NMR and AFM investigations of nanocavities on the double rare-earth fluoride crystal surface

Nanocavities with a size of less than 30 nm on the crystal surface of superfine LiYF₄ powders and single crystals of the Van Vleck paramagnet LiTmF₄ were detected by nuclear magnetic resonance (NMR) cryoporometry and atomic-force microscopy (AFM) and liquid³He NMR cryoporometry. NMR investigations show that the nanocavities are evidently present on the surface of the LiYF₄ powder particles. The distribution of nanocavities has two maxima corresponding to the specific porous sizes of 3–5 and 10–15 nm. AFM investigations detect the presence of the nanocavities with sizes in the range 50–300 nm on the surface of powder particles and single crystals. The cases of powder microparticles and of a single crystal differ in the value of the parameter “surface roughness”, which is 14.5 and 11 nm, respectively. The mechanism of creation of nanocavities is proposed and verified by additional investigations with CaF₂ powders.     

NMR investigations of ion distribution in real structure of manganese-zinc ferrites

Using the nuclear magnetic resonance, X-ray and electromagnetic methods we investigated structure and properties of the single- and polycrystalline manganous and manganese-zinc ferrites: MnFe₂O₄, Mn_0.8Zn_0.2Fe₂O₄, Mn_0.6−xCu_xZn_0.3Fe_2.1O₄ (0 ≤x ≤ 0.3). The real structure of the manganous and manganese-zinc ferrites is found to contain simultaneously vacancies in cationic and anionic sublattices. The analysis of the NMR spectra of⁵⁵Mn,⁵⁷Fe,^63,65Cu and⁶⁷Zn ions allowed us to find the magnetic and valence states of all ions and their distribution in the real spinel structure. It was shown that the Fe³⁺ paramagnetic ions, Zn²⁺ diamagnetic ions and cationic vacancies in tetrapositions are distributed statistically. The Mn³⁺ and Cu²⁺ Jahn-Teller ions and presumably Fe²⁺ in octahedral positions are distributed nonstatistically forming cluster-type mesoscopic inhomogeneities. The correlation between distribution of ions and vacancies in the real structure of manganese-zinc ferrites and their functional electromagnetic properties was found.     

Quantum chemical studies of semiconductor surface chemistry using cluster models

Modern quantum chemical methods can be used to investigate many properties of novel molecules and materials with predictive power. We have carried out accurate quantum chemical calculations with cluster models to investigate chemical reactions on semiconductor surfaces. The structure–property relationships that emerge from these studies are illustrated with particular emphasis on silicon as well as indium phosphide surface chemistry. Some new strategies that we have developed to provide a proper balance between covalent and dative bonding in compound semiconductors are discussed. Embedded cluster models have been used in some cases to include the effects of the surroundings on the active region. The structural and mechanistic understanding that emerges from our studies is illustrated by selected results on atomic layer deposition of Al₂O₃ on silicon and hydrogenation of P-rich and In-rich indium phosphide surfaces.     

Physical and chemical investigations on natural dyes

Natural dyes have been used extensively in the past for many purposes, such us to colour fibers and to produce inks, watercolours and paints, but their use declined rapidly after the discovery of synthetic colours. Nowadays we witness a renewed interest, as natural dyes are neither toxic nor polluting.     

高温超导体的核磁共振研究

文章回顾了用核磁共振技术研究高温超导体的进展,着重介绍了局域电荷分布、d波超导能隙以及赝能隙的性质.除了CuO2面上空穴密度总数,Cu和O轨道上面空穴数的分布是决定Tc的重要参数.转变温度Tc取决于自旋晶格弛豫率,奈特位移可以显示出d波配对,而d波配对使得准粒子可以从涡线中心 “漏”到外面.超导能隙函数里存在节点,这也是非磁性杂质以及晶体无序会导致Tc明显降低的原因.高温超导体零温正常态在45T的强磁场下变成“费米弧”金属态,这说明赝能隙态和超导态共存于高温超导物质中.     

Quantum chemical interpretation of 13C NMR spectra of adsorbed molecules

In order to assist the interpretation of carbon-13 NMR spectra for molecules sorbed on solid surfaces, a theoretical analysis of resonance shifts was performed which were measured for simple alkenes and arenes in zeolites of type X and Y and on silica gel surfaces. Quantum chemical computations for simple models of adsorption complexes on the basis of CNDO and PCILO methods were combined with resonance shift calculations by means of the Ramsey theory and by employing an empirical formula to find out the most probable structures for the adsorption complexes.     

High-speed 1H MAS NMR investigations of the weathered surface of a phosphate glass

Solid-state high-speed 1H MAS NMR spectroscopy was used to investigate the weathered surface of a potassium aluminum phosphate (KAP) glass exposed to a humid environment (30K2O10Al2O360P2O5, mol%). Through the combination of spin-spin relaxation and double quantum (DQ) filtering it was possible to resolve seven or eight different proton environments within the weathered surface of the KAP glass. Two-dimensional (2D) DQ and 2D NOESY NMR correlation experiments were performed to probe the spatial proximity of these different proton species. These 1H-1H correlation experiments helped confirm the spectral assignments. The analysis of these different 1H environments provides additional information about the chemical processes that occur at the weathered glass surface.     

Structural investigations of DNA-polycation complexes

The internal structure of DNA-polycation complexes is investigated by synchrotron small-angle X-ray scattering (SAXS). Hexagonal packing of DNA is observed for DNA complexed with poly-L-lysine (PL), poly-L-arginine (PA), spermine (Sp), and linear and branched polyethyleneimine (lPEI and bPEI, respectively). Variations in the internal spacings and degree of long-range ordering are dependent on both polycation type and concentration of added salt. With increasing concentration of monovalent salt, a discontinuous phase transition is observed from compact to loose bundles and finally to an isotropic network phase. This salt-induced melting transition was found to be universal for all polyplexes studied and is in quantitative agreement with a simple free energy model based solely on electrostatic and entropic contributions. Using the osmotic stress method, bulk modulus (K) is measured for PL-DNA and PA-DNA polyplexes at various salt concentrations. With increasing osmotic force, we show that the salt-induced melting transition is shifted and compression in the loose bundle regime is in qualitative agreement with our model.     

在［311］面上直接生长GaAs／AlAs的量子线结构及其光学性质

本文介绍在分子束外延系统中，沿［３１１］面原位生长ＧａＡｓ／ＡＩＡｓ量子线结构的一种简单方法及其拉曼和发光特性。这一方法能克服目前纳米尺度光刻技术的困难，因此，是一种制备低维样品的有希望的方法。     

Quantum Monte Carlo investigations of adsorption energetics on graphene

We have performed calculations of adsorption energetics on the graphene surface using the state-of-the-art diffusion quantum Monte Carlo method. Two types of configurations are considered in this work: the adsorption of a single O, F, or H atom on the graphene surface and the H-saturated graphene system (graphane). The adsorption energies are compared with those obtained from density functional theory with various exchange-correlation functionals. The results indicate that the approximate exchange-correlation functionals significantly overestimate the binding of O and F atoms on graphene, although the preferred adsorption sites are consistent. The energy errors are much less for atomic hydrogen adsorbed on the surface. We also find that a single O or H atom on graphene has a higher energy than in the molecular state, while the adsorption of a single F atom is preferred over the gas phase. In addition, the energetics of graphane is reported. The calculated equilibrium lattice constant turns out to be larger than that of graphene, at variance with a recent experimental suggestion.     

EPR investigations of bonding and activation of nitrogen oxides by surface complexes of transition metal ions

EPR spectroscopy has been used to investigate changes occurring at the molecular level during coordination and activation of NO and N₂O small pollutant molecules on Cu/ZSM-5 and Mo/SiO₂ catalysts, respectively. It is shown that the activation of nitric oxide occurs by formation of a bent η¹{CuNO}¹¹ surface adduct with rehybridization of the NO Orbitals. The spin Hamiltonian parameters of this species have been analyzed in detail, leading to a molecular and electronic picture of the complex. The mechanism along which N₂O is decomposed upon interaction with surface Mo⁵⁺ species involves dissociative metal-to-ligand electron transfer. The reaction was monitored by EPR using naturally abundant and⁹⁵Mo-enriched molybdenum. The dynamics of the N-O bond cleavage is discussed in terms of two-dimensional Morse potential energy surfaces. From the EPR parameters of the resultant O⁻ radical, the spin density distribution and the stabilization energy were calculated.     

取代硝基苯炸药的理论研究

在133LYP/6.31 G*理论水平上优化了一系列取代硝基苯类化舍物的几何构型,计算了它们的电子结构和Wiberg键级,结果表明,随着氨基的引入,起爆引发键C-NO2键的强度有可能得到加强,炸药的撞击感度减小.讨论了分子内氢键与其撞击感度的关系,结果表明氢键的形成可能对分子的撞击感度起钝化作用.讨论了C-NO2键的强弱与其位置的关系.     

Quantum logics and chemical kinetics

A statistical theory of chemical kinetics is presented based on the quantum logical concept of chemical observables. The apparatus of Boolean algebra $\text{B}$ is applied for the construction of appropriate composition polynomials referring to any stipulated arrangement of the atomic constituents. A physically motivated probability measure μ(F) is introduced on the field $\text{B}$ of chemical observables, which considers the occurrence of the yes response of a given F ? $\text{B}$. The equations for the time evolution of the species density operators and the master equations for the corresponding number densities are derived. The general treatment is applied to a superposition of elementary substitution reactions (AB)_α + C ? (AC)_β + B. The expressions for the reaction rate coefficients are established.     

量子算法的NMR实现方法

量子计算机是一种以量子耦合方式进行信息处理的装置[1 ] 。原则上 ,它能利用量子相干干涉方法以比传统计算机更快的速度进行诸如大数的因式分解、未排序数据库中的数据搜索等工作[2 ] 。建造大型量子计算机的主要困难是噪音、去耦和制造工艺。一方面 ,虽然离子陷阱和光学腔实验方法大有希望 ,但这些方法都还没有成功实现过量子计算。另一方面 ,因为隔离于自然环境 ,核自旋可以成为很好的“量子比特” ,可能以非传统方式使用核磁共振 (NMR)技术实现量子计算。本文介绍一种用NMR方法实现量子计算的方法 ,该方法能够用比传统方法少的步骤解决一个纯数学问题。基于该方法的简单量子计算机使用比传统计算机使用更少的函数“调用”判断一未知函数的类别。     

七量子位Deutsch-Josza量子算法的核磁共振实验实现

近年来 ,量子计算机的研究有了很大的发展 ,在目前提出的各种量子计算的方案中 ,核磁共振技术对模拟和演示量子算法以及验证量子计算机的优越性做出了巨大的贡献 .Deutsch Jozsa算法是一种研究较为广泛的量子算法 ,它可以用核磁共振实验予以验证 ,并可根据Cirac等人提出的方案予以简化 .报道了在核磁共振量子计算机上实验实现七位Deutsch Jozsa算法的过程和结果. Recent years, remarkable progresses in experimental realization of quantum information have been made, especially based on nuclear magnetic resonance (NMR) theory. In all quantum algorithms, Deutsch-Jozsa algorithm has been widely studied. It can be realized on NMR quantum computer and also can be simplified by using the Cirac s scheme. In this paper, at first the principle of Deutsch-Jozsa quantum algorithm is analyzed, then we implement the seven-qubit Deutsch-Jozsa algorithm...     

Determination of the 19F NMR chemical shielding tensor and crystal structure of 5-fluoro-dl-tryptophan

5-Fluoro-dl-tryptophan (5F-Trp) is a very sensitive probe used to investigate orientation and dynamics of biomacromolecules at the in situ level. In order to establish a (19)F NMR strategy, the crystal structure and (19)F chemical shielding tensor of 5F-Trp are reported. A novel approach was developed to use F-F homonuclear dipole-dipole coupling information to analyze single-crystal NMR data without determining crystal orientations. The measured values for the principal components of the shielding tensor are sigma(11)=0.9, sigma(22)=-63.3, and sigma(33)=-82.9 ppm relative to TFA in D(2)O. The principal axes of the shielding tensors coincide with the indole ring symmetry, which makes it a straightforward and powerful tool to monitor protein alignment in oriented environments. Hartree-Fock (HF) and density functional theory (DFT) calculations of the chemical shielding tensors are also reported.     

NMR methods for molecular structure studies of paramagnetic lanthanide complexes in solutions. Applications to crown ether complexes

The literature data on NMR spectroscopic methods for investigation of the spatial structure and molecular dynamics of lanthanide complexes with macrocyclic polyethers in solutions are reviewed. Translated fromZhurnal Strukturnoi Khimii, Vol. 39, No. 4, pp. 714–730, July–August, 1998.