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51.
The increasing application of positron emission tomography (PET) in nuclear medicine has stimulated the extensive development of a multitude of novel and versatile techniques to introduce fluorine-18, especially for the radiolabelling of biologically or pharmacologically active molecules. Taking into consideration that the introduction of fluorine-18 (t1/2=109.8 min) mostly proceeds under harsh conditions, radiolabelling of such molecules represents a challenge and is of enormous interest. Ideally, it should proceed in a regioselective manner under mild physiological conditions, in an acceptable time span, with high yields and high specific activities. Special attention has been drawn to 2-fluoroethyl and 3-fluoropropyl groups, which are often the active sites of radiofluorinated compounds. Precursors containing an ammonium leaving group – such as a strained azetidinium or aziridinium moiety – can help to overcome these obstacles leading to a convenient and mild introduction of [18F]fluoride with high radiochemical yields. 相似文献
52.
Peng Sun Zihan Xia Zhongfang Li Ziwei Fan Fengqi He Qiang Liu Xiaoyan Yin 《Materials Today Chemistry》2022
A high-temperature proton exchange membrane with high proton conductivity over a wide humidity range still remains a challenge. PBI dendrimer containing triazine rings (TPBI) was synthesized to approach this aim considering its high content of hygroscopic terminal groups and of larger free volume. A novel proton conductor previously synthesized (zirconium 3-sulfopropyl phosphonate, ZrSP) was doped due to its good proton conductivity over a wide humidity range. TPBI was post-crosslinked with a tetrafunctional epoxy resin (N,N,N′,N′-tetraglycidyl-4,4′-diaminodiphenylmethane, TGDDM) to enhance the mechanical stability at low cross-linking degrees, which allowed high doping levels of ZrSP, and thus, high conductivity. The prepared membranes (TPBI-TGDDM/ZrSP) showed good thermal stability, high proton conductivity over wide humidity range, and good dimensional stability. At suitable degrees of branching, TPBI-TGDDM/ZrSP exhibited superior mechanical property, oxidative stability, methanol barrier property, and membrane selectivity than its linear analog (mPBI-TGDDM/ZrSP). As ZrSP instead of PA was applied as the proton conductor, TPBI-TGDDM/ZrSP showed good durability of proton conductivity, especially in comparison with TPBI-TGDDM/PA, which highly retarded decline in conductivity caused by PA leaking. The proton conductivity at 180 °C of TPBI(20)-TGDDM(10)/ZrSP(50) achieved 142, 84.2 and 23.6 mS cm?1 at 100%, 50%, and 0 RH, respectively. 相似文献
53.
Gaetano D'Avino 《Electrophoresis》2021,42(21-22):2293-2302
The migration of a spherical particle immersed in a viscoelastic liquid flowing in a microchannel with a triangular cross-section is investigated by direct numerical simulations under inertialess conditions. The viscoelastic fluid is modeled through two constitutive equations to investigate the effect of the second normal stress difference and the resulting secondary flows on the migration phenomenon. The results are presented in terms of trajectories followed by the particles released at different initial positions over the channel cross-section in a wide range of Weissenberg numbers and confinement ratios. Particles suspended in a fluid with a negligible second normal stress difference migrate toward the channel centerline or the closest wall, depending on their initial position. A much more complex dynamics is found for particles suspended in a fluid with a relevant second normal stress difference due to the appearance of secondary flows that compete with the migration phenomenon. Depending on the Weissenberg number and confinement ratio, additional equilibrium positions (points or closed orbits) may appear. In this case, the channel centerline becomes unstable and the particles are driven to the corners or “entrapped” in recirculation regions within the channel cross-section. The inversion of the centerline stability can be exploited to design efficient size-based separation devices. 相似文献
54.
In light of the generator polynomials of constacyclic codes over finite chain rings, the depth spectrum of constacyclic codes can be determined if (n,p)=1. 相似文献
55.
We study electron energies in a double concentric quantum ring with anisotropy in the rims heights in the presence of the external magnetic field applied along the symmetry axis. To this end, we consider a model in which the thickness grows linearly from the axis up to the inner rim with a slope different from one between the inner and the outer rims. The anisotropy in the rims heights originated by the presence in the structure of various valleys we simulate by periodic dependence of the slope on the radial direction. We show that the wave functions of the electron confined in such structure can be found analytically if the slopes in all radial directions are the same, and by using a simple exact diagonalization procedure otherwise. The behavior of the electron energies as functions of the magnetic field, rings radii and rims heights, as well as the number of the valleys and their depths is consistently described with our formalism. The entanglement of the states with different radial and orbital quantum numbers, the period and the amplitude of the Aharonov–Bohm oscillations are very sensible to any variations of the rims heights. 相似文献
56.
Minghui Zhou Marianne Lankelma Dr. Ir. Jarl Ivar van der Vlugt Prof. Dr. Bas de Bruin 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(27):11166-11172
The metalloradical activation of o-aryl aldehydes with tosylhydrazide and a cobalt(II) porphyrin catalyst produces cobalt(III)-carbene radical intermediates, providing a new and powerful strategy for the synthesis of medium-sized ring structures. Herein we make use of the intrinsic radical-type reactivity of cobalt(III)-carbene radical intermediates in the [CoII(TPP)]-catalyzed (TPP=tetraphenylporphyrin) synthesis of two types of 8-membered ring compounds; novel dibenzocyclooctenes and unprecedented monobenzocyclooctadienes. The method was successfully applied to afford a variety of 8-membered ring compounds in good yields and with excellent substituent tolerance. Density functional theory (DFT) calculations and experimental results suggest that the reactions proceed via hydrogen atom transfer from the bis-allylic/benzallylic C−H bond to the carbene radical, followed by two divergent processes for ring-closure to the two different types of 8-membered ring products. While the dibenzocyclooctenes are most likely formed by dissociation of o-quinodimethanes (o-QDMs) which undergo a non-catalyzed 8π-cyclization, DFT calculations suggest that ring-closure to the monobenzocyclooctadienes involves a radical-rebound step in the coordination sphere of cobalt. The latter mechanism implies that unprecedented enantioselective ring-closure reactions to chiral monobenzocyclooctadienes should be possible, as was confirmed for reactions mediated by a chiral cobalt-porphyrin catalyst. 相似文献
57.
通过利用半单环的结构定理、环的单位群的性质、环的阶及环的单位群的阶的关系等,确定了当n为素数,m=2~(t+1),t为正整数时,群环z_nD_m的代数结构和性质. 相似文献
58.
O.A. Shegai V.I. Mashanov H.-H. Cheng O.P. Pchelyakov 《Physica E: Low-dimensional Systems and Nanostructures》2009,42(1):22-24
At low temperatures a lateral photoconductivity (PC) of Ge/Si (1 0 0) self-organized quantum rings (QRs) structures as a function of interband light intensity has been investigated for different values of lateral voltage and temperature. In contrast to self-organized Ge/Si quantum dots (QDs) structures (grown at the same conditions) where the stepped PC was registered, for QRs structures essential smoothing of PC steps was observed. Such behavior is determined by decreasing of strain potential around QRs in conductive Si matrix due to a transfer of Ge atoms from the center of QDs to its periphery accompanied by Ge/Si intermixing. 相似文献
59.
T. A. Nguyen H. T. Dung 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2008,46(1):173-177
Using the Born expansion of the Green tensor, we consider the
spontaneous decay rate of an excited atom placed in the vicinity of
a rectangular plate. We discuss the limitations of the commonly used
simplifying assumption that the plate extends to infinity in the lateral
directions and examine the effects of the atomic dipole moment
orientation, atomic position, and plate boundary and thickness on the atomic
decay rate. In particular, it is shown that
due to the plate finite size, the spontaneous decay may be inhibited
even when the atom is situated very close to the surface, and that
in the boundary region, the
spontaneous decay rate can be strongly modified. 相似文献
60.
Let be a complete local domain containing the integers with maximal ideal such that is at least the cardinality of the real numbers. Let be a nonmaximal prime ideal of such that is a regular local ring. We construct an excellent local ring such that the completion of is , the generic formal fiber of is local with maximal ideal and if is a nonzero ideal of , then is complete.