Dynamics of [C3H5N2]6[Bi4Br18] by means of (1)H NMR relaxometry and quadrupole relaxation enhancement.

(1)H spin-lattice field cycling relaxation dispersion experiments in the intermediate phase II of the solid [C3H5N2]6[Bi4Br18] are presented. Two motional processes have been identified from the (1)H spin-lattice relaxation dispersion profiles and quantitatively described. It has been concluded that these processes are associated with anisotropic reorientations of the imidazolium ring, characterized by correlation times of the order of 10(-8) s-10(-9) s and of about 10(-5) s. Moreover, quadrupole relaxation enhancement (QRE) effects originating from slowly fluctuating (1)H-(14)N dipolar interactions have been observed. From the positions of the relaxation maxima, the quadrupole coupling parameters for the (14)N nuclei in [C3H5N2]6[Bi4Br18] have been determined. The (1)H-(14)N relaxation contribution associated with the slow dynamics has been described in terms of a theory of QRE [Kruk et al., Solid State Nucl. Magn. Reson. 40, 114 (2011)] based on the stochastic Liouville equation. The shape of the QRE maxima (often referred to as "quadrupole peaks") has been consistently reproduced for the correlation time describing the slow dynamics and the determined quadrupole coupling parameters.

Simultaneous measurement of very small magnetic fields and spin-lattice relaxation.

A field cycling (FC) NMR experiment is presented which allows for the simultaneous determination of very small magnetic fields down to about 3 µT and the concomitant measurement of nuclear spin-lattice relaxation times in these fields. The technique will enable broadband spin-lattice relaxation dispersion experiments down to about 100 Hz (1)H Larmor frequency. Limitations of its applicability are discussed.

NMR field-cycling at ultralow magnetic fields.

The paper describes some significant technical improvements of a home built NMR field cycling relaxometer [O. Lips, A. Privalov, S. Dvinskikh, F. Fujara, J. Magn. Reson. 149 (2001) 22-28] now allowing for fast switching of polarization fields (up to more than 1T) to evolution fields down to the sub-µT range. The most important instrumental details such as the description of an involved 3-dimensional resistive coil setup are given. Fields below about 5 µT can only be stabilized by incorporation of an active field drift and fluctuation compensation tool. In this way, the smallest 1H Larmor frequency obtained and measured so far has been 12 Hz.

One dimensional magnetic resonance microscopy with micrometer resolution in static field gradients.

Magnetic resonance microscopy (MRM) is a valuable tool for spatially resolved studies. While it is desirable to address voxels in the general case, it is sufficient to resolve slices of the sample in many cases of practical importance, e.g., for layered structures or at planar surfaces. We demonstrate that use of high static field gradients of 73 T/m in combination with a specially designed probe head enable MRM with an ultrahigh resolution of ∼2 µm in one dimension. The key feature of the built probe head is a precise computer controlled adjustment of the sample position and orientation, which allows for an accurate alignment of the samples with respect to the gradient of the magnetic field. Since slice-wise scanning of extended samples with this high spatial resolution is time-consuming, we introduce a methodology to reduce the experimental time significantly. Unlike the usual approach, which involves elaborate hardware and software correction, experimental imperfections are removed by stepwise moving the sample in our case. We demonstrate the capabilities of high-resolution 1D MRM for a solid sample with a layered structure and a liquid droplet on a planar solid substrate.

1H NMR at Larmor frequencies down to 3Hz by means of Field-Cycling techniques.

Field-Cycling (FC) NMR experiments were carried out at 1H Larmor frequencies down to about 3Hz. This could be achieved by fast switching a high polarizing magnetic field down to a low evolution field which is tilted with respect to the polarization field. Then, the low frequency Larmor precession of the nuclear spin magnetization about this evolution field is registered by means of FIDs in a high detection field. The crucial technical point of the experiment is the stabilization of the evolution field, which is achieved by compensating for temporal magnetic field fluctuations of all three spatial components. The paper reports on some other basic low field experiments such as the simultaneous measurement of the Larmor frequency and the spin-lattice relaxation time in such small fields as well as the irradiation of oscillating transversal magnetic field pulses at very low frequencies as a novel method for field calibration in low field FC NMR. The potential of low field FC is exemplified by the 1H relaxation dispersion of water at frequencies below about 2kHz stemming from the slow proton exchange process.

Perspectives of Deuteron Field-Cycling NMR Relaxometry for Probing Molecular Dynamics in Soft Matter.

Due to the single-particle character of the quadrupolar interaction in molecular systems, (2)H NMR poses a unique method for probing reorientational dynamics. Spin-lattice relaxation gives access to the spectral density, and its frequency dependency can be monitored by field-cycling (FC) techniques. However, most FC NMR studies employ (1)H; the use of (2)H is still rare. We report on the application of (2)H FC NMR for investigating the dynamics in molecular liquids and polymers. Commercial as well as home-built relaxometers are employed accessing a frequency range from 30 Hz to 6 MHz. Due to low gyromagnetic ratio, high coupling constants, and finite FC switching times, current (2)H FC NMR does not reach the dispersion region in liquids (toluene and glycerol), yet good agreement with the results from conventional high-field (HF) relaxation studies is demonstrated. The pronounced difference at low frequencies between (2)H and (1)H FC NMR data shows the relevance of intermolecular relaxation in the case of (1)H NMR. In the case of the polymers polybutadiene and poly(ethylene-alt-propylene), very similar relaxation dispersion is observed and attributed to Rouse and entanglement dynamics. Combination with HF (2)H relaxation data via applying frequency-temperature superposition allows the reconstruction of the full spectral density reflecting both polymer as well as glassy dynamics. Transformation into the time domain yields the reorientational correlation function C2(t) extending over nine decades in time with a long-time power law, C2(t) ∝ t(-0.45±0.05), which does not conform to the prediction of the tube-reptation model, for which ∝ t(-0.25) is expected. Entanglement sets in below C2(t = τe) ≅ S(2) = 0.001, where τe is the entanglement time and S the corresponding order parameter. Finally, we discuss the future prospects of the (2)H FC NMR technique.

Long-Time Diffusion in Polymer Melts Revealed by 1H NMR Relaxometry.

We demonstrate that field-cycling 1H NMR relaxometry can be used as a straightforward method of determining translational diffusion coefficient D = D(M) in polymer systems. The 1H spin-lattice relaxation dispersion for polybutadiene of different molecular masses M (446 < M/(g mol-1) < 9470) is measured at several temperatures (233 < T/K < 408) in a broad frequency range. The diffusion coefficient D(T) is determined from the intermolecular contribution to the overall spin-lattice relaxation rate R1(ω), which dominates in the low-frequency range and follows a universal dispersion law linear in √ω. The extracted diffusion coefficients are in good agreement with the values obtained previously by field gradient NMR. The molecular mass dependence D = D(M) exhibits two power laws: D ∝ M-1.3±0.1 and ∝M-2.3±0.1. They show a crossover for M = 2300, a value that is close to the entanglement molecular mass Me of polybutadiene. The corresponding power-law exponents are close to the prediction of the tube-reptation model.