Nuclear magnetic resonance frequency scale

Abstract We use Nuclear Magnetic Resonance relaxometry (i.e. the frequency variation of the NMR relaxation rates) of quadrupolar nucleus ( Na) and H Pulsed Gradient Spin Echo NMR to determine the mobility of the counterions and the water molecules within aqueous dispersions of clays. The local ordering of isotropic dilute clay dispersions is investigated by NMR relaxometry. In contrast, the NMR spectra of the quadrupolar nucleus and the anisotropy of the water self-diffusion tensor clearly exhibit the occurrence of nematic ordering in dense aqueous dispersions. Multi-scale numerical models exploiting molecular orbital quantum calculations, Grand Canonical Monte Carlo simulations, Molecular and Brownian Dynamics are used to interpret the measured water mobility and the ionic quadrupolar relaxation measurements. 

The temperature regime of particular interest is 100-300 K. Within these temperature limits several transitions occur which are only partially understood. The transitions occur at 250 20 K, at 195 10 K, and at 150dz5K (4). The methods which have been used to observe these relaxations cover the entire frequency scale from dilatometry to electron spin resonance (ESR) and nuclear magnetic resonance (NMR). The... 

A) Schematic diagram of a simple nuclear magnetic resonance (NMR) spectrometer. The sample is placed in solution in a long, thin tube and spins in a probe sitting in a magnetic and surrounded by radio-frequency (RF) coils B) proton NMR spectrum of ethanol (QH O) with tetramethylsilane (TMS) added as internal standard. On the 8-scale of chemical shifts,... 

Finally, the values of the dielectric relaxation of hydroxyl groups measured by NMR (NMR= Nuclear Magnetic Resonance) and the frequencies of the vibrational modes of adsorbed molecules (OH or NH3), recorded by infra-red absorption, are related to the adsorption strength. From experimental values of OH stretching frequencies and from the acidity scale relevant for acids in solutions. Hair and Hertl (1970) were, for example, able to determine the pKa of adsorbed OH on surfaces. We will come back to the characteristics of hydroxyl groups adsorbed on oxide surfaces in the last section of this chapter. 

The translational motions and spin dynamics of conduction electrons in metals produce fluctuating local magnetic hyperfine fields. These couple to the nuclear magnetic moments, inducing transitions between nuclear spin levels and causing nuclear spin relaxation. The translational motions of electrons occur on a very rapid time scale in metals (<10 s), so the frequency spectrum of hyperfine field fluctuations is spread over a wide range of w-values. Only a small fraction of the spectral intensity falls at the relatively low nuclear resonance frequency (ojq 10 s ). Nevertheless, the interaction is so strong that this process is usually the dominant mode of relaxation for nuclei in metallic systems, either solid or liquid. 

The SIN defined by Equation 7.6 for a given NMR resonance is proportional to the square of the nuclear precession frequency (mo, rad/s), the magnitude of the transverse magnetic field (Bi) induced in the RE coil per unit current (/), the number of spins per unit volume (Ns), the sample volume (Vs), and a scaling constant that accounts for magnetic field inhomogeneities. The SIN is inversely proportional to the noise generated in the RE receiver and by the sample (Vnoise) as defined by the Nyquist theorem,... 

Chemical shift relates the Larmor frequency of a nuclear spin to its chemical environment l 3. The Larmor frequency is the precession frequency v0 of a nuclear spin in a static magnetic field (Fig. 1.1). This frequency is proportional to the flux density Bo of the magnetic field (v0 B0 = const.) 3. It is convenient to reference the chemical shift to a standard such as tetramethylsilane [TMS, (C//j)4Si] rather than to the proton ft. Thus, a frequency difference (Hz) is measured for a proton or a carbon-13 nucleus of a sample from the H or 13C resonance of TMS. This value is divided by the absolute value of the Larmor frequency of the standard (e.g. 400 MHz for the protons and 100 MHz for the carbon-13 nuclei of TMS when using a 400 MHz spectrometer), which itself is proportional to the strength B0 of the magnetic field. The chemical shift is therefore given in parts per million (ppm, 5 scale, SH for protons, 5C for carbon-13 nuclei), because a frequency difference in Hz is divided by a frequency in MHz, these values being in a proportion of 1 106. 

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