-to-metal ion ratios were consistent with the rapid formation of a diprotonated intermediate, Ln(H(2)PCTA)(2+). The stabilities of the intermediates as determined from the kinetic data were 2.81, 3.12, 2.97, and 2.69 log K units for Ce(H(2)PCTA), Eu(H(2)PCTA), Y(H(2)PCTA), and Yb(H(2)PCTA), respectively. Rearrangement of these intermediates to the fully chelated complexes was the rate-determining step, and the rate constant (k(r)) for this process was found to be inversely proportional to the proton concentration. The formation rates (k(OH)) increased with a decrease in the lanthanide ion size [9.68 x 10(7), 1.74 x 10(8), 1.13 x 10(8), and 1.11 x 10(9) M(-1) s(-1) for Ce(PCTA), Eu(PCTA), Y(PCTA), and Yb(PCTA), respectively]. These data indicate that the Ln(PCTA) complexes exhibit the fastest formation rates among all lanthanide macrocyclic ligand complexes studied to date. The acid-catalyzed dissociation rates (k1) varied with the cation from 9.61 x 10(-4), 5.08 x 10(-4), 1.07 x 10(-3), and 2.80 x 10(-4) M(-1) s(-1) for Ce(PCTA), Eu(PCTA), Y(PCTA), and Yb(PCTA), respectively. 相似文献
72.
73.
Franziska Debatin Arne Thomas Prof. Dr. Alexandra Kelling Niklas Hedin Dr. Zoltan Bacsik Dr. Irena Senkovska Dr. Stefan Kaskel Prof. Dr. Matthias Junginger Holger Müller Uwe Schilde Prof. Dr. Christian Jäger Prof. Dr. Alwin Friedrich Dr. Hans‐Jürgen Holdt Prof. Dr. 《Angewandte Chemie (International ed. in English)》2010,49(7):1258-1262
74.
Dragan A. Marković Zoltan Djarmati Ratko M. Jankov Dragan M. Marković Ljubiša M. Ignjatović 《Mikrochimica acta》1996,124(3-4):219-226
The electrochemical behaviour and mechanism of the redox process of the natural antioxidant rosmanol 9-ethyl ether, isolated fromSaliva officinalis L., were studied. The cyclic voltammograms of rosmanol 9-ethyl ether (R9EE), at characteristic pH values, and the electrochemical parameters for all investigated pH values were measured. Three characteristic pH regions, each with different behaviour of R9EE, were identified. In regions of pH < 4 and pH > 5 only one anodic peak appeared, whereas in the solutions of pH 4–5 two anodic peaks could be noted. The overall oxidation mechanism at pH < 4 is an e.H.e.H. oxidation mechanism, which as a final product gives a quinonic molecule. The influence of pH on the second oxidation peak potential tends towards zero in accordance with the preceding dissociation of the one of phenolic groups, thus suggesting an e.e.H. mechanism at pH > 5. This means that at the pH values expected in plant cells, R9EE has an unexpected structure, making this substance a potent antioxidant. Electrochemical and spectrophotometric measurements enabled us to establish an extremely low pK
a value (4.35) for R9EE. 相似文献
75.
Luis M. De León-Rodríguez Zoltan Kovacs Ana Cristina Esqueda-Oliva 《Tetrahedron letters》2006,47(39):6937-6940
A high yielding N-trans diprotection procedure for cyclen has been developed by using succinimide carbamate derivates of Boc and Cbz and chloroformate protecting groups. 相似文献
76.
De-Cai Fang P. Fabian Zoltan Szekely Xiao-Yuan Fu Ting-Hua Tang Imre G. Csizmadia 《Journal of Molecular Structure》1998,430(1-3):161-170
Ab initio (HF/6-31G** and B3LYP/6-31 + + G**) methods have been used to study the stability and structure of complexes between CH3SO−3 and CH3NH+3 or C(NH2)+3. Results show that no hydrogen jump is involved in the complex formations, which is different from previous work studying complexes between CH3COO− and CH3NH+3. In addition, we have studied complexes between CH3SO−3 and HC(NH2)+3 or +H3NC(NH2)3, all of which have a cage structure. 相似文献
77.
Zeng H Schelly ZA Ueno-Noto K Marynick DS 《The journal of physical chemistry. A》2005,109(8):1616-1620
The structures of (PbS)n (n = 1-9) clusters are investigated with density functional theory at the B3LYP level. Various pseudopotential basis sets on lead and the 6-31+G basis set on sulfur were employed. Full geometry optimization and extensive searches of the potential energy surface were carried out for clusters with n = 1-6. We find that even small PbS clusters (n > 2) start to take on the characteristic features of the rock salt structure of solid-state PbS (galena). The origin of some of the structural aspects of these crystals is shown to be associated with the partial covalent nature of the Pb-S bond. The magnitude of the HOMO-LUMO gap oscillates with increasing size of the clusters, in agreement with the observed behavior of the corresponding UV absorption bands of ultrasmall PbS quantum dots. Direct conformation of this oscillation was found by CIS(D) calculations, for which the absorption with the largest oscillator strength oscillates as the clusters grow from PbS to (PbS)9. 相似文献
78.
79.
Tel T Karolyi G Pentek A Scheuring I Toroczkai Z Grebogi C Kadtke J 《Chaos (Woodbury, N.Y.)》2000,10(1):89-98
We review and generalize recent results on advection of particles in open time-periodic hydrodynamical flows. First, the problem of passive advection is considered, and its fractal and chaotic nature is pointed out. Next, we study the effect of weak molecular diffusion or randomness of the flow. Finally, we investigate the influence of passive advection on chemical or biological activity superimposed on open flows. The nondiffusive approach is shown to carry some features of a weak diffusion, due to the finiteness of the reaction range or reaction velocity. (c) 2000 American Institute of Physics. 相似文献
80.