魔角旋转情形下固体的非均匀相互作用

各向异性非均匀相互作用的慢旋转MAS-NMR谱是一系列的旋转边带,这些边带的强度包含了该相互作用张量的全部信息,只要计算和分析少数几条边带就可以得到这些有用的结构信息,本文用不可约球谐张量算符表示非均匀相互作用,采用Bessel函数展开法分析计算非均匀相互作用产生的旋转边带,这种分析计算方法适用于各种非均匀相互作用。以化学位移和四极相互作用为例的实验与理论符合很好。 关键词：     

应用相位循环方法抑制TOSS谱的残余旋转边带

对π脉冲宽度的误差与频率偏置效应引起的TOSS谱的残余旋转边带进行了分析,证实了相位循环方法能够在一级近似下有效地抑制这种残余旋转边带。在比较宽的频率偏置范围内从实验上检验了相位循环方法的有效性。     

二维旋转边带分解实验

描述了一类新的二维旋转边带分解实验,准备期包括交叉极化、若干个以TOSS序列的延迟加上的π脉冲和一个π/2脉冲,混合期包括一个π/2脉冲和一个TOSS序列,在演化期内,磁化矢量位于z方向。本实验能给出完全分解的各向同性化学位移与各向异性化学位移,它与Kolbert等人^[5]的实验方法相互补充。     

具有Dzyaloshinskii-Moriya相互作用的三量子比特海森伯模型中的对纠缠

研究了一维三量子比特海森伯模型中的对纠缠的提高和控制问题,在该系统中引入了Dzyaloshinskii-Moriya（DM)相互作用,通过求解共生来计算两量子比特之间的热纠缠,结果表明：对于XXX模型,引入DM相互作用D,可以诱导铁磁和反铁磁自旋链产生热纠缠,尽管它们产生热纠缠所需的D值大小不同.对于XXZ模型,引入DM相互作用后,可以使原本不存在热纠缠的反铁磁自旋链产生纠缠,而且对于铁磁和反铁磁这两种XXZ自旋链,DM相互作用和各向异 关键词： 纠缠 XXX模型')" href="#">XXX模型 XXZ模型')" href="#">XXZ模型 Dzyaloshinskii-Moriya相互作用     

两位置交换问题魔角旋转核磁共振时域信号的准确解

已知两位置交换分子运动速率对于静态¹³CNMR谱的影响是由一简单代数关系决定,它已成为分析固体中分子运动的常规方法.慢速的魔角旋转可以保留边带,所产生的边带型原则上也可用来分析分子运动,只是缺乏象静态谱那样的简单的关系,以至于在实际应用上是不可行的.本文给出作者所发现的存在分子运动时魔角旋转核磁共振自由诱导衰减信号的解析表达式的理论推导,提供分析固体分子运动的理论谱计算公式.我们选择二甲矾(dimethylsulfone)作为模型化合物,记录其不同温度下¹³CNMR谱,又计算了两个甲基做两位置交换运动的理论谱.理论谱与实验谱定性一致.定量上,边带强度存在一些差别,但边带宽度符合很好.通过理论谱与实验谱的比较,得到交换速率与温度的对应关系.虽然本文限于化学位移各向异性相互作用两位置交换问题,但容易推广到偶极作用多位置交换问题.     

生物流体中声参量效应的研究

本文利用Fenlon理论分析了流体中低频泵浦波与高频弱声波非线性相互作用而产生的参量效应,计算了和差频边带声的声压表式,并由此导出计及媒质衰减时的非线性参量B/A的公式。理论计算表明和差频边带声压幅值随泵浦声压线性地增加,由此提供了一个测量非线性参量的方法,用此方法测量了若干生物流体和混合物的B/A值。     

魔角旋转核磁共振波谱学

样品绕与外加恒定磁场方向成魔角β_M=cos^-1(1)/√3的轴向快速旋转(MAS)可以消除固体NMR的多种谱增宽。本文首先介绍了固体中的几种主要的各向异性相互作用,即偶极-偶极,核四极,自旋-自旋或称间接偶极-偶极以及化学位移相互作用。然后用平均哈密顿理论阐述了MAS对这些相互作用的影响。现今的固体高分辨方法有四种,即多脉冲(MP),多量子(MQ),交叉极化和大功率异核去耦合(CP)以及MAS,在这些方法中MAS方法是唯一可以获得各向同性化学位移的。因而要得到类似于液体谱的固体高分辨,MAS往往是最后的手段。以上这些方法的联合运用以及极为广泛的应用研究使得固体高分辨NMR兴旺发达起来了。本文引述了大量原始和最新的文献,力图概述当前该领域中的大部分成就。评论这一领域的理论和实验技术的发展。     

一维扩展t-J模型中密度-自旋相互作用诱导的相分离

t-J模型是研究高温超导电性的重要理论模型之一. 最近的冷分子实验表明可用极性分子模拟t-J模型. 实验模拟的t-J模型除了引进长程的偶极相互作用外, 还引进了密度-自旋相互作用. 本文使用密度矩阵重整化群方法研究了密度-自旋相互作用对一维t-J 模型基态性质的影响. 选取了t-J模型基态相图中不同相区的三个点, 计算了不同密度-自旋相互作用强度下的粒子数和自旋的实空间分布,以及密度-密度关联函数和自旋-自旋关联函数与相应的结构因子. 计算结果表明, 密度-自旋相互作用强度较弱时, 对系统的性质不会产生定性影响;当其强度足够大时, 系统会进入相分离, 该相分离与传统t-J模型的相分离有很大区别.     

大规模宽频弹性动力学分析的快速定向压缩边界元法

发展一种大规模宽频弹性动力学分析的快速定向压缩边界元法.证明弹性动力学核函数具有定向低秩特性，为采用快速定向压缩算法提供理论基础.根据S波的波数，将节点之间的相互作用划分为低频相互作用和高频相互作用，并将后者进一步划分为与多个楔形区.在楔形区上，可以采用核函数的定向低秩特性进行快速计算.低频相互作用与核无关快速多极边界元法中计算方法相同，不同方向楔形区上的变换矩阵可以采用坐标系旋转的方法进行快速计算.可对任意频率进行快速谐响应分析.数值算例表明：该方法可以将宽频弹性动力学问题计算复杂度降低到O（N log^αN）.与卷积求积边界元法相结合，也可以应用于弹性动力学瞬态分析.     

质子核磁共振谱的偶极魔角旋转边带强度的矩的展开

对于包含分子和分子基团绕至少一个轴高速运动的固体体系,本文推导出其质子核磁共振谱的偶极魔角旋转边带强度的理论计算表达式,建立了用其静态粉末谱的矩的展开的计算方法,计算出旋转边带强度按三十阶矩展开的系数,它可以处理包含高达十五阶边带的谱     

A new sensitive isotropic-anisotropic separation experiment-SPEED MAS

A new sensitive 2D isotropic-anisotropic separation experiment that utilizes stroboscopic phase encoding in the evolution dimension (SPEED) under magic angle sample spinning is presented. This 2D experiment consists of a train of 2N - 1 pi pulses that are applied over 2N rotor periods. The pi pulse train effectively reduces the apparent spinning speed in the evolution dimension by a factor of 1 / (2N) from the mechanical spinning speed. Thus, problems commonly associated with magic angle turning such as stable slow spinning, different matching and TPPM proton decoupling conditions are avoided. Data replication similar to the five pi replicated magic angle turning (FIREMAT) and pseudo 2D sideband suppression (P2DSS) experiments transfers resolution from the acquisition dimension to the evolution dimension. Hence, large spectral windows with good digital resolution are obtained with a few evolution increments. Here, slow spinning sideband patterns are extracted from the replicated 2D dataset with TIGER processing. Nevertheless, 2D Fourier transformation is also applicable. The extracted sideband patterns are identical to magic angle turning sideband pattern allowing for easy extraction of principal shift components. Accurate (13)C principal shift components are obtained for 3-methylglutaric acid using SPEED and FIREMAT experiments to validate the method. Furthermore, SPEED spectra for calcium acetate and alpha santonin are reported to show the wide applicability of this new experiment.     

Removal of sidebands in double-rotation NMR in real time

Double-rotation (DOR) is the only technique generally capable of yielding high-resolution NMR spectra of half-integer quadrupolar nuclei in one dimension for solids without the need for sophisticated coherence pathway selection. Unfortunately, due to the low outer rotor spinning frequencies currently available, the spectra often contain a large number of spinning sidebands which may overlap with the resonances of interest. We implement a simple, robust, and easy to use family of pulse sequences, which in practice are fully analogous to the 'total suppression of sidebands' (TOSS) sequences, to suppress all sidebands arising from the spinning of the outer rotor in DOR experiments. By removing the rotor phase dependence of the evolution of the sidebands, the sidebands destructively interfere with one another during the course of signal averaging to yield 'solution-like' spectra of half-integer quadrupolar nuclei in solids. Advantages and shortcomings of the method compared to other DOR sideband suppression methods are explored with the aid of simulations.     

The evaluation of different MAS techniques at low spinning rates in aqueous samples and in the presence of magnetic susceptibility gradients

It was recently demonstrated that the nuclear magnetic resonance (NMR) linewidths for stationary biological samples are dictated mainly by magnetic susceptibility gradients, and that phase-altered spinning sideband (PASS) and phase-corrected magic angle turning (PHORMAT) solid-state NMR techniques employing slow and ultra-slow magic angle spinning (MAS) frequencies can be used to overcome the static susceptibility broadening to yield high-resolution, spinning sideband (SSB)-free 1H NMR spectra [Magn. Reson. Med. 46 (2001) 213; 47 (2002) 829]. An additional concern is that molecular diffusion in the presence of the susceptibility gradients may limit the minimum useful MAS frequency by broadening the lines and reducing SSB suppression at low spinning frequencies. In this article the performance of PASS, PHORMAT, total sideband suppression (TOSS), and standard MAS techniques were evaluated as a function of spinning frequency. To this end, 300MHz (7.05T) 1H NMR spectra were acquired via PASS, TOSS, PHORMAT, and standard MAS NMR techniques for a 230-microm-diameter spherical glass bead pack saturated with water. The resulting strong magnetic susceptibility gradients result in a static linewidth of about 3.7kHz that is larger than observed for a natural biological sample, constituting a worst-case scenario for examination of susceptibility broadening effects. RESULTS: (I) TOSS produces a distorted centerband and fails in suppressing the SSBs at a spinning rate below approximately 1kHz. (II) Standard MAS requires spinning speeds above a few hundred Hz to separate the centerband from the SSBs. (III) PASS produces nearly SSB-free spectra at spinning speeds as low as 30Hz, and is only limited by T(2)-induced signal losses. (IV) With PHORMAT, a SSB-free isotropic projection is obtained at any spinning rate, even at an ultra-slow spinning rate as slow as 1Hz. (V) It is found empirically that the width of the isotropic peak is proportional to F(-x), where F is the spinning frequency, and x=2 for MAS, 0.84 for PASS, and 0.5 for PHORMAT.     

Inhomogeneous interactions in solids under slow off-magic angle spinning.

The central band of an inhomogeneous interaction under slow off-magic angle spinning (off-MAS) is no longer a scaled static spectrum with scaling factor P2(cos upsilon), that is, the spectrum distortion occurs in comparison with that under the rapid rotation condition. The dependence of this distortion in the chemical shift spectrum, for instance, on anisotropy, the asymmetry parameter, spinning rate and the angle between the spinning axis and the applied magnetic field is studied in this paper. It is shown that the distortion can be partly eliminated by using TOSS in certain circumstances. Experimental evidence is presented in this study.     

Analytical solutions to several magic-angle spinning NMR experiments

Using the Anderson–Weiss (AW) formalism, analytical expressions of the NMR signal are obtained for the following magic-angle spinning (MAS) experiments: total suppression of sidebands (TOSS); phase adjusted spinning sidebands (PASS); rotational-echo double-resonance (REDOR); rotor-encoded REDOR (REREDOR); cross-polarization magic-angle spinning (CPMAS); exchange induced sidebands (EIS); one-dimensional exchange spectroscopy by sideband alternation (ODESSA); time-reverse ODESSA (trODESSA); centerband-only detection of exchange (CODEX). In order to test the validity of the AW approach, the Gaussian powder approximation is compared with exact powder calculations. A quantitative study of the effect of molecular dynamics on the efficiency of the TOSS and REDOR pulse sequences is then presented.     

High-performance solvent suppression for proton detected solid-state NMR

High-sensitivity proton detected experiments in solid-state NMR have been recently demonstrated in proton diluted proteins as well as fully protonated samples under fast magic-angle spinning. One key element for performing successful proton detection is effective solvent suppression achieved by pulsed field gradients (PFG) and/or saturation pulses. Here we report a high-performance solvent suppression method that attenuates multiple solvent signals simultaneously by more than a factor of 10,000, achieved by an optimized combination of homospoil gradients and supercycled saturation pulses. This method, which we call Multiple Intense Solvent Suppression Intended for Sensitive Spectroscopic Investigation of Protonated Proteins, Instantly (MISSISSIPPI), can be applied without a PFG probe. It opens up new opportunities for two-dimensional heteronuclear correlation spectroscopy of hydrated proteins at natural abundance as well as high-sensitivity and multi-dimensional experimental investigation of protein-solvent interactions.     

A comparative study of jet formation and nanofiber alignment in electrospinning and electrocentrifugal spinning systems

Electrocentrifugal spinning is a recently developed spinning system whose performance is still under investigation by researchers. In this study the process of jet formation in electrocentrifugal spinning is explored and compared to the same process in electrospinning and centrifuge spinning. The results show that the incorporation of the electrical and the centrifugal forces in the electrocentrifugal spinning system leads to the formation of a more stable jet at lower viscosities. It is also shown that the electrocentrifugal spinning method is an efficient technique for the production of aligned nanofiber bundles with enhancement in the mechanical properties.     

Low noise spinning wheel technique for THz material parameter extraction

Double-modulated terahertz differential time-domain spectroscopy (double-modulated THz-DTDS), is a technique that is based on dithering the sample under test. In this paper, we report a measurement technique based on mounting the sample on a spinning wheel, in order to overcome fundamental limitations imposed by linear dithering. We demonstrate a proof-of-principle showing that noise decreases as a function of the spinning wheel modulation frequency. This technique does not suffer the mechanical noise limitation of traditional linear dithering and thus opens up future scope for further noise reduction via hardware advances in the modulation frequency of the wheel. The spinning wheel technique enables a rapid succession of measurements between the reference and sample signals with a single mechanical delay scan. As a result, an improvement in measurement time by at least a factor of two, as compared to the conventional THz-TDS measurement technique is observed. The spinning wheel technique is experimentally verified by measuring the dielectric properties of a thick polymer material.     

To what extent does the centerband and each spinning sideband contain contributions from different crystallites of the powder sample?

It is shown that due to the destructive interference of the magnetization paths of crystallites taking place during the rotor period at slow spinning regime, the contribution of different crystallites to the centerband and to each spinning sideband is strongly weighted. For this, a separated-local-field experiment is used to tag the crystallites contributing to a given spinning sideband at different spinning speeds. The orientation dependence of spinning sidebands is also responsible for different lineshapes of the centerband and of each spinning sidebands observed under conditions of off-magic angle spinning.     

F/Si distance determination in fluoride-containing octadecasil by Hartmann–Hahn cross-polarization under fast magic-angle spinning

¹⁹F/²⁹Si Hartmann–Hahn continuous wave cross-polarization (CP) has been applied under fast magic-angle spinning (MAS) to a powder sample of octadecasil. Strong oscillations occur during CP on a sideband matching condition between the isolated ²⁹Si–¹⁹F spin pairs formed by the silicons in the D4R units and the fluoride anions. The magnitude of the dipolar coupling constant was deduced directly from the line-splitting between the intense singularities of the Pake-like patterns obtained by Fourier transformation of the oscillatory polarization transfer. The corresponding Si–F internuclear distance, r=2.62±0.05 Å, is found to be in very good agreement with the X-ray crystal structure and the value of 2.69±0.04 Å recently reported from rotational echo double resonance (REDOR) and transferred echo double resonance (TEDOR) nuclear magnetic resonance (NMR) experiments. Furthermore, the CP technique is still reliable under fast MAS where both REDOR and TEDOR sequences suffer from severe artefacts due to finite pulse lengths. In octadecasil, a spinning frequency of 14 kHz is shown to be necessary for an effective suppression of ¹⁹F–¹⁹F spin diffusion. The influences of experimental missettings and radiofrequency (RF) field inhomogeneity are taken into account.