A dielectric material distorts the microwave field inside an EPR resonator, which results in distortion of the EPR signal from spins inside the material. In this paper, the effects of a spherical bulb filled with a dielectric liquid such as water or a water–ethanol mixture were examined. EPR spectra were recorded for small samples inside and outside of the sphere. The studies include CW and ESE experiments at two microwave frequencies, X band (9.2 GHz) and L band (1.03 GHz). The double integral (area) of an EPR signal depends on[formula]at the position of the sample, causing a large difference in EPR signal intensities between samples in regions of different dielectrics. The phase of the EPR signal also is affected by the presence of the dielectric. These results were compared with three methods of calculating electromagnetic fields (quasi-static method, plane-wave-superposition method, and numerical analysis). Good agreement was found between experimental and calculated results.
The precession of nuclear magnetization about tilted axes in the rotating frame and its consequences for phase-sensitive NMR detection are examined in detail in this work. Expressions for the frequencies and phases of a NMR spectrum acquired during tilted-axis precession are derived, and utilized to show how phase-cycling schemes that eliminate undesired artifacts in such spectra can be determined. The explicit use of these expressions is demonstrated in a calculation of efficient new phase cycles for three line-narrowing experiments (MREV-8, BR-24, and C-24) exemplifying precession of nuclear magnetization about axes not aligned with the static field direction. Experimental and simulated results that confirm the effectiveness of the phase cycles and illustrate their applicability under conditions of practical interest are reported. An analysis of artifacts not totally eliminated by these phase-cycling schemes is described. The source of these residual artifacts in multiple-pulse applications is shown to be large deviations of the effective offset Hamiltonian parameters from canonical values calculated using zeroth-order coherent-averaging theory. These deviations are an intrinsic characteristic of multiple-pulse line-narrowing techniques, and are not eradicated even when the experiments are performed under ideal conditions. The consequences of this shortcoming of the zeroth-order theory for the interpretation of multiple-pulse spectra are discussed.
In a preceding paper [P. Mansfield and B. Issa,J. Magn. Reson. A122, 137–148 (1996)], a stochastic model of fluid flow in porous rocks based upon the experimental observation of water flow through a Bentheimer sandstone core was proposed. The flow maps were measured by NMR-imaging techniques. The stochastic theory led to a Gaussian velocity distribution with a mean value in accord with Darcy's law. Also predicted was a linear relationship between flow variance and mean fluid flow through rock, the Mansfield–Issa equation, originally proposed as an empirical relationship. In the present work a flow coupling mechanism between voxels is proposed. Examination of the flow coupling between isolated voxel pairs leads to a complementary explanation of the Gaussian velocity distribution, and also gives further details of the Mansfield–Issa equation. These details lead to a new expression for the connectivity, 〈C〉, between voxels with an experimental value of 〈C〉 = 5.64 × 10−9for Bentheimer sandstone.
Detailed multinuclear magnetic resonance studies on phenylphosphonic dichloride oriented in mixed and individual nematic liquid crystals have been carried out. Analysis of the1H,13C, and31P NMR spectra provided precise geometrical and orientational parameters and the signs and the magnitudes of the indirect spin–spin couplings between heteronuclei.
A pulse-sequence optimization procedure that uses a gradient-search method with analytical derivatives is demonstrated. It can be implemented for arbitrarily complex spin-system Hamiltonians and arbitrary pulse sequences. The computational method presented is time-efficient and more accurate than difference methods. It is applied to the optimization of a pulse sequence proposed by K. V. Schenker, D. Suter, and A. Pines [J. Magn. Reson.73, 99 (1987)] for deuterium decoupling in oriented phases. The optimized sequence is tested experimentally on pentadeuterobenzene dissolved in nematic phase.
A new crossed-loop resonator (CLR) structure is described that uses two orthogonal resonators to isolate the EPR signal from the microwave source. Resonators of this type are usually referred to as bimodal, which is descriptive of cavity or distributed element resonators. However, it is more useful to think of the new resonator as two virtually independent lumped-element resonators that have a common sample volume where the loops of the two resonators meet orthogonally. The first resonator excites the spins, and the second resonator (or resonant section of the CLR) acts like an antenna that detects only the signal caused by the spin system. In this manner, the very phenomenom that is being studied is used to separate the desired EPR signal from the microwave source and performs the function of the circulator. The signal coupled into the second resonator is due to the spin system. The phase noise of the source is reflected from the first resonator back toward the source and is efficiently isolated from the second resonator and the EPR signal. This resonator structure eliminates the need for the circulator, simplifies the spectrometer circuit, virtually eliminates source phase noise in the detected signal, and allows dispersion spectra to be measured with the same high signal-to-noise (S/N) as absorption spectra. In addition, for pulse experiments, this resonator greatly decreases the dead time of the instrument and allows measurement of a portion of the signal previously inaccessible with other resonators. The CLR is rugged, easy to tune, does not have to be critically coupled, and is easily adjusted to maintain source isolation when the sample is changed.
The exact solution was found for inverting pulses with constant adiabaticity for spin ½. The analytical relationship between the time-varying frequency of the microwave resonant field (or RF field in the case of NMR) and its amplitude time dependence such that the adiabaticity parameter remains constant for the single isochromat throughout the pulse is found. Comparison with EPR (hyperbolic tangent)-(hyperbolic secant) pulse method was carried out. On the basis of the analytical solution the pulses with different dependences of the microwave field amplitude conserving the constant adiabaticity have been constructed. The pulses exhibit rather sharp inversion selectivity that can be used in the field of EPR, NMR and MRI.
A new method for obtaining one- and two-dimensional spectral information for NMR spin systems from a set ofNcomplex excitation–response data pairs is presented. The spin system is excited with RF pulses, the flip angles and phases of which are represented by a discrete stochastic process. The relation between complex excitation and complex response is analyzed by perturbation theory and is shown to be a special functional series. The 1D spectrum is obtained by Fourier transformation of the complex cross correlation between response and excitation time functions in first order. A 2D spectrum of the E. COSY type is derived with a new algorithm which performs the complex cross correlation of response with excitation in third order, the 3D Fourier transformation, and the selection of the 2D diagonal spectral plane (ω1, ω2, ω3= −ω2) in an integrated, rapid manner. It is shown that the complex approach yields 2D quadrature detection. The validity of this new approach is demonstrated with simulation calculations.
A slice-selection method developed within the frame of the sequence forindirectmagicangle in therotatingframe (I-MARF) imaging of solids is proposed. The pulse sequence is based on the Lee–Goldburg sequence, adapted to imaging by a dedicated gradient setup, and it is left unchanged, except for the insertion of a dc “selection-plane magnetic field.” The selection-plane field makes the magic-angle coherent-averaging condition spatially varying and delimits the selected slice to only the sample portion where the residual dipolar coupling has been preserved.
An important aspect of HCCH-TOCSY-based multidimensional experiments of13C-labeled proteins and nucleic acids is the optimization of the13C–13C TOCSY mixing time. For this purpose, one needs a description of the mixing-time dependence of the magnetization transfer between various spins. Here, analytical expressions have been obtained for three- and four-spin systems found in amino acids, assuming isotropic-mixing conditions and a uniform one-bond13C–13C coupling constant. These results, which require analytical determination of eigenvalues and eigenvectors of the isotropic-mixing Hamiltonian, were obtained using the programMathematicafor performing the essential linear algebra. The expressions are in complete agreement with numerical calculations carried out by Eatonet al.(J. Magn. Reson.90, 462–463, 1990). From these expressions, optimum magnetization transfer efficiencies for various spin topologies are easily obtained as a function ofJand the mixing timet. The isotropic-mixing results agree reasonably well with simulations employing mixing sequences such as IICT-1.
Flow techniques have been used in NMR for over 50 years, and there has been a lot of growth and interest in the field over the last decade. Most of that growth has focused on LC-NMR type techniques, but it has also led to the development of other flow-NMR methods, each of which is suited to different applications. Some of these methods use different kinds of chromatography, whereas others use different plumbing schemes. All of these variations will be discussed here, with particular emphasis being given to the techniques of Flow-Injection-Analysis NMR (FIA-NMR), Direct-Injection NMR (DI-NMR), Solid-Phase-Extraction NMR (SPE-NMR), and loop collection. By understanding how and why each flow-NMR method was developed, a user can more easily select the best tool for any given application.
In solid-state proton-dipolar-decoupled19F MAS NMR spectroscopy,19F chemical-shift data need to be corrected for the Bloch–Siegert shift. Assigning the single sharp19F resonance of 2-fluoroadamantane to its proton-coupled19F shift of −174.4 ppm results in chemical-shift referencing that is independent of the amplitude of the proton-decoupling field. The Bloch–Siegert shift is also a useful tool to characterize the amplitude and homogeneity of the proton-decoupling field,H1H, and to monitor probe performance. Considerable inhomogeneity inH1Halong the long axis of the right-cylinder sample rotor was detected. In our commercial 7 mm H– F MAS probe, the proton field strength,[formula], decreases to 25% of the maximum value across the usable sample volume. Measurement of the Bloch–Siegert shift revealed that the proton-decoupling field strength decreases during the first few scans of an acquisition. Reductions in the proton field strengths can exceed 10%, and they are explained by the heating of the RF coil circuitry which is caused by high-power proton decoupling. The extent of reduction in field amplitude is a function of the decoupling duty cycle. Losses in[formula]can be avoided by tuning the probe proton RF circuitry at the operating temperature of the probe, using the Bloch–Siegert shift as an optimization parameter.
It is shown that the presence of thermal convection in a sample tube may lead to a variety of anomalous phenomena in prolonged multiple-pulse NMR experiments. They are investigated by applying inversion-recovery pulse sequences to129Xe of xenon gas dissolved in deuterated cyclohexane and acetonitrile, and to19F in xenon difluoride (XeF2) dissolved in deuterated acetonitrile. If convection is present, the recovery of the magnetization after the π pulse may be very different from the recovery due to the spin–lattice relaxation alone. It may be much faster, very sensitive to temperature, and nonexponential, exhibiting even oscillatory behavior. In addition, the shape of the spectral lines may be seriously distorted. The results show that convection and the resulting anomalies can be effectively eliminated by rotating the sample tube at a spinning speed on the order of 10 Hz. These phenomena may provide novel methods for investigating thermal convection.
The fabrication and analysis of19F-based and1H-based thin films and the NMR techniques employed for studying them are discussed. The heteronuclear magnetization transfer results in disordered media that motivated the development of the ordered thin-film techniques are summarized. The experimental setup and sample geometry for the thin-film experiments are described, and the techniques for thin-film preparation and thickness measurement are discussed. For the NMR spectrometer, coil tuning via changing the applied magnetic field and the techniques employed for lowering the dead time are described. No signal averaging was required to obtain spin echoes from a sample composed of 0.5 μm thin films.T1values on the order of one minute were measured in the thin films at a temperature of 0.5 K. These techniques should be of particular use in studying cross relaxation at surfaces and NMR of membrane proteins.
NMR relaxation parameters are potentially a valuable source of information on molecular dynamics. However, due to the complexity of the processes which can occur, studies are usually restricted to those cases in which relatively few processes can be considered to contribute to relaxation. These include15N and13C, but usually exclude the most abundant source of relaxation information, protons. General expressions are derived for all elements of the dipolar relaxation matrix that are valid for a system consisting of an arbitrary number of inequivalent weakly coupled spins. The similarities apparent between many of the diagonal elements are exploited to isolate the mutual relaxation of a restricted set of spins by taking linear combinations of relaxation rates. The experimental procedures necessary for making the measurements required to determine the mutual relaxation of a single pair of spins are discussed and are demonstrated for protons.
New symmetry-adapted autocorrelation functions are used in the theoretical description of the nuclear magnetic dipole–dipole relaxation in molecular crystals. They are obtained in the model frame that a molecule rotates by means of the finite angular jumps between the hindered states of the same symmetry in different potential wells. The experimental data of the temperature dependence of the proton relaxation timesT1andT1ρin the mono- and polycrystalline NH4Cl are revised. As a result, it is proposed to classify the NH+4ion motion by two kinds of the hindered states corresponding to the two-dimensional (E) and three-dimensional (F) irreducible representations of a point group of the cubic system. The dynamical weight of the stateFis defined to be equal to 0.25 and that of the stateEto be equal to 0.73 from the experiments in the ordered phase of NH4Cl. Based on abstract geometric groups, the respective weights are 0.4 and 0.6. This discrepancy in the weights of the states is explained by the effect of the tetragonal distortion of the tetrahedral site symmetry of the NH+4ion. The result of the prevalence of aC3reorientation in comparison to aC2reorientation of the ion is also justified. Because of the absence of the experimental temperature minima ofT1orT1ρ, assumptions are only made about the dynamical parameters of the NH+4ion motion in the disordered phase of NH4Cl.
A new scheme is suggested for the automated recognition of cross peaks in two-dimensional correlation spectra. It makes use of prior knowledge of the chemical shifts derived from a symmetry-based search of the two-dimensionalJspectrum, using this information to construct a two-dimensional “chemical-shift grid” that may be overlaid on the corresponding correlation spectrum (COSY, TOCSY, EXSY, or NOESY). The search for local symmetry (or antisymmetry) in the correlation spectrum may then be restricted to intersections on the chemical-shift grid instead of a point-by-point examination of the entire two-dimensional array. This greatly accelerates the search program. Applied to the 400 MHz proton correlation (DQ-COSY) spectrum of 1-dehydrotestosterone, this automated routine locates 28 pairs of cross peaks, and in particular, identifies a tight cluster ofeightinterpenetrating spin multiplets. This latter finding is confirmed by a different method that reconstructs individual cross peaks from partially exposed segments and then strips them away one at a time.
