Nuclear Magnetic Resonance Absorption Spectra Research Articles

Investigation of Condensed Media in Weak Fields by the Method of Nuclear Magnetic Resonance

A compact design of a rapid-response nuclear magnetic spectrometer for investigation of condensed media in weak fields is reported. As a result of investigation of different condensed media, special features of recording a nuclear magnetic resonance (NMR) signal in a weak magnetic field from a small volume of the medium under study are established. For the first time the NMR absorption spectra of condensed media in a weak field are collected. Based on the results of experimental studies, the potential of using a compact NMR-spectrometer for condensed media monitoring in a rapid response mode is determined.

Multiferroic coreshell magnetoelectric nanoparticles as NMR sensitive nanoprobes for cancer cell detection

Magnetoelectric (ME) nanoparticles (MENs) intrinsically couple magnetic and electric fields. Using them as nuclear magnetic resonance (NMR) sensitive nanoprobes adds another dimension for NMR detection of biological cells based on the cell type and corresponding particle association with the cell. Based on ME property, for the first time we show that MENs can distinguish different cancer cells among themselves as well as from their normal counterparts. The core-shell nanoparticles are 30 nm in size and were not superparamagnetic. Due to presence of the ME effect, these nanoparticles can significantly enhance the electric field configuration on the cell membrane which serves as a signature characteristic depending on the cancer cell type and progression stage. This was clearly observed by a significant change in the NMR absorption spectra of cells incubated with MENs. In contrast, conventional cobalt ferrite magnetic nanoparticles (MNPs) did not show any change in the NMR absorption spectra. We conclude that different membrane properties of cells which result in distinct MEN organization and the minimization of electrical energy due to particle binding to the cells contribute to the NMR signal. The nanoprobe based NMR spectroscopy has the potential to enable rapid screening of cancers and impact next-generation cancer diagnostic exams.

Reconstruction of nuclear quadrupole interaction in (In,Ga)As/GaAs quantum dots observed by transmission electron microscopy

A microscopic study of the individual annealed (In,Ga)As/GaAs quantum dots is done by means of high-resolution transmission electron microscopy. The Cauchy-Green strain-tensor component distribution and the chemical composition of the (In,Ga)As alloy are extracted from the microscopy images. The image processing allows for the reconstruction of the strain-induced electric-field gradients at the individual atomic columns extracting thereby the magnitude and asymmetry parameter of the nuclear quadrupole interaction. Nuclear magnetic resonance absorption spectra are analyzed for parallel and transverse mutual orientations of the electric-field gradient and a static magnetic field.

Majorana states and longitudinal NMR absorption in a3He-Bfilm

The topological superfluid $^3$He-B supports massless Dirac spectrum of surface bound states which can be described in terms of the self-conjugated Majorana field operators. We discuss here the possible signature of surface bound states in nuclear magnetic resonance absorption spectrum in a $^3$He-B film. It is shown that transitions between different branches of the surface states spectrum lead to the non-zero absorption signal in longitudinal NMR scheme when the frequency is larger than the Larmour one.

Supramolecular Aggregation/Deaggregation in Amphiphilic Dipolar Schiff-Base Zinc(II) Complexes

The synthesis, characterization, optical, and fluorescent properties of an amphiphilic Schiff-base bis(salicylaldiminato)zinc(II) complex are reported. Detailed (1)H nuclear magnetic resonance (NMR), diffusion-ordered spectroscopy (DOSY) NMR, optical absorption, and fluorescence spectroscopy studies indicate the existence of aggregate species in noncoordinating solvents. The degree and type of aggregation are related to the concentration and the polarity of the noncoordinating solvent. Dilute solutions are likely characterized by the presence of defined dimers, whereas larger oligomeric aggregates are conceivably formed at higher concentrations. The concentration needed to observe the formation of larger species depends upon solvent polarity. In coordinating solvents or in the presence of coordinating species, a complete deaggregation of the system occurs, because of the axial coordination to the Zn(II) ion, accompanied by considerable changes of (1)H NMR and optical absorption spectra. A dramatic enhancement of fluorescence emission is observed in dichloromethane solutions upon deaggregation with a coordinating agent. The formation of a defined 2:1 supramolecular structure is demonstrated in the case of a ditopic ligand as coordinating species. Overall, these complexes are promising systems for the development of new supramolecular architectures and supramolecular fluorescent probes.

On the coordinate of a singular point of time correlation functions for the system of nuclear magnetic moments of a crystal

The hypothesis concerning the existence of singular points on the imaginary time axis for a correlation function of a system with the dipole-dipole interaction of nuclear spins of a crystal is verified. Within the framework of the self-consistent fluctuating field theory taking into account the principal corrections related to the correlation of local fields, a result for this coordinate is obtained in terms of the ratios of lattice sums. Experimental values of this coordinate are calculated from the wings of the nuclear magnetic resonance absorption spectrum of a BaF2 crystal for the magnetic field directions along the three crystallographic axes. Good agreement of the theoretical and experimental results justifies this hypothesis.

Low-temperature 1H nuclear magnetic resonance study of crystal and electronic structures of the nearly stoichiometric yttrium dihydride

Abstract Proton nuclear magnetic resonance absorption spectra and spin-lattice relaxation rates in yttrium dihydride have been measured in the temperature range from 4.2 to 310K at 36.0MHz. The second moment of the resonance line corresponds to the rigid-lattice regime for YH1.99 and its value agrees with the anticipated CaF2 type of structure. The main contribution to the spin-lattice relaxation rate R 1 arises from conduction electrons and is characterized by R le = 2.77 × 10−3s−1 K−1 × T. Evidence of proton self-diffusion was seen in the linewidth and in R 1 for YH1.99+0.1. The onset temperature, about 250 K, of the self-diffusion, is close to the metal-semiconductor transition reported earlier for that hydride. Below that temperature the linewidth becomes temperature independent and the second moment of the line is explained in terms of different structure models. The fit to the temperature dependence of R 1 in the temperature range 70–310K gives R le = 2.1 × 10−3s−1K−1 × T. The relaxation becomes almost temperature independent below 50 K. Various mechanisms for this behaviour are discussed. In addition, the R 1 data for the sample prepared with yttrium of 99.9% purity are presented. In contrast with the previous case, where pure yttrium from the Ames Laboratory was used, R 1 has a large contribution of spin diffusion to the paramagnetic Gd3+ ions.

Low-temperature1H nuclear magnetic resonance study of crystal and electronic structures of the nearly stoichiometric yttrium dihydride

Abstract Proton nuclear magnetic resonance absorption spectra and spin-lattice relaxation rates in yttrium dihydride have been measured in the temperature range from 4.2 to 310K at 36.0MHz. The second moment of the resonance line corresponds to the rigid-lattice regime for YH1.99 and its value agrees with the anticipated CaF2 type of structure. The main contribution to the spin-lattice relaxation rate R 1 arises from conduction electrons and is characterized by R le = 2.77 × 10−3s−1 K−1 × T. Evidence of proton self-diffusion was seen in the linewidth and in R 1 for YH1.99+0.1. The onset temperature, about 250 K, of the self-diffusion, is close to the metal-semiconductor transition reported earlier for that hydride. Below that temperature the linewidth becomes temperature independent and the second moment of the line is explained in terms of different structure models. The fit to the temperature dependence of R 1 in the temperature range 70–310K gives R le = 2.1 × 10−3s−1K−1 × T. The relaxation becomes alm...

Orientational dependence of the tails of dipole-broadened NMR spectra in crystals

This paper describes experimental and theoretical studies of the tails of the dipole-broadened nuclear magnetic resonance (NMR) absorption spectra of 19F in isomorphic single crystals of BaF2 and CaF2 with the magnetic field directed along three crystallographic axes. The results obtained by directly measuring the derivative of the tail of the NMR absorption spectrum and the falloffs of the Engelsberg-Lowe free precession after Fourier transformation qualitatively agree. It is shown that the shape of the tail is well described by an exponential function in which the orientational dependence of the exponent does not reduce to variation of the second moment. The observed shape of the tail and the orientational dependence of its parameters are explained on the basis of a self-consistent fluctuating-local-field theory. Nonlinear integral equations are derived for the correlation functions, taking into account the changes of the actual number of nearest neighbors caused by the anisotropy of the dipole-dipole interaction and the contribution of lattice sums with loops. The equations are solved numerically. Good agreement is obtained for the computed dropoffs of the free precession, the NMR spectra, and the cross-polarization rates with the experimental results.

Two-dimensional nuclear magnetic resonance study of phason and amplitudon relaxation mechanisms in structurally incommensurate Rb2ZnCl4

In structurally incommensurate systems phason and amplitudon thermal fluctuations of the frozen-in modulation wave provide for two different nuclear magnetic resonance (NMR) spin-lattice relaxation mechanisms. These two relaxation mechanisms influence different parts of the inhomogeneously broadened NMR absorption spectrum differently. They can be resolved in principle by a conventional one-dimensional measurement of the spin-lattice relaxation time T1 over the NMR lineshape. Here the authors show that a two-dimensional NMR spin-lattice relaxation technique provides a very convenient way of determining the variation of T1 over the lineshape by the use of normalized contour plots. The results for the central transition of 87Rb in Rb2ZnCl4 show that the measured phason-induced spin-lattice relaxation time is temperature-independent. The much less efficient amplitudon relaxation mechanism is bypassed in part by the cross-relaxation process of the amplitudon-relaxed parts to the phason-relaxed parts of the NMR lineshape. The theoretical prediction for the total variation of T1 over the lineshape for the cross-relaxation process agrees with the experimental value of three.

The dipolar fluctuation spectrum and the shape of the wings of nuclear magnetic resonance absorption spectra in solids

A theoretical approach providing the shape of the spectrum of dipolar fluctuations (Fourier transform of the longitudinal component of magnetization) and shape of the wings of NMR absorption in condensed matter has been elaborated. It is shown that the spectral contours are exponentials with the same decay constant. The results obtained explain for the first time the known experimental data and establish a connection between two groups of different experiments. The theoretical prediction is in good agreement with experimental results.

Nuclear spin relaxation in Li2BeF4

Nuclear spin lattice relaxation time T1 of 19F and 7Li in the solid and liquid phases of lithium tetrafluoroberyllate (Li2BeF4) were measured in the temperature range between 250 and 630 °C by means of pulse NMR technique. Nuclear magnetic resonance absorption spectra were also measured by use of a spectrometer. T1 of 19F and 7Li in the solid phase decreases exponentially as the temperature is increased. The temperature dependence of T1 of 7Li can be attributed to the diffusion of Li nuclei with an activation energy of 1.03 eV while T1 of 19F can be explained by the dipole interaction modulated by the diffusion of F ions with an activation energy of 1.23 eV. In the liquid phase T1 of 19F goes through a minimum at about 550 °C and is expressed as T1−1∝τ/(1+ω2τ2) in terms of a correlation time, τ. We obtained τ=5×10−16 exp (1.08/kT) s. It is suggested that the relaxation mechanism of 19F in the liquid phase is due to the dissociation of F ions from BeF4 complex ions and the long distance diffusion of the F ions in the liquid.

Effects of Red and Far Red Light on the Initiation of Cold Acclimation in Cornus stolonifera Michx.

Red and far red light distinctly influence the initial phytochrome-mediated phase of cold acclimation in red-osier dogwood (Cornus stolonifera). Under controlled conditions, short days and end-of-day far red light exposure after long days promote growth cessation, cold acclimation, and subsequent cold hardening of dogwood stems in response to low temperature. Nuclear magnetic resonance absorption spectra of the water in internode stem sections imply that the short day-induced phase of cold acclimation involves a change in tissue hydration, at least in part, due to a substantial reduction in bulk phase water as a result of senescence and loss of water from the pith. Seasonal responses to light and an attempt to induce early acclimation under natural conditions with end-of-day far red light are discussed.

Studies on absorbed water in cellulose by broad-line NMR

Nuclear magnetic resonance absorption spectra of linter cellulose containing various amounts of water were studied to clarify the relation between the amount of absorbed water in cellulose fiber and the molecular motion in such a cellulose–water system. An amorphous cellulose sample was used for the sake of comparison. The changes in line width and second moment of the spectra were measured at various temperatures. From the variation with temperature of the first-derivative curves and the second moments, it has been inferred that the proton movement of absorbed water changes markedly over the approximate range 180°–200°K and that the absorbed water lowers the glass transition temperature of cellulose to room temperature.

Fluorine magnetic resonance of solid chloropentafluorobenzene

The nuclear magnetic resonance absorption spectrum of solid chloropentafluorobenzene has been studied at temperatures between 77 °K and the melting point. The experimental values of the second moment have been divided into two parts, one due to broadening by nuclear dipole-dipole interactions and the other due to broadening by the anisotropy of the fluorine chemical shift. The absorption line begins to narrow at 190 °K, close to the temperature of a specific heat anomaly. Consideration of the effects of molecular motion on both parts of the second moment show that the narrowing is due to rotation about an axis perpendicular to the plane of the molecule.

Spin Relaxation of a Rotating Pair of Nuclei

Rapid rotation of an assembly of dipolar coupled nuclei has a profound effect on the nuclear magnetic resonance absorption spectrum. In turn, this affects the broadening caused by random fields even when the correlation frequency greatly exceeds the rotation rate. This broadening is considered for a rotating pair of I = ½ nuclei and it is concluded that rotation may reduce the broadening due to very rapidly fluctuating fields. In this case the broadening depends on the degree of overlap of the frequency spectra of off-diagonal elements of the density matrix. When the angle between the rotation axis and the external magnetic field is 54.7° a continuous transition from almost no overlap to complete coincidence is obtained with rising rotation speed. For slowly varying fields, on the other hand, broadening may be increased by rotation.

Dilatometric and nuclear magnetic resonance studies of polyethylene with different branching and crystallinity

AbstractHighly branched (DUPONT) and nearly unbranched (Marlex 50) polyethylenes were investigated. The samples were partly crystallized. Marlex was also prepared in pure crystalline form. Dilatometric investigations revealed a sharp melting point and practically no relaxation phenomena in pure crystals. Nuclear magnetic resonance absorption spectra of branched samples definitely show remains of narrow line even at – 170°C., but it disappears completely with Marlex. In pure crystals the narrow line is extremely weak below 70°C. and changes reversibly with temperature. After heating over 70°C. irreversible changes occur, the samples approaches the structure of Marlex crystallized from melt.

Molecular Structure of cyclo-Octatetraene

AN investigation of the nuclear magnetic resonance absorption spectrum of solid cyclo-octatetraene has been made with the object of obtaining information about its molecular structure. Previous determinations of the molecular structure of cyclo-octatetraene have used the techniques of infra-red and Raman spectroscopy, and of electron diffraction. The results have been explained in terms of three different molecular models : (1) the ‘crown’ (D 4) structure with alternate double and single bonds1; (2) the ‘crown’ (D 4d) structure with equivalent bonds2; (3) the ‘tub’ (D 2d) structure with double and single bonds3. Subsequent investigations have supported one or other of these models, but with no real agreement as to which is the most likely structure. Theoretical predictions of the vibrational spectra and diffraction patterns of these three molecular configurations show little difference.

A nuclear magnetic resonance investigation of three solid benzenes

The nuclear magnetic resonance absorption spectrum and the spin-lattice relaxation time have been measured for the protons in three isotopic species of benzene in polycrystalline form between 75 and 278° K. The three species were C 6 H 6 , C 6 H 5 D and 1. 3. 5 - C 6 H 3 D 3 . For all three species the measured spectrum has its full rigid lattice width below 90° K. A method of analysis is developed which makes it possible to derive separately the intramolecular and the intermolecular contributions to the second moment (mean square width) of the spectrum from the measured second moments, without the necessity of knowing the crystal structure. From the intramolecular contribution it is found that the separation of neighbouring protons in the C 6 H 6 molecule is 2.495 ± 0.018 Å. The intermolecular contribution is in agreement with a value calculated from a knowledge of the crystal structure. On warming from 90 to 120°K the spectrum for all three species narrows considerably. From 120°K to the melting-point (278.7° K) the second moments remain almost constant. The second moment separation procedure is also applied in this range and leads to the conclusion that the narrowing is caused by reorientation of the molecules about their hexad axes in the crystal lattice. Analysis of the measurements of the spin-lattice relaxation time shows that for all three species the reorientation process is governed by an activation energy of 3.7 ± 0.2 kcal/mole. The reorientation frequency is of the order of 10 4 c/s at 85° K and rises to a value of the order of 10 11 c/s just below the melting-point. The relationship between the present experimental results and recent measurements of the Raman spectrum of solid benzene is discussed. Finally, consideration is given to the application to other materials of methods of separating the intra- and intermolecular contributions to the second moment.

A nuclear magnetic resonance investigation of solid cyclo hexane

The nuclear magnetic resonance absorption spectrum and the spin-lattice relaxation time have been measured for the protons in polycrystalline cyclo hexane between 100° K and its freezing-point (279·6° K). It has been found that the second moment (mean square width) of the measured spectrum for temperatures at which the lattice is effectively rigid, namely, below 150° K, is consistent with a molecular structure having D 3d symmetry, tetrahedral bond angles, C—C bond lengths of 1.54 Å and C—H bond lengths of 1.10 A. If the HCH angle is treated as a parameter to be determined, it is found to be 1071/2 ± 3°. On warming from 155 to 180° K the second moment decreases to a value which indicates the reorientation of the molecules about their triad axes. Analysis of the spin-lattice relaxation time, which falls rapidly in this temperature range, shows that the height of the barrier hindering this reorientation is 11 ± 1 kcal/mole. Just below 186° K, the temperature at which there is a polymorphic change, the reorientation frequency is of the order 10 6 c/s. The polymorphic transformation is accompanied by discontinuous changes in the second moment and the relaxation time. It is concluded that in the higher temperature modification the molecules have a considerable freedom of reorientation, such that the intramolecular contribution to the second moment becomes negligibly small. Just above 186° K the mean reorientation frequency exceeds 3 x 10 7 c/s. A final narrowing of the line between 220 and 240° K is thought to be due to vacancy diffusion of the molecules within the lattice, causing the intermolecular contribution to the second moment to vanish also. Details are given of the gas-flow cryostat used in this work. The theoretical formulation of the second moment has been extended to include the modification of the intermolecular contribution during reorientation.