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41.
Lothar Jaenicke 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》1989,101(3):365-366
42.
The oxygen-exchange reaction of V10O with bulk water has been followed by time-dependent 17O-NMR spectroscopy (buffered solutions, pH ~ 5.5, [V10]total ~ 0.17m, T = 298 K). It is shown that all seven structurally different sites of O-atoms are kinetically similar but, in contrast to earlier studies, not identical (6 h ? ‘t1/2’ ? 11 h). The kinetic similarity of the various structural sites implies the some (but not full) O scrambling is involved. Two possible mechanisms with a ‘half-bonded’ and an ‘open’ intermediate are discussed in detail to interpret the experimental results. A computer simulation of the exchange reaction based on these models is presented. It is shown that the ‘half-bonded-intermediate’ mechanism is consistent with the experimental data and the following parameters are calculated: formation of the intermediate: k1 = 5.8 · 10?3 s?1, k?1 = 6.7 · 10?2 s?1, [intermediate]∞ ≈ 8%; all activated O-atoms exchange within the lifetime of the intermediate (τ ~ 15 s), and the calculated exchange rate of the intermediate (k2 ? 0.60 s?1) is consistent with earlier assumptions (k2 ≈ 0.5 s?1). It is shown that a simulation based on the ‘open-intermediate’ mechanism results in kinetic parameters which are not consistent with the kinetics of the formation of cyclic metavanadates ((VO)n, n = 4,5) from decavanadate, since the required formation rate is by a factor ~ 102 too fast, and the equilibrium concentration of metavanadates is by a factor of ~ 2 too large (under the conditions of the O-exchange experiments of decavanadate (T = 298 K, [V10]total ≈ 0.17m, pH ~ 5.55) the total amount of metavanadates present is ~ 8%, with [(VO)4]/[(VO)5] ~ 4:1; a qualitative analysis of the kinetics of the formation of metavanadates (vo kinetics; the exact mechanism of the back-reaction (at least second-order) is not known with certainty) leads to k1 ? 4·10?5 s?1). O exchange of decavanadates via equilibrated metavanadates would lead to full scrambling of the O sites and is not consistent with the observed differences in the exchange rates. From the qualitative kinetic parameters of the metavanadate formation kinetics, it can be concluded that any contribution of an ‘open’ or an ‘metavanadate’ mechanism is of the order of 1–2% at most. 相似文献
43.
Static properties of an isotropic magnet are calculated in the whole critical region including the magnetization curve. The method proposed is a resummation of renormalized perturbation theory without use of recursion relations. This is possible because only special diagrams or subdiagrams show infrared divergencies at the magnetization curve due to Goldstone modes. The arguments given are heavily based on Ward identities. The resulting perturbation theory is well behaved in the total critical region and exhibits explicitely the form of the Goldstone mode singularities at the magnetization curve. The equation of state is calculated including two-loop contribution. Resulting effective exponents are then correct in order in the whole critical region. In various limits agreement with known results is found. A one-loop calculation of the correlation functions is also given. 相似文献
44.
Wolfgang K. Schubert J.F. Southern Lothar Schäfer 《Journal of Molecular Structure》1973,16(3):403-415
The structure of cis-1,4-ditertiarybutylcyclohexane(DTBC) was investigated by combined electron diffraction, conformational and vibrational analyses in order to obtain results which are more conclusive than those previously obtained by electron diffraction alone. In this study, first the minimum energy conformations for DTBC were calculated by the Westheimer-Hendrickson procedure using various force fields described in the literature; the same fields and the minimum energy conformations were used in subsequent vibrational analyses to calculate the mean amplitudes of vibration for each minimum energy conformation of DTBC; these mean amplitudes and the corresponding internuclear distances were then used to calculate the theoretical electron diffraction radial distribution curves which were compared to the experimental curves. The results indicate that the conformational energies of all the minimum energy chair and non-chair forms of DTBC are very similar. In excellent agreement with this, the theoretical radial distribution curves of all minimum energy forms have to be mixed for a best fit to the experimental radial distribution curve. A least squares analysis of the mixture under the described conditions yields for 110 °C a composition of approximately one third chair and two thirds non-chair forms. The quality of the empirical conformational force fields has a definite influence on the reliability of these results. 相似文献
45.
46.
Lothar Metzger Günter G. Willems und Rolf Neeb 《Fresenius' Journal of Analytical Chemistry》1978,292(1):20-22
Zusammenfassung Die starke Temperaturabhängigkeit des polarographischen Zinn(II/0)-Signals in Lösungen des vierwertigen Zinns ermöglicht bei –32° C in methanolischer salzsaurer Grundlösung die Bestimmung des Bleis neben hohem Zinnüberschuß. Eine Anwendung des Verfahrens zur wechselstrompolarographischen Bestimmung des Bleis in Rohzinn wird beschrieben.
Investigations on the simultaneous electrochemical determination of lead and tinII. Polarographic determination of lead in the presence of tin at lower temperatures
Summary The strong temperature dependence of the polarographic tin(II/0) signal in solutions containing tin(IV) renders possible the determination of lead in the presence of a high excess of tin at working temperatures of –32° C in hydrochloric methanol as supporting electrolyte solution. An application of the procedure to the ac-polarographic determination of lead in crude tin is described.
Die vorliegende Arbeit wurde in dankenswerter Weise durch Mittel der Deutschen Forschungsgemeinschaft und des Verbandes der Chemischen Industrie ermöglicht. 相似文献
47.
In chloroform, [ZrCl4·2(MeO)3PO] exists in both cis- and trans-isomeric forms. Three reactions can be envisaged in the presence of excess (MeO)3PO = L: (1) cis-[ZrCl4·2L] + *L?cis-[ZrCl4·L*L]+ L; (2) trans-[ZrCl4·2L] + *L ? trans-[ZrCl4·L*L] + L; (3) cis-[ZrCl4·2L]? trans-[ZrCl4·2L]. To distinguish between these possible reaction pathways, we have used 2D 1H-NMR spectroscopy. For the first time, variable-pressure 2D exchange spectra were used for mechanistic assignments. cis/trans-Isomerisation was found to be the fastest reaction (in CHCl3/CDCl3), with a small acceleration at higher pressure: it is concluded to be an intramolecular process with a slightly contracted six-coordinate transition state. The intermolecular (MeO)3PO exchange on the cis- and trans-isomer are second-order processes and are strongly accelerated by increased pressure: Ia mechanisms are suggested without ruling out limiting A mechanisms. 相似文献
48.
The vibrational structure of the endohedral cluster fullerene Sc(3)N@C(78) is studied by FTIR spectroscopy, Raman spectroscopy and DFT-based quantum chemical calculations. Remarkably good agreement between experimental and calculated spectra is achieved and a full assignment of the Sc(3)N-based vibrational modes is given. Significant differences in the vibrational structure of the endohedral cluster fullerene Sc(3)N@C(78) and the empty, charged C(78) (6-): 5 (D(3h)') are rationalized by the strong coupling between the Sc(3)N cluster and the fullerene cage. This coupling has its origin in a significant overlap of the Sc(3)N and C(78) molecular orbitals, and causes atomic-charge and bond-length redistributions compared to the neutral C(78) and the C(78) (6-) anion. An ionic model is not sufficient to describe the electronic, geometric and vibrational structure of the Sc(3)N@C(78) nitride cluster fullerene. 相似文献
49.
Kinetic studies of cyanide exchange on [M(CN)(4)](2-) square-planar complexes (M = Pt, Pd, and Ni) were performed as a function of pH by (13)C NMR. The [Pt(CN)(4)](2-) complex has a purely second-order rate law, with CN(-) as acting as the nucleophile, with the following kinetic parameters: (k(2)(Pt,CN))(298) = 11 +/- 1 s(-1) mol(-1) kg, DeltaH(2) (Pt,CN) = 25.1 +/- 1 kJ mol(-1), DeltaS(2) (Pt,CN) = -142 +/- 4 J mol(-1) K(-1), and DeltaV(2) (Pt,CN) = -27 +/- 2 cm(3) mol(-1). The Pd(II) metal center has the same behavior down to pH 6. The kinetic parameters are as follows: (k(2)(Pd,CN))(298) = 82 +/- 2 s(-1) mol(-1) kg, DeltaH(2) (Pd,CN) = 23.5 +/- 1 kJ mol(-1), DeltaS(2) (Pd,CN) = -129 +/- 5 J mol(-1) K(-1), and DeltaV(2) (Pd,CN) = -22 +/- 2 cm(3) mol(-1). At low pH, the tetracyanopalladate is protonated (pK(a)(Pd(4,H)) = 3.0 +/- 0.3) to form [Pd(CN)(3)HCN](-). The rate law of the cyanide exchange on the protonated complex is also purely second order, with (k(2)(PdH,CN))(298) = (4.5 +/- 1.3) x 10(3) s(-1) mol(-1) kg. [Ni(CN)(4)](2-) is involved in various equilibrium reactions, such as the formation of [Ni(CN)(5)](3-), [Ni(CN)(3)HCN](-), and [Ni(CN)(2)(HCN)(2)] complexes. Our (13)C NMR measurements have allowed us to determine that the rate constant leading to the formation of [Ni(CN)(5)](3-) is k(2)(Ni(4),CN) = (2.3 +/- 0.1) x 10(6) s(-1) mol(-1) kg when the following activation parameters are used: DeltaH(2)() (Ni,CN) = 21.6 +/- 1 kJ mol(-1), DeltaS(2) (Ni,CN) = -51 +/- 7 J mol(-1) K(-1), and DeltaV(2) (Ni,CN) = -19 +/- 2 cm(3) mol(-1). The rate constant of the back reaction is k(-2)(Ni(4),CN) = 14 x 10(6) s(-1). The rate law pertaining to [Ni(CN)(2)(HCN)(2)] was found to be second order at pH 3.8, and the value of the rate constant is (k(2)(Ni(4,2H),CN))(298) = (63 +/- 15) x10(6) s(-1) mol(-1) kg when DeltaH(2) (Ni(4,2H),CN) = 47.3 +/- 1 kJ mol(-1), DeltaS(2) (Ni(4,2H),CN) = 63 +/- 3 J mol(-1) K(-1), and DeltaV(2) (Ni(4,2H),CN) = - 6 +/- 1 cm(3) mol(-1). The cyanide-exchange rate constant on [M(CN)(4)](2-) for Pt, Pd, and Ni increases in a 1:7:200 000 ratio. This trend is modified at low pH, and the palladium becomes 400 times more reactive than the platinum because of the formation of [Pd(CN)(3)HCN](-). For all cyanide exchanges on tetracyano complexes (A mechanism) and on their protonated forms (I/I(a) mechanisms), we have always observed a pure second-order rate law: first order for the complex and first order for CN(-). The nucleophilic attack by HCN or solvation by H(2)O is at least nine or six orders of magnitude slower, respectively than is nucleophilic attack by CN(-) for Pt(II), Pd(II), and Ni(II), respectively. 相似文献
50.
Chatterton N Gateau C Mazzanti M Pécaut J Borel A Helm L Merbach A 《Dalton transactions (Cambridge, England : 2003)》2005,(6):1129-1135
The ligand N,N'-bis[(6-carboxy-2-pyridylmethyl]ethylenediamine-N,N'-diacetic acid (H(4)bpeda) was synthesised using an improved procedure which requires a reduced number of steps and leads to a higher yield with respect to the published procedure. It was obtained in three steps from diethylpyridine-2,6-dicarboxylate and commercially available ethylenediamine-N,N[prime or minute]-diacetic acid with a total yield of approximately 20%. The crystal structure of the hexa-protonated form of the ligand which was determined by X-ray diffraction shows that the four carboxylates and the two amines are protonated. The crystal structure of the polynuclear complex [Gd(bpeda)(H(2)O)(2)](3)[Gd(H(2)O)(6)](2)Cl(3)(2), isolated by slow evaporation of a 1:1 mixture of GdCl(3) and H(4)bpeda at pH approximately 1, was determined by X-ray diffraction. In complex three [Gd(bpeda)(H(2)O)(2)] units, containing a Gd(III) ion ten-coordinated by the octadentate bpeda and two water molecules, are connected in a pentametallic structure by two hexa-aquo Gd(3+) cations through four carboxylato bridges. The protonation constants (pK(a1)= 2.9(1), pK(a2)= 3.5(1), pK(a3)= 5.2(2), and pK(a4)= 8.5(1)) and the stability constants of the complexes formed between Gd(III) and Ca(II) ions and H(4)bpeda (log beta(GdL)= 15.1(3); log beta(CaL)= 9.4(1)) were determined by potentiometric titration. The unexpected decrease in the stability of the gadolinium complex and of the calcium complex of the octadentate ligand bpeda(4-) with respect to the hexadentate ligand edta(4-) has been interpreted in terms of an overall lower contribution to stability of the metal-nitrogen interactions. The EPR spectra display very broad lines (apparent DeltaH(pp) approximately 800-1200 G at X-band and 90-110 G at Q-band depending on the temperature), indicating a rapid transverse electron spin relaxation. At X-band, Gd(bpeda) is among the fastest relaxing Gd(3+) complexes to date suggesting that the presence of pyridinecarboxylate chelating groups in itself does not lead to slow electron relaxation. 相似文献