Determination of quadrupole parameters with a composite pulse for spurious signal suppression

Spurious signals such as the piezoelectric signal from a ferroelectric crystal or the ringing signal from the NMR probe head tuned for low gyromagnetic ratio nuclei are often observed in pulsed NMR. Both signals are cancelled using the Hahn echo sequence with appropriate phase cyclings. The present paper applies a composite-pulse sequence to cancel the ringing signal. The main advantage of this sequence over the Hahn echo sequence is in the simplicity of optimizing the line intensity: the optimization of only one pulse duration for this sequence but of two pulse durations and the interpulse delay for the Hahn echo sequence. We are interested in half-integer quadrupole spins (I = 3/2, 5/2, 7/2, and 9/2), which means that we must consider the first-order quadrupole interaction during the pulses. For simplicity, we deal mainly with spin I = 3/2 nuclei. Since the central-line intensity depends on the ratio of the quadrupole coupling constant (QCC) to the amplitude of the RF pulse, we can determine the QCC from a featureless lineshape by fitting the variation of the experimental central-line intensity for increasing pulse duration with theoretical results. Contrary to the one-pulse sequence where the central-line intensity is proportional to the pulse duration if the latter is short, there is no such condition with the composite-pulse sequence. In other words, this sequence does not allow us to quantify the relative spin populations in powders. The size of the sample must be much smaller than that of the RF coil in order for the RF magnetic field to become homogeneous for the sample. We used (87)Rb (I = 3/2) in an aqueous solution of RbCl and in RbNb2O5F powder, (131)Xe (I = 3/2) of xenon gas physisorbed in Na-Y zeolite, and (23)Na (I = 3/2) in two well-known powders (NaNO3 and NaNO2) to support our theoretical result.     

Sensitivity enhancement of HCACO by using an HMQC magnetization transfer scheme

Previous theoretical calculations have demonstrated that the multiquantum relaxation rate of (1)H(alpha)-(13)C(alpha)(R(MQ)) is, on average, 1.3 +/- 0.4 or 1.7 +/- 0.6 times slower than the single-quantum relaxation rate of (13)C(alpha)(R(C)) for a sample with or without, respectively, amide protons. By taking advantage of this fact and by using the PEP sensitivity enhancement scheme, an HMQC version of the HCACO experiment has been developed. We demonstrate that this new experiment is 23 and 55% more sensitive than the original HSQC version of the HCACO experiment, at constant times of 7 and 27 ms, respectively, for a sample of the BC domain of the ciliary neurotrophic factor receptor protein dissolved in D(2)O at 20 degrees C.     

Vibrational entropy-driven dealloying of Mo(1 0 0) and W(1 0 0) surface alloys

The formation and stability of Cu, Ag and Au-induced c(2 × 2) alloys at the Mo(1 0 0) and W(1 0 0) surfaces have been investigated with low-energy electron microscopy and diffraction. The ordered alloys transform to disordered overlayer structures at elevated temperature. Comparison of the transformation temperatures with energetics obtained from first principles calculations reveals the vibrational entropic contribution to the system free energy that defines alloy thermal stability. Effective Debye temperatures for metal adatoms are determined that exhibit the expected mass and bond strength dependence.     

阿姆斯特朗电解液液桥的实验现象及断裂机理研究

阿姆斯特朗液桥是电流体力学领域中的一种有趣现象,它悬在空中抵抗重力流动的神奇状态引起了人们的广泛关注.近十几年来,去离子水液桥已经通过理论和实验得到深入研究,但是对于电解液液桥的研究依然十分有限.本文以Na2SO4电解液作为形成液桥的实验液体,利用高速摄相机和红外热像仪研究了电解液液桥的形成过程、焦耳效应、流动中的热气泡产生及其破碎导致的液桥断裂等现象,提出了不同于去离子水液桥的断裂机理,有助于进一步加深人们对液桥这一复杂现象的理解.     

Naphthodithiophene-2,1,3-benzothiadiazole copolymers for bulk heterojunction solar cells

Two newly synthesized naphthodithiophene-based copolymers, PNB, exhibit a low optical bandgap of ～1.64 eV with which the solar cells fabricated from the blend of PNB and PC(71)BM afforded a power conversion efficiency of 5.3% with external quantum efficiency over 60% in a broad spectral range.     

A dipicolinic acid tag for rigid lanthanide tagging of proteins and paramagnetic NMR spectroscopy

A new lanthanide tag was designed for site-specific labeling of proteins with paramagnetic lanthanide ions. The tag, 4-mercaptomethyl-dipicolinic acid, binds lanthanide ions with nanomolar affinity, is readily attached to proteins via a disulfide bond, and avoids the problems of diastereomer formation associated with most of the conventional lanthanide tags. The high lanthanide affinity of the tag opens the possibility to measure residual dipolar couplings in a single sample containing a mixture of paramagnetic and diamagnetic lanthanides. Using the DNA-binding domain of the E. coli arginine repressor as an example, it is demonstrated that the tag allows immobilization of the lanthanide ion in close proximity of the protein by additional coordination of the lanthanide by a carboxyl group of the protein. The close proximity of the lanthanide ion promotes accurate determinations of magnetic susceptibility anisotropy tensors. In addition, the small size of the tag makes it highly suitable for studies of intermolecular interactions.     

Nonlinear Eigenvalues for a Quasilinear Elliptic System in Orlicz–Sobolev Spaces

Using an Orlicz–Sobolev Space setting, we consider an eigenvalue problem for a system of the form

Asymmetric synthesis of homo-apioneplanocin A from d-ribose

Homo-apioneplanocin A was efficiently synthesized via stereoselective hydroxymethylation, regio- and chemoselective hydroboration, and chemoselective oxidation as key steps from d-ribose.     

Bridged silsesquioxanes containing a chiral diphosphine substructure. Catalytic properties in asymmetric hydrogenation

Bridged silsesquioxanes with asymmetric catalytic properties are described. These new silica-based materials are obtained by the sol-gel hydrolysis of an organosilylated chiral compound bearing rhodium-complexed diphosphine ligands. The incorporation of the organometallic species in various hybrid networks was achieved upon co-hydrolysis of the latter silylated ligands with TEOS or with 1,4-bis(trimethoxysilyl)benzene. These amorphous hybrids have been tested as enantioselective catalysts for the hydrogenation of (Z)α-(acetamido)cinnamic acid to the corresponding aminoacid and the results were compared with that obtained from the complexed precursor in homogeneous medium and related grafted silica. Enantioselectivities slightly higher than for homogeneous reaction were obtained in the case of the heterogeneous catalysts prepared by the direct hydrolysis of the rhodium-complexed diphosphine compound or by its co-hydrolysis with 1,4-bis(trimethoxysilyl)benzene. Conversely, a significant decrease in selectivity was observed when the organometallic species was immobilised in silica or grafted at the surface of silica.     

高维空间中的有界凸域

证明n维空间中的有界凸域D能被拟共形映射到n维单位球B~n(0,1),即D是拟球,从而说明拟共形映射中的黎曼定理在n维空间中的有界凸域类中是成立的．