1H,1H,2H,2H-perfluoroalkyl-functionalization of Ni(II), Pd(II), and Pt(II) mono- and diphosphine complexes: minimizing the electronic consequences for the metal center

A series of fluorous derivatives of group 10 complexes MCl(2)(dppe) and [M(dppe)(2)](BF(4))(2) (M = Ni, Pd or Pt; dppe = 1,2-bis(diphenylphosphino)ethane) and cis-PtCl(2)(PPh(3))(2) was synthesized. The influence of para-(1H,1H,2H,2H-perfluoroalkyl)dimethylsilyl-functionalization of the phosphine phenyl groups of these complexes, as studied by NMR spectroscopy, cyclovoltammetry (CV), XPS analyses, as well as DFT calculations, points to a weak steric and no significant inductive electronic effect. The steric effect is most pronounced for M = Ni and leads in the case of NiCl(2)(1c) (3c) and [Ni(1c)(2)](BF(4))(2) (7c) (1c = [CH(2)P[C(6)H(4)(SiMe(2)CH(2)CH(2)C(6)F(13))-4](2)](2)) to a tetrahedral distortion from the expected square planar geometry. The solubility behavior of NiCl(2)[CH(2)P[C(6)H(4)(SiMe(3-b)(CH(2)CH(2)C(x)F(2x+1)b)-4](2)](2) (3: b = 1-3; x = 6, 8) in THF, toluene, and c-C(6)F(11)CF(3) was found to follow the same trends as those observed for the free fluorous ligands 1. A similar correlation between the partition coefficient (P) of complexes 3 and free 1 was observed in fluorous biphasic solvent systems, with a maximum value obtained for 3f (b = 3, x = 6, P = 23 in favor of the fluorous phase).     

Interlaboratory reproducibility of mobility parameters in capillary electrophoresis for substance identification in systematic toxicological analysis

An interlaboratory pilot study was performed to determine the reproducibility of mobility parameters in capillary zone electrophoresis (CZE) and micellar electrokinetic chromatography (MEKC). The study was performed by an intended small number of laboratories (three) that used different brands of instruments (two). The effective mobility was corrected using standards by a method that was recently introduced to obtain a more reproducible migration parameter. A test set of 20 acidic test compounds and 5 reference compounds were analyzed during five days in each laboratory using CZE and MEKC. Buffers used consisted of 90 mM borate set at pH 8.4 (CZE) and 20 mM phosphate, 50 mM sodium dodecyl sulfate set at pH 7.5 (MEKC). Analyses were carried out using fused-silica capillaries at an electric field strength of either 52.6 kV/m or 37.5 kV/m. The interlaboratory reproducibility (mean RSD) of the effective mobility was 3.0% for CZE and 6.7% for MEKC. After applying the correction method, these values became 3.0% for CZE and 3.3% for MEKC, which is adequate for systematic toxicological analysis (STA) applications. A significant improvement of reproducibility for the calculated corrected effective mobility mu(eff)c was observed when variations are high. Therefore, it is recommended to use the correction method in interlaboratory situations, especially when instruments and capillaries from different manufacturers are used.     

Valence bond descriptions of benzene and cyclobutadiene and their counterparts with localized bonds

Geometry optimizations have been performed for benzene and cyclobutadiene and for the corresponding moieties with nonresonating double bonds, viz. 1,3,5‐cyclohexatriene and 1,3‐cyclobutadiene. The calculations were done using the valence bond self‐consistent field method including orbital optimization. Both strictly local and delocalized p‐like orbitals were used for the π system, which influences the strengths of the π bonds. The calculations result in geometries and resonance and stabilization energies for benzene and cyclobutadiene, which are compared with theoretical models of aromaticity. The importance of resonance is discussed. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003     

On the rate of convergence to a stable limit law. II

Summary In [5] Feller states the following assertion: As soon as we leave the domain of applicability of the central limit theorem we find ourselves on practically unknown terrain; the problems receive an entirely new aspect and no systematic tools have as yet been developed for treating the theory, After the appearance of Volume II [6], many papers were published on stable distributions. The reader will find several references in [8].In the same paper [5] Feller states the following assertions: The theory becomes the simpler the fewer moments are finite. This assertion was stated in a paper in which he proved a law of the iterated logarithm for the heavy tail of stable distributions. (See also Chapter 8 in [8].) We shall show that the second assertion also holds in the study of the rate of convergence to a stable limit distribution. In several examples we shall compare our results with the results of Christoph [1–3] and Dubinskaite [4].Department of Mathematics, University of Leiden, Holland. Published in Litovskii Matematicheskii Sbornik (Lietuvos Matematikos Rinkinys), Vol. 26, No. 3, pp. 482–487, July–September, 1986.     

NMR transversal relaxivity of aqueous suspensions of particles of Ln(3+)-based zeolite type materials

A series of zeolite-type silicates containing stoichiometric amounts of Ln(3+) ions in the framework (Ln-AV-9 materials), with composition (Na(4)K(2))(Ln(2)Si(16)O(38)).10H(2)O (Ln = Nd, Sm, Eu, Tb, Gd, Dy) has recently been synthesized and characterized. They form paramagnetic microparticles, which as aqueous suspensions have negligible water (1)H longitudinal relaxivities (r(1)) for all Ln(3+) ions studied and quite large transverse relaxivities (r(2)). In this work we further analysed the size distribution of the Ln-AV-9 particles and their r(2)* and r(2) relaxivities. The r(2)* relaxivity effects are explained by the static dephasing regime (SDR) theory. The r(2) relaxivities appear to be strongly dependent on the interval between two consecutive refocusing pulses (tau(CP)) in the train of 180 degrees pulses applied. For long tau(CP) values, the r(2) of the systems saturates at a value, which is always an order of magnitude smaller than r(2)*. These features are explained by a crude model, which takes into account the residual diffusion effect in the static dephasing regime. The large microparticles, although not efficient in T(1) relaxation, are quite effective in enhancing T(2) relaxation, particularly at high magnetic fields. The r(2)* values and the saturation values for r(2) were found to increase linearly with B(0) and mu(0)(2). The largest transversal relaxation rate enhancements were observed for Dy-AV-9 with a saturation value of r(2) of 60 s(-1) mM(-1) and a r(2)* value of 566 s(-1) mM(-1) at 9.4 T and 298 K.     

Complexes of DOTA-bisphosphonate conjugates: probes for determination of adsorption capacity and affinity constants of hydroxyapatite

The adsorption on hydroxyapatite of three conjugates of a bisphosphonate and a macrocycle having C1, C2, and C3 spacers and their terbium complexes was studied by the radiotracer method using 160Tb as the label. The radiotracer-containing complex of the conjugate with the C3 spacer was used as a probe for the determination of the adsorption parameters of other bisphosphonates that lack a DOTA unit. A physicochemical model describing the competitive adsorption was successfully applied in the fitting of the obtained data. The maximum adsorption capacity of bisphosphonates containing bulky substituents is determined mainly by their size. For bisphosphonates having no DOTA moiety, the maximum adsorption capacity is determined by the electrostatic repulsion between negatively charged bisphosphonate groups. Compounds with a hydroxy or amino group attached to the alpha-carbon atom show higher affinities. Macrocyclic compounds containing a short spacer between the different bisphosphonic acid groups and the macrocyclic unit exhibit high affinities, indicating a synergic effect of the bisphosphonic and the macrocyclic groups during adsorption. The competition method described uses a well-characterized complex and allows a simple evaluation of the adsorption behavior of bisphosphonates. The application of the macrocycle-bisphosphonate conjugates allows easy radiolabeling via complexation of a suitable metal isotope.     

Tuning of the size of Dy2O3 nanoparticles for optimal performance as an MRI contrast agent

The transverse 1H relaxivities of aqueous colloidal solutions of dextran coated Dy2O3 nanoparticles of different sizes were investigated at magnetic field strengths (B) between 7 and 17.6 T. The particle size with the maximum relaxivity (r2) appears to vary between 70 nm at 7 T (r2 approximately = 190 s(-1) mM(-1)) and 60 nm at 17.6 T (r2 approximately = 675 s(-1) mM(-1)). A small difference between r2 and r2* was observed, which was ascribed to the effect of the dextran coating. The value of r2 is proportional to B2 up to 12 T after which it saturates. Independent magnetization measurements on these particles at room temperature at magnetic field strengths up to 30 T, however, show a typical paramagnetic behavior with a magnetization of the particle that is proportional to the field strength. The saturation in the curve of r2 as a function of B2 was tentatively explained by the presence of an extremely fast relaxing component of the signal at high field strengths, which is not observable on the NMR time scale. The results of this study can be exploited for the rational design of MRI contrast agents, based on lanthanide oxide particles, with high efficiencies at magnetic field strengths of more than 1.5 T.     

Asymptotic inference for AR(1) processes with (nonnormal) stable errors

This is the first of several papers in which we consider problems related to the asymptotic distribution of the least squares estimate of the parameter γ in theAR(1) model $$X_k = \gamma X_{k - 1} + \varepsilon _k , k = 1,...,n,$$ where ε_k are independent identically distributed (i.i.d.) random variables in the domain of attraction of a stable law. In §1 we give a summary in the case ε_k is in the domain of attraction of the normal distribution. In §2 we consider errors in the domain of attraction of a (nonnormal) stable distribution. In §3 we prove a result in the case of the completely asymmetric stable distribution with α=β=1.