Chemical Shift Components Research Articles

Modeling the Structure of Crystalline Alamethicin and Its NMR Chemical Shift Tensors.

Alamethicin (ALM) is an antimicrobial peptide that is frequently employed in studies of the mechanism of action of pore-forming molecules. Advanced techniques of solid-state NMR spectroscopy (SSNMR) are important in these studies, as they are capable of describing the alignment of helical peptides, such as ALM, in lipid bilayers. Here, it is demonstrated how an analysis of the SSNMR measurements can benefit from fully periodic calculations, which employ the plane-wave density-functional theory (PW DFT) of the solid-phase geometry and related spectral parameters of ALM. The PW DFT calculations are used to obtain the structure of desolvated crystalline ALM and predict the NMR chemical shift tensors (CSTs) of its nuclei. A variation in the CSTs of the amidic nitrogens and carbonyl carbons along the ALM backbone is evaluated and included in simulations of the orientation-dependent anisotropic 15N and 13C chemical shift components. In this way, the influence of the site-specific structural effects on the experimentally determined orientation of ALM is shown in models of cell membranes.

125Te NMR Probes of Tellurium Oxide Crystals: Shielding-Structure Correlations.

The local environments around tellurium atoms in a series of tellurium oxide crystals were probed by 125Te solid-state NMR spectroscopy. Crystals with distinct TeOn units (n from 3 to 6), including Na2TeO3, α-TeO2 and γ-TeO2, Te2O(PO4)2, K3LaTe2O9, BaZnTe2O7, and CsYTe3O8 were studied. The latter four were synthesized through a solid-state process. X-ray diffraction was used to confirm the successful syntheses. The 125Te chemical shift was found to exhibit a strong linear correlation with the Te coordination number. The 125Te chemical-shift components (δ11, δ22, and δ33) of the TeO4 units were further correlated to the O-Te-O-bond angles. With the aid of 125Te NMR, it is likely that these relations can be used to estimate the coordination states of Te atoms in unknown Te crystals and glasses.

Correlation between electronic parameters and corrosion inhibition of benzothiazole derivatives- NMR parameters as important and neglected descriptors

The relation between electronic properties and corrosion inhibitive performance of three benzothiazole derivatives 1,3-benzothiazol-2-amine (BTA), 6-methyl-1,3-benzothiazol-2-amine (MBTA) and 2-amino-1,3-benzthiazole-6-thiol (TBTA) has been investigated by density functional theory. The electronic properties including EHOMO, ELUMO and related parameters were calculated at the B3LYP/6-311++G(d,p) level. The chemical shielding CS tensors were introduced as important and neglected descriptors to evaluate inhibitive efficiency of corrosion inhibitors. Nuclear independent chemical shift (NICS) components, as an aromaticity criterion, were also investigated as local descriptor. Polarizability and CS descriptors, as second rank tensors, show the best correlations with inhibition efficiencies of studied inhibitors.

Natural bond orbital, nuclear magnetic resonance analysis and hybrid-density functional theory study of σ-aromaticity in Al2F6, Al2Cl6, Al2Br6 and Al2I6

Natural bond orbital (NBO), nuclear magnetic resonance (NMR) analysis and hybrid-density functional theory based method (B3LYP/Def2-TZVPP) were used to investigate the correlation between the nucleus-independent chemical shifts [NICS, as an aromaticity criterion], σ Al(1)-X2(b) → σ*Al(3)-X₄(b) electron delocalizations and the dissociation energies of Al₂F₆, Al₂Cl₆, Al₂Br₆ and Al₂I₆ to 2AlX₃ (X = F, Cl, Br, I). The results obtained showed that the dissociation energies of Al₂F₆, Al₂Cl₆, Al₂Br₆ and Al₂I₆ decrease from Al₂F₆ to Al₂I₆. Like aromatic molecules, these compounds have relatively significant negative NICSiso(0) values. Clearly, based on magnetic criteria, they exhibit aromatic character and make it possible to consider them as σ-delocalized aromatic species, such as Möbius σ-aromatic species. The σ-aromatic character which is demonstrated by their NICSiso(0) values decreases from Al₂F₆ to Al₂I₆. The NICSiso values are dominated by the in-plane σ₂₂ (i.e., σyy, the plane containing halogen atoms bridged) chemical shift components. The increase of the NICSiso values explains significantly the decrease of the corresponding dissociation energies of Al₂F₆, Al₂Cl₆, Al₂Br₆ and Al₂I₆. Importantly, the NBO results suggest that in these compounds the dissociation energies are controlled by the stabilization energies associated with σ Al(1)-X2(b) →σ*Al(3)-X4(b) electron delocalizations. The decrease of the stabilization energies associated with σ Al(1)-X₂(b) →σ*Al(3)-X4(b) electron delocalizations is in accordance with the variation of the calculated NICSiso values. The correlations between the dissociation energies of Al₂F₆, Al₂Cl₆, Al₂Br₆ and Al₂I₆, σ Al(1)-X₂(b) →σ*Al(3)-X4(b) electron delocalizations, natural atomic orbitals (NAOs) and NICSiso values have been investigated.

Void and Nanostructure Formations during Thermal Decomposition of 20-nm-Thick Silicon Oxide Layer on Si(100)

We investigate the thermal decomposition of 20-nm-thick silicon oxide layers on Si(100) at a temperature of 1050 °C in vacuum by X-ray photoelectron spectroscopy, scanning Auger electron microscopy, and atomic force microscopy. Time evolutions of chemical-shift components in Si 2p core-level spectra are analyzed with the SiO2-decomposition model based on void formation. The experimental data can be reproduced well if the void area is proportional to the square of the annealing time, being consistent with the model in which the reaction at the void periphery is the rate-limiting step for void growth. Microscopic images show that the void periphery is square with rounded corners, and many silicon nanostructures are formed inside the void. These observations reflect reaction processes of the thermal decomposition of the silicon oxide layer.

Magnetic resonance tensors in Uracil: Calculation of 13C, 15N, 17O NMR chemical shifts, 17O and 14N electric field gradients and measurement of 13C and 15N chemical shifts

The experimental 13C NMR chemical shift components of uracil in the solid state are reported for the first time (to our knowledge), as well as newer data for the 15N nuclei. These experimental values are supported by extensive calculated data of the 13C, 15N and 17O chemical shielding and 17O and 14N electric field gradient (EFG) tensors. In the crystal, uracil forms a number of strong and weak hydrogen bonds, and the effect of these on the 13C and 15N chemical shift tensors is studied. This powerful combination of the structural methods and theoretical calculations gives a very detailed view of the strong and weak hydrogen bond formation by this molecule. Good calculated results for the optimized cluster in most cases (except for the EFG values of the 14N3 and 17O4 nuclei) certify the accuracy of our optimized coordinates for the hydrogen nuclei. Our reported RMSD values for the calculated chemical shielding and EFG tensors are smaller than those reported previously. In the optimized cluster the 6-311+G** basis set is the optimal one in the chemical shielding and EFG calculations, except for the EFG calculations of the oxygen nuclei, in which the 6-31+G** basis set is the optimal one. The optimal method for the chemical shielding and EFG calculations of the oxygen and nitrogen nuclei is the PW91PW91 method, while for the chemical shielding calculations of the 13C nuclei the B3LYP method gives the best results.

Ring Current Effects in Crystals. Evidence from 13C Chemical Shift Tensors for Intermolecular Shielding in 4,7-Di-t-butylacenaphthene versus 4,7-Di-t-butylacenaphthylene

13C chemical shift tensor data from 2D FIREMAT spectra are reported for 4,7-di-t-butylacenaphthene and 4,7-di-t-butylacenaphthylene. In addition, calculations of the chemical shielding tensors were completed at the B3LYP/6-311G** level of theory. While the experimental tensor data on 4,7-di-t-butylacenaphthylene are in agreement with theory and with previous data on polycyclic aromatic hydrocarbons, the experimental and theoretical data on 4,7-di-t-butylacenaphthene lack agreement. Instead, larger than usual differences are observed between the experimental chemical shift components and the chemical shielding tensor components calculated on a single molecule of 4,7-di-t-butylacenaphthene, with a root mean square (rms) error of +/-7.0 ppm. The greatest deviation is concentrated in the component perpendicular to the aromatic plane, with the largest value being a 23 ppm difference between experiment and theory for the 13CH2 carbon delta11 component. These differences are attributed to an intermolecular chemical shift that arises from the graphitelike, stacked arrangement of molecules found in the crystal structure of 4,7-di-t-butylacenaphthene. This conclusion is supported by a calculation on a trimer of molecules, which improves the agreement between experiment and theory for this component by 14 ppm and reduces the overall rms error between experiment and theory to 4.0 ppm. This intermolecular effect may be modeled with the use of nuclei independent chemical shieldings (NICS) calculations and is also observed in the isotropic 1H chemical shift of the CH2 protons as a 4.2 ppm difference between the solution value and the solid-state chemical shift measured via a 13C-1H heteronuclear correlation experiment.

Solid-State NMR Spectra and Long Intradimer Bonds in the π-[TCNE]22- Dianion

The principal (13)C chemical-shift values for the pi-[TCNE](2)(2-) dimer anion within an array of counterions have been measured to understand better the electronic structure of these atypical chemical species in several related TCNE-based structures. The structure of pi-[TCNE](2)(2-) is unusual as it contains two very long C-C bond lengths (ca. 2.9 Angstroms) between the two monomeric units and has been found to exist as a singlet state, suggestive of a (1)A(1g) (b(2u)(2)b(1g)(0)) electronic configuration. A systematic study of several oxidation states of [TCNE](n) (n = 0, 1-, 2-) was conducted to determine how the NMR chemical-shift tensor values change as a function of electronic structure and to understand the interactions that lead to spin-pairing of the monomer units. The density functional theory (DFT) calculated nuclear shielding tensors are correlated with the experimentally determined principal chemical-shift values. Such theoretical methods provide information on the tensor magnitudes and orientations of their principal tensor components with respect to the molecular frame. Both theoretical and experimental ethylenic chemical-shielding tensors reveal high sensitivity in the component, delta(perpendicular), lying in the monomer molecular plane and perpendicular to the pi-electron plane. This largest shift dependence on charge density is observed to be about -111 ppm/e(-) for delta(perpendicular). The component in the molecular plane but parallel to the central C=C bond, delta(parallel), exhibits a sensitivity of approximately -43 ppm/e(-). However, the out-of-plane component delta'(perpendicular) shows a minimal dependence of -2.6 ppm/e(-) on the oxidation state (n) of [TCNE](n). These relative values support the claim that it is changes within the ethylenic pi-electrons and not the sigma-electrons that best account for the dramatic variations in bonding and shift tensors in this series of compounds. Concerning the intraion bonding, relatively weak Wiberg bond orders between the two monomeric components of the dimer correlate with the long bonds linking the two [TCNE(*)](-) monomers. The chemical-shift tensors for the cyano group, compared to the ethylene shifts, exhibit a reduced sensitivity on the TCNE oxidation state. The experimental principal chemical-shift components agree (within typical errors) with the calculated quantum mechanical shieldings used to correlate the bonding. The embedded ion model (EIM) was used to investigate the typically large electrostatic lattice potential in these ionic materials. Chemical-shielding principal values calculated with the EIM model differ from experiment by +/-3.82 ppm on average, whereas in the absence of an electrostatic field model, the experimental and theoretical results agree by +/-4.42 ppm, which is only a modest increase in error considering the overall ionic magnitudes associated with the tensor variations. Apparently, the effects of the sizable long-range electrostatic fields cancel when the shifts are computed because of lattice symmetry.

Relativistic component of chemical shift of Uranium X-ray emission lines

Twelve most intensive hard X-ray emission lines belonging to two multiplets (2p-3d and 2p-4d) and three doublets (2s-3p, 2s-4p, and 2p-3s) of the uranium L-series were studied by using a precise crystal-diffraction X-ray spectrometer. Chemical shift (CS) values were obtained for the first time for uranium (VI) oxide and uranium fluoride with respect to uranium (IV) oxide. The first attempt was made to separate CS of hard X-ray lines of an actinide atom into two components: the non-relativistic CS part characterizing the charge distribution of valence electrons between the atom and its partners in chemical bonding and the relativistic part reflecting the effects which are not related to electron removal from the atom. A method for calculating the relativistic CS component is proposed. It is concluded that it is the relativistic component that provides information about the changes in the valence state of the actinide atom in a compound. On the basis of model Dirac–Hartree–Fock calculations, it is shown that in the case of the uranium atom the relativistic component of CS is a direct consequence of intra-atomic electrons redistribution between 5f 7/2- and 5f 5/2-split levels of the uranium atom fine structure without changes in its charge state.

リアルタイム光電子分光によるTi(0001)表面酸化反応の「その場」観察

Temperature dependence of the initial oxidation kinetics of Ti(0001) surface was investigated by low energy electron diffraction (LEED) and real-time photoelectron spectroscopy using synchrotron radiation of surface- and bulk-sensitive photon energies. LEED observation revealed that oxide layers grow epitaxially with different surface structures depending on temperature: 1×1 at 200oC and √3×√3 at 400oC. From the oxygen uptake curve measured by O 1s photoelectron intensity, it was clarified that oxygen diffusion through the epitaxially grown oxide layer is significantly enhanced with raising temperature, making the oxide layer thicker than 70 A at 400oC. The chemical shift components observed for Ti 2p showed that TiO2 becomes predominant at the subsurface with O2 dose, while the stoichiometry of oxide near the interface is maintained as TiO and Ti2O3, for both cases at 200oC and 400oC. Thus it is concluded that the epitaxial growth of a very thin oxide on the Ti(0001) surface proceeds not only at the interface but also in the oxide layer by increasing the oxidation number to that of TiO2.

Cobalt-59 chemical shift and quadrupolar tensors of simple octahedral cobalt(III) complexes

Analysis of the solid-state powder59Co NMR spectra of ten simple inorganic cobalt(III) complexes at 11.75, and in most cases, 4.7 T have permitted the assignment of specific ligand planes to ranges of values of the observed chemical shift principal components. The relevant chemical shift components were determined from the simulations of the powder line shapes. These simulations also provided the relative orientations of the chemical shift (CS) and electric field gradient (efg) tensors, as well as magnitude and asymmetry of the59Co quadrupolar coupling. Using symmetry arguments and ab initio calculations, as appropriate or necessary, the orientations of the efg tensors in the molecular frame were deduced. This allowed the determination of the CS tensors in the molecular frame and thus assignment of the ligand planes responsible for the observed values of chemical shifts.Key words: cobalt, chemical shift, quadrupolar coupling, solid state NMR.

Fat- and water-selective MR cine imaging of the human heart: assessment of right ventricular dysplasia.

The purpose of this study was to develop and implement MR sequences for chemical shift-selective breath-hold cine imaging of the heart. Fibroadipose conversion of myocardium in cases suspected of right ventricular dysplasia should be revealed in fat- and water-selective MR images of high quality. Frequency-selective saturation of one chemical shift component was applied in modified k-space-segmented, electrocardiography-gated sequences, allowing high-quality cine imaging of the human heart in a single breath-hold. Phantom studies and human examinations in eight normal subjects (aged 24-62 years) and in seven patients (aged 31-47 years) with suspected right ventricular dysplasia were performed. The patients showed suspicious findings, such as a dyskinetic and dilated right ventricle combined with ventricular arrhythmia, and underwent MR imaging after exclusion of other possible reasons (eg, coronary artery disease or pulmonary hypertension). High selectivity to the desired chemical shift component was confirmed by test measurements in a phantom containing water and lipids. In the human subjects, minor problems with magnetic field inhomogeneities appeared in the thoracic walls only. Four patients with suspected right ventricular dysplasia showed clearly abnormal signal behavior of the right myocardial wall in both fat- and water-selective cine images. Bright transmural structures were exhibited in fat-selective images, but the origin of the fat (epicardium or infiltrated myocardium) was often difficult to assess. Right ventricular areas with fibrosis and fatty degeneration often show normal signal intensity in standard T1-weighted images but can be differentiated from normal tissue by the new chemical shift-selective breath-hold cine techniques.

GIAO-CHF study of magnetic shielding tensors of carbon and nitrogen nuclei in cyano groups

13C and 15N magnetic shielding has been investigated for the sp-hybridizations in simple nitrile and isonitrile molecular structures. It is shown that GIAO-CHF calculations reproduce important features of shielding tensors and that the calculated chemical shifts for carbons and nitrogens correlate with appropriate NMR measurements. The present theoretical and experimental results are jointly used to show the structure independence of chemical shift components δ∥(13C)∝−90ppm and δ∥(15N)∝−480ppm for linear molecules and the correlation between carbon and nitrogen chemical shifts in isomeric structures XCN and XNC. The expected chemical shift changes during dissociation of the hypothetical isolated NaCN and CNNa molecules have been calculated.

Improved clinical echo-planar MRI using spatial-spectral excitation.

Echo-planar imaging (EPI) is markedly susceptible to B0 field distortions and to frequency differences caused by chemical shift, because the phase of the signals is accumulating during the acquisition train. Thus, only water proton signals are usually recorded after frequency-selective suppression of the fat portions of magnetization. Otherwise, a shifted signal frequency from fat results in ghosting artifacts. In this article, a technique is presented working with spatial-spectral excitation for highly selective water or fat EPI. The proposed method allows recording in multislice operation on EPI scanners without irregular gradient or RF pulse shapes. Examples of gradient-echo and spin-echo EPI using spatial-spectral excitation by series of two to eight single slice-selective RF pulses are demonstrated. The method is not sensitive to misadjustments or inhomogeneities of the B1 field, but sufficient homogeneity of the static magnetic field B0 is required. Especially the quality of diffusion-weighted echo-planar images can be markedly improved by the new technique compared to conventional EPI, because artifacts from undesired chemical shift components are completely avoided.

Highly selective water and fat imaging applying multislice sequences without sensitivity to B1 field inhomogeneities.

Improved selectivity to one chemical shift component was obtained using simultaneous slice-selective and chemical shift-selective excitation in sequences with usual spin-echo refocusing. The new type of sequences can be applied on modern whole-body units and permits multislice operation. Spatial-spectral excitation is based on prior research in this field, but the proposed improved version provides off-center slice excitation by the usual processing of the RF pulse envelopes. In addition, no irregular gradient shapes are necessary. The required B0 homogeneity of the new method is similar to conventional "fat-sat" techniques. In contrast to fat-sat methods, selectivity to water is not reduced by unavoidable misadjustments of the transmitter or B1 field inhomogeneities in the newly developed approach. Thus, the reported method has the potential to replace standard frequency selective fat-sat sequences for most applications.

Magnetic Resonance Bone Densitometry

Three different magnetic resonance techniques for the assessment of the density of trabecular bone are presented and compared. All methods are based on the susceptibility difference between marrow and bony components. Localized spectroscopic (PRESS single voxel spectroscopy) and two different imaging approaches (FLASH gradient echo imaging and MAGSUS imaging) are demonstrated to be sensitive to the trabecular structures on application to the distal femur of a healthy volunteer. This region was chosen because a continuous rarefication of the trabecular density from the epiphysis to the metaphysis occurs physiologically. In addition, results from the heel-bones of five young healthy volunteers were compared with data from three patients with severe osteoporosis. All methods allowed differentiation between the two groups. Advantages and disadvantages of the methods concerning spatial resolution, preparation and measuring time, necessity of postprocessing, problems with shim adjustment, and occurrence of several chemical shift components are reported. The MAGSUS technique combines simple application and avoidance of postprocessing for at least qualitative assessment of bone density with sufficient spatial resolution. A reliable assessment of osteoporosis by magnetic resonance is possible in peripheral marrow regions of adults. Further studies are needed to establish standard measuring protocols.

Fast 19F-NMR imaging in vivo using FLASH-MRI

The goal of this study was the development of a method that allows the acquisition of chemical shift selective 19F NMR images within a short measuring time. A train of small tip angle frequency selective pulses, each followed by a dephasing gradient of random strength, was used as a presaturation experiment for the suppression of the unwanted chemical shift components. Imaging was performed after presaturation using a SNAPSHOT-FLASH experiment. The acquisition time for a 64 × 128 image including the presaturation experiment amounted to only 470 ms. The sequence was applied in a 4.7 Tesla magnet to perform a longitudinal pharmacological study on the clearance of perfluorooctylbromide (PFOB) in the liver and spleen of rats.

4996480 Image reconstructing device for use in magnetic resonance imaging apparatus

A magnetic resonance signal is detected at a plurality of different periods of time in accordance with the phase shift of a plurality of chemical shift components while varying the intensity of a gradient magnetic field with time, a plurality of magnetic resonance signal thus obtained are used for estimating a magnetic resonance signal due to only a predetermined chemical shift component, the phase rotation of the predetermined chemical shift component is corrected in the estimated resonance signal, and further a blur due to chemical shift components other than the predetermined one is corrected to obtain a tomogram formed of only the predetermined chemical shift component. Alternatively, one measured resonance signal is modified by an image reconstruction algorithm peculiar to each of a plurality of chemical shift components, to obtain tomograms corresponding to the chemical shift components.

4901021 Image constructing device included in a magnetic resonance imaging apparatus

A magnetic resonance signal is detected at a plurality of different periods of time in accordance with the phase shift of a plurality of chemical shift components while varying the intensity of a gradient magnetic field with time, a plurality of magnetic resonance signals thus obtained are used for estimating a magnetic resonance signal due to only a predetermined chemical shift component, the phase rotation of the predetermined chemical shift component is corrected in the estimated resonance signal, and further a blur due to chemical shift components other than the predetermined one is corrected to obtain a tomogram formed of only the predetermined chemical shift component. Alternatively, one measured resonance signal is modified by an image construction algorithm peculiar to each of a plurality of chemical shift components, to obtain tomograms corresponding to the chemical shift components.

Single-acquisition chemical-shift imaging of a binary system with use of stimulated echoes.

A method for separating binary chemical-shift components with a single image data acquisition by means of stimulated echoes is demonstrated. With a strategy analogous to the modified Dixon method, three stimulated echoes were acquired to form three complex images. In each of the images, the complex pixel intensities were imparted, by design of the pulse sequence, with a phase factor carrying chemical-shift or field inhomogeneity information. With these three images, true fat/water separation can be obtained in biologic tissues. Studies at high field strength (4.7 T) on a toluene phantom, a pseudo-binary chemical-shift system, were used to evaluate the applicability of the method. Its clinical feasibility was demonstrated on a healthy human subject in a 0.6-T whole-body imaging system.