J. W. M. van Os

NMR images of rotating solids

It is demonstrated that NMR images may be obtained from solids rotating at the magic angle. This approach leads to line narrowing in favorable samples and is easily implemented. In very rigid systems it is suggested that imaging be performed with a combination of MAS and multiple-pulse line narrowing.

NMR images of rigid solids

The first two-dimensional 1H NMR image of a solid whose linewidth is not narrowed by motional averaging is presented. The image of a polyethylene phantom was acquired with line narrowing by multiple-pulse coherent averaging of the homonuclear dipolar broadening, and by magic-angle sample spinning of the chemical shift anisotropy. To obtain the image, a magnetic field gradient was rotated synchronously with the spinner, and a series of spectra were recorded at incremented phase differences between the spinner and the magnetic field gradient. An image was calculated from these spectra by the filtered back projection method.

Adiabatic homonuclear polarization transfer in magic-angle-spinning solid-state NMR

Abstract An adiabatic homonuclear polarization-transfer scheme is introduced. It involves an adiabatic passage through the rotational-resonance condition (APRR) experimentally realized by a ramp of the MAS speed. The APRR experiment is demonstrated to lead to a more complete transfer and to be more broadbanded than the rotational-resonance experiment. APRR can be looked at as a prototype for a whole class of homonuclear adiabatic polarization-transfer schemes.

Strategies for solid-state NMR in high-field Bitter and hybrid magnets

Abstract Strategies for successful solid-state NMR experiments in high-field Bitter and hybrid magnets are discussed and implemented on an older generation 24 T system. In general, the stability and homogeneity of this type of magnets do not meet the requirements for most NMR experiments. Nevertheless, we will show that a new type of shimming with ferromagnetic inserts, combined with feedback control of the power-supply and NMR reference deconvolution can be used to obtain well-resolved solid-state NMR spectra. We will demonstrate that this approach is especially useful for the study of half-integer quadrupolar nuclei. Experimental results include static and Magic Angle Spinning (MAS) 27 Al NMR spectra of the minerals sillimanite and andalusite.

Broadband heteronuclear decoupling

Abstract It is shown experimentally and by calculations, using average Hamiltonian theory, that the decoupling efficiency of the composite decoupling schemes MLEV-4 and MLEV-16, introduced by Levitt, Freeman, and Frenkiel, can be improved by inserting delays and changing one pulse length of the composite pulse.

A high speed spinner for magic angle spinning NMR

A high speed spinner for high resolution solid state NMR is described. The cylindrical rotor is supported by two air bearings which makes the orientation of the spinning axis independent of small imbalances and rotation speed. With compressed air speeds up to 7 kHz have been obtained with a phi =10 mm rotor.

Direct digital frequency modulation in NMR spectrometers

An instrument is described that allows fast phase‐continuous frequency switching and modulation. The instrument is designed to switch to a new frequency for a period of time or to a series of frequencies, and then switch back to the original frequency with the phase of the original output signal as if no frequency switching occurred (phase‐coherent switching). The described device is implemented in Bruker NMR spectrometers.

Programmable digital pulse generator for proton enhanced 13C high resolution NMR in solids

A programmable pulse generator is described for NMR spectrometers. The pulse generator can operate either in a single cycle mode where up to 16 pulses can be given sequentially or, in the double cycle mode, a programmable number (</=15) of these pulses can be repeated n times (n</=99) before the rest of the 16 pulses are completed. The length of all these pulses can be programmed, manually or by computer. As an example the programming of the pulse generator for a multiple contact proton enhanced (13)C NMR experiment is given.

Feasibility studies for the implementation of nuclear magnetic resonance in a 25T hybrid magnet

As an exploratory study for NMR experiments in the future 20 MW Nijmegen high field magnet laboratory, the possibilities of field stabilization and field gradient compensation in a 25 T hybrid magnet in the present installation were evaluated. High frequency field fluctuations from the power supply can be compensated to better than 10-3 ppm in the 10 Hz-10 kHz range using a computer controlled feedback system. Field mapping by 2H magnetic resonance using a homebuilt device not only showed that there are substantial axial but also strong radial field gradients. It can be shown that for any cylindrical multicoil Bitter magnet the main components of these gradients can be compensated with simple ferromagnetic inserts. In this way we achieved a linewidth under 5 ppm in 1 mm3 without further shimming or optimization. The low frequency drift of the field due to instabilities of the present power supply and the effects of temperature fluctuations of the coil are determined by simultaneous acquisition of an in-situ deuterium reference signal together with the signal of interest. This allows for a full compensation of the field fluctuations by deconvolution techniques. We will report preliminary NMR results on solid27 Al samples in fields up to 25 T.

Time-encoded nuclear magnetic resonance spectrometer control

A device is described that enables time control of many external devices on any nuclear magnetic resonance spectrometer even if it has only one control output available for switching external appliances. Furthermore, it allows activation of extra devices during so‐called ‘‘wait states’’ which cannot be served by most standard spectrometer setups. The basic idea is to decode the pulse length applied to one ‘‘standard’’ control output in order to create a whole new set of ‘‘nonstandard’’ controls.