Sample Rotation Frequency Research Articles

Automation in solid state NMR

Automation in solid state NMR (ssNMR) requires appropriate hardware, from rotor loading mechanisms over highly stable rf-transmitters and probe circuitry to automatic tuning and matching capabilities including automatic magic angle adjustment for ssNMR probes. While these hardware capabilities are highly desirable and are, to various degrees, provided by manufacturers, we focus herein on automating experiment setup using radio frequency (rf) fields, which are key parameters in solid state NMR experiments. Specifically, these include spinlock fields during cross polarization (CP), or rf-fields for homo- or heteronuclear spin recoupling or decoupling. Often, these fields have specific relationships to the magic angle spinning (MAS) frequency. Relying on a well-maintained spectrometer, the experiment setup shifts from traditionally required optimization of rf-power values for each element of an experiment sequence to automatically setting all parameters correctly without any need for optimization. The proposed approach allows executing an experiment by reading its rf-amplitude requirements based on the actual MAS rotation frequency just before starting data acquisition, while all other hardware-related parameters are automatically provided through global tables and scripts. Under modest MAS frequencies, no further rf-power optimization is required while providing optimal sensitivity of better than 90% of the optimal signal to noise. Any optional parameter optimization relates only to adjusting rf-nutation frequencies to the requirements of the sample and the sample rotation frequency rather than the spectrometer hardware. Fast MAS CP experiments with MAS frequencies above 40 kHz require a semi-automated setup by optimizing Hartmann-Hahn (HH) matched rf-fields that are synchronously varied relative to the MAS-frequency. This allows for a significant reduction of setup steps by up to one order of magnitude for such experiments, avoiding the traditional grid search for optimal CPMAS conditions. The approach presented here can also be applied to decoupling or recoupling sequences, requiring rotor synchronized rf-fields, reducing the setup to a few steps addressing the spin system’s properties rather than the spectrometer hardware. Our approach permits automating all basic solid state NMR experiments for high throughput analytical tasks.

Performance of the cross-polarization experiment in conditions of radiofrequency field inhomogeneity and slow to ultrafast magic angle spinning (MAS).

In this paper, we provide an analytical description of the performance of the cross-polarization (CP) experiment, including linear ramps and adiabatic tangential sweeps, using effective Hamiltonians and simple rotations in 3Dspace. It is shown that radiofrequency field inhomogeneity induces a reduction in the transfer efficiency at increasing magic angle spinning (MAS)frequencies for both the ramp and the adiabatic CPexperiments. The effect depends on the ratio of the dipolar coupling constant and the sample rotation frequency. In particular, our simulations show that for small dipolar couplings (1 ) and ultrafast MAS (above 100 ) the transfer efficiency is below 40 % when extended contact times up to 20 are used and relaxation losses are ignored. New recoupling and magnetization transfer techniques that are designed explicitly to account for inhomogeneous radiofrequencyfields are needed.

Reduced sample rotation frequency in hospitals and household utilities

Reduced sample rotation frequency in hospitals and household utilities

Formula omitted]-NMR solid state investigations of monovalent alkali metal fluorides and tetra-alkylammonium fluorides

The 19 F -NMR chemical shifts of the alkali metal fluorides and of monovalent tetra-alkylammonium fluorides were measured under strictly anhydrous conditions. For reducing the line widths of the broad resonance signals, conventional and ultrafast MAS at 32 kHz rotational frequency as well as CRAMPS (combined rotation and multiple-pulse spectroscopy) techniques were used. Averaging the strong homonuclear dipolar interactions of fluorine by MAS, the dependence of line width on the frequency of sample rotation is given for (CH 3) 4N + F −. The data obtained are compared with CRAMPS results. The broad distribution in shielding of the fluorine nucleus in the cubic alkali fluorides in the column from LiF to CsF is discussed in terms of cation–anion polarization effects, creating a partial covalent bond. The experimental chemical shift values were correlated with the Allred–Rochow electronegativities and with the polarizabilities of ions in the alkali fluoride crystals. The linear fit of both gave R-values better than 0.96. From the latter, a chemical shift of the “free F-ion” at −293 ppm, referred to CCl 3F, was extrapolated.

Slow-down of 13C spin diffusion in organic solids by fast MAS: a CODEX NMR Study.

One- and two-dimensional 13C exchange nuclear magnetic resonance experiments under magic-angle spinning (MAS) can provide detailed information on slow segmental reorientations and chemical exchange in organic solids, including polymers and proteins. However, observations of dynamics on the time scale of seconds or longer are hampered by the competing process of dipolar 13C spin exchange (spin diffusion). In this Communication, we show that fast MAS can significantly slow down the dipolar spin exchange effect for unprotonated carbon sites. The exchange is measured quantitatively using the centerband-only detection of exchange technique, which enables the detection of exchange at any spinning speed, even in the absence of changes of isotropic chemical shifts. For chemically equivalent unprotonated 13C sites, the dipolar spin exchange rate is found to decrease slightly less than proportionally with the sample-rotation frequency, between 8 and 28 kHz. In the same range, the dipolar spin exchange rate for a glassy polymer with an inhomogeneously broadened MAS line decreases by a factor of 10. For methylene groups, no or only a minor slow-down of the exchange rate is found.

Double-quantum excitation in the NMR of spinning solids by pulse-assisted rotational resonance.

We describe a new technique for double-quantum excitation in magic-angle-spinning NMR of powdered solids. The technique is designed to efficiently excite double-quantum coherence in the vicinity of a rotational resonance condition. The offset from rotational resonance allows the double-quantum filtered signals to be observed with high resolution and sensitivity. The method uses rotational excitation of zero-quantum coherence, assisted by radiofrequency pulse cycles. The zero-quantum coherence is converted into double-quantum coherence by a frequency-selective inversion sequence. Experiments on [(13)C(2), (15)N]-glycine demonstrate a double-quantum filtering efficiency of approximately 41% at a sample rotation frequency of 8.300 kHz, which is 1.600 kHz away from the n = 1 rotational resonance. We achieve 32% double-quantum filtering efficiency at a spinning frequency of 9.250 kHz, which is 2.550 kHz away from rotational resonance.

Longitudinal rotational resonance echoes in solid state nuclear magnetic resonance: Investigation of zero quantum spin dynamics

In the magic angle spinning NMR of rotating solids, it is possible to drive nuclear spin transitions by the macroscopic sample rotation. This rotational resonance phenomenon arises when the chemical shift frequency difference between two neighboring spin sites matches a small integer multiple of the sample rotation frequency. The mechanically driven spin evolution may be reversed by applying a radio-frequency pulse cycle, leading to the formation of rotational resonance echoes. A detailed theory based on a Liouville space formalism is developed. Experimental demonstrations are given for a polycrystalline sample of zinc C213 acetate. The decay of zero quantum coherence is examined at two different magnetic fields.

Stability and oscillation of MTG high-T/sub c/ superconductors in inhomogeneous magnetic field

The dependencies of the oscillation amplitude on the oscillation frequency of high-T/sub c/ superconducting (HTSC) melt-texture growth (MTG) ball-sample suspended or levitated in an inhomogeneous magnetic field have been investigated. The dependence of the oscillation amplitude versus the oscillation frequency has maxima. The rotation of the levitated or suspended MTG ring-sample in the rotating magnetic field have been studied. The curve of the sample rotation frequency versus the magnet rotation frequency has maxima which correlate with maxima of the dependence of the oscillation amplitude of the levitated HTSC MTG ball-sample on the frequency of the external magnetic field.

Large gyromagnetic effect in FeSiB amorphous wires

Results on a new effect observed in FeSiB amorphous wires-the large gyromagnetic effect-are reported for the first time. This effect consists in the appearance of a rotation of the sample around its axis with frequencies from 1 to 60 Hz when the sample is subjected to an axial alternating magnetic field of /spl sim/10/sup 2/ A/m having a frequency of /spl sim/10/sup 4/ Hz. Both the sample rotation frequency and the frequency of the field at which the effect appears depend on the sample length, the characteristics of this dependence indicating that the appearance of the effect is closely related to the formation of a standing wave associated to the oscillation of the magnetic moments. The magnitude of the wire's magnetostriction constant is also essential as concerns the appearance of the large gyromagnetic effect.

NMR Velocity Mapping of Couette Flow Using Oscillating Magnetic Field Gradients

A method for obtaining spin-density distributions of selected layers in annular shear flow is introduced. A transverse rotating magnetic field gradient is generated by oscillating orthogonal gradients in quadrature. The equivalence of the frames of reference of a rotating sample with a fixed magnetic field gradient and a rotating magnetic field gradient with a fixed sample is demonstrated with modulation sideband patterns. When the magnetic field gradient and sample rotation frequency differ, sidebands appear at integer multiples of thedifferencebetween the two rotation frequencies. When the gradient is synchronized with the sample rotation, the spectrum is a spin-density projection of the sample onto the gradient. When the sample and gradient are counterrotating, high audio-frequency sidebands appear with consequent spectral simplification. When the orthogonal components used to generate the rotating gradient are imperfectly matched, sidebands appear at integer multiples of the sum and difference frequencies of Ωsand Ωg. A novel, low-cost electronic system is described to create the rotating gradient on a standard high-resolution spectrometer. Spectra of layers of fluid moving at a chosen angular frequency in a sample undergoing Couette flow shearing are shown. The radial fluid layer is selected by stroboscopically sampling with a Hahn-echo refocusing pulse applied at integer multiples of the rotation period. A two-dimensional analysis is given that separates the signal arising from the layer that is synchronized with the rotating gradient from the signals arising from the rest of the sample. Complex phase distortions occur when the echo delays do not match the rotation period. A radial frequency map of the Couette streamline flow is generated using the 2D technique which compares favorably with the calculated flow field.

Analytical expression for the magic-angle spinning nuclear magnetic resonance line shape in the presence of the second-order quadrupole interaction for the non-zero asymmetry parameter and large sample rotation frequency

An analytical expression for the magic-angle spinning nuclear magnetic resonance (MAS NMR) line shape function for the non-zero asymmetry parameter and large sample rotation frequency has been derived using the time-independent part of the quadrupole transition energy based on the stationary perturbation theory. The averaging over Euler angles in the case of a powdered sample is also done analytically, which is so far implemented in the literature by a numerical simulation procedure. While the peak positions obtained from analytical and numerical averaging procedures agree well, the peak heights are significantly different, showing the need for an analytical solution for the averaging over Euler angles, as this would otherwise lead to erroneous values for the quadrupole parameters. The MAS NMR line shape function derived in this paper can be compared directly with the experimental MAS NMR spectrum for the large sample rotation frequency, and the values of the quadrupole parameters can be obtained. We have also described a method of evaluating the quadrupole parameters for the small sample rotation frequency, using the peak heights and peak separation of the central band of the experimental MAS NMR spectrum and the MAS NMR line shape function derived in the present paper. As an example, the quadrupole parameters for vitreous B 2O 3 are computed.

The diazo group as an example of an isolated nuclear spin pair. Nitrogen-15 rotational resonance

This solid-state nuclear magnetic resonance investigation is concerned with the 15N spin pair present in the non-associated molecular solid, 5-methyl-2-diazobenzenesulphonic acid hydrochloride. The 15N spectrum is investigated under both magic-angle and off-magic-angle conditions. For the magic-angle spinning case, when the isotropic shift difference of the two spins, or a submultiple thereof, precisely coincides with the frequency of sample rotation, spectral broadening is predicted. We observe unusually distinct and striking lineshape features for this compound under this rotational resonance condition from which we estimate a 15N 15N separation of 1.110±0.004Å. A simplified description of the rotational resonance phenomenon is presented. The off-magic-angle spinning experiment allows us to deduce the relative orientations of the shielding tensors and the dipolar interaction. The experimental shielding parameters are compared with qualitative theoretical prediction.

Double-quantum homonuclear rotary resonance: Efficient dipolar recovery in magic-angle spinning nuclear magnetic resonance

We describe an efficient method for the recovery of homonuclear dipole–dipole interactions in magic-angle spinning NMR. Double-quantum homonuclear rotary resonance (2Q-HORROR) is established by fulfilling the condition ωr=2ω1, where ωr is the sample rotation frequency and ω1 is the nutation frequency around an applied resonant radio frequency (rf) field. This resonance can be used for double-quantum filtering and measurement of homonuclear dipolar interactions in the presence of magic-angle spinning. The spin dynamics depend only weakly on crystallite orientation allowing good performance for powder samples. Chemical shift effects are suppressed to zeroth order. The method is demonstrated for singly and doubly 13C labeled L-alanine.

Influence of sample rotation on NMR relaxation in liquids

It is shown that spin—lattice and spin—spin relaxation times T 1 and T 2 of methyl group protons in some liquids depend on the sample rotation. The observed dependence of T 1 on sample rotation frequency is interpreted on the basis of the relaxation theory.

The origin of rotational remanent magnetization

Summary. Rotational remanent magnetization, RRM, is the magnetization acquired when a sample is rotated during alternating field demagnetization. Although the existence of RRM has been well documented in different laboratories, until now no physical mechanism explaining its origin has been given. We propose that the RRM originates from thermal fluctuations biased by a precessional torque associated with the alternating field. Our theory is consistent with the observation that no directional preference exists in the experimental situation until the sample is rotated relative to the alternating field. Moreover, our theory predicts that the combined sample rotation and precession will produce a RRM that switches direction when the frequency of sample rotation increases from any value below the frequency of the alternating field to any value above that frequency as observed in experiments. Although no precise theory is given for the intensity of RRM, the model presented here can qualitatively explain previous intensity observations.

NIRSE: Normal-incidence rotating-sample ellipsometer

We describe a normal-incidence rotating-sample ellipsometer (NIRSE) which is capable of measuring the ratio of the complex amplitude reflection coefficients ϱ of an anisotropic sample for incident light that is linearly polarized along two orthogonal principal axes in its surface. In the NIRSE, ϱ is derived from the normalized amplitudes of the Fourier components of a photoelectric signal at double and four times the angular frequency of sample rotation. The application of PIE (perpendicular-incidence ellipsometry) to anisotropic film-substrate systems is also discussed.

Investigation of the 1H Spin-Lattice Relaxation in Polycrystalline NH4ReO4 Induced by Mechanical Sample Rotation

Abstract Measurements of the proton spin-lattice relaxation time, T1 , in polycrystalline NH4ReO4 have shown that mechanical sample rotation in the static magnetic field reduces T1 for proton magnetic resonance frequencies within the range 30 - 42 MHz. The rotation induced proton spin-lattice relaxation times, T1 ′, were measured at different Larmor frequencies, sample rotation frequencies, and temperatures. At room temperature T1 ′ increases with increasing proton Larmor frequency. The dependence of T1 ′ on sample rotation frequency and temperature is small. The "normal" proton T1 times were determined as a function of temperature at 30 and 42 MHz. From these measurements an average activation energy of 2.4 kcal/mole was obtained for the NH4 + reorientation. The "normal" spin-lattice relaxation behavior was observed to be non-exponential at 30 MHz above 200 K and at 42 MHz above 260 K and is nearly independent on temperature above 300 K.