Magnetic fields NMR and

Figure Bl.12.13. MAS NMR spectra from kyanite (a) at 17.55 T along with the complete simulation and the individual components, (b) simulation of centreband lineshapes of kyanite as a fiinction of applied magnetic field, and tire satellite transitions showing (c) the complete spiimmg sideband manifold and (d) an expansion of individual sidebands and their simulation.

Nuclear magnetic resonance, NMR (Chapter 13 introduction) A spectroscopic technique that provides information about the carbon-hydrogen framework of a molecule. NMR works by detecting the energy absorptions accompanying the transitions between nuclear spin states that occur when a molecule is placed in a strong magnetic field and irradiated with radiofrequency waves. 

Many atomic nuclei behave like small bar magnets, with energies that depend on their orientation in a magnetic field. An NMR spectrometer detects transitions between these energy levels. The nucleus most widely used for NMR is the proton, and we shall concentrate on it. Two other very common nuclei, those of carbon-12 and oxygen-16, are nonmagnetic, so they are invisible in NMR. 

To study NMR spectra of compounds, apparatus is required that consists of three sets of components. These are a radio-frequency transmitter, a homogeneous magnetic field and a radio-frequency receiver. In addition to these, the apparatus includes a unit to sweep the magnetic field over a small range, a mere few parts per million. 

The difference in resonance NMR frequency of a chemically shielded nucleus measured relative to that of a suitable reference compound is termed chemical shift [164,165], and is a measure of the immediate electromagnetic environment of a nucleus. While the chemical shift depends on the Bo field, J does not. Chemical shifts, which cover a range of about 10 ppm for protons (i.e. 600 Hz in case of a 14.1 kG magnetic field) and 250 ppm for 13C, are the substance of NMR. 

A disadvantage in the utilisation of 13C NMR is the intrinsic low relative sensitivity (Table 5.14). However, higher magnetic fields and better probe design have... 

Ishikawa etal. proposed an approach for the determination of the ligand-field (LF) parameters of a set of isostructural lanthanide complexes. This method consists of a simultaneous fit of the temperature dependence of magnetic susceptibilities and NMR spectra for the whole isostructural series [18]. In order to avoid over-parametrization a key restriction is imposed each parameter is expressed as a linear function of the number of f electrons, n ... 

Macrocycles and other concave structures, acid-base behaviour in, 30,63 Macromolecular systems in biochemical interest, 13C NMR spectroscopy in, 13,279 Magnetic field and magnetic isotope effects on the products of organic reactions, 20,1 Mass spectrometry, mechanisms and structure in a comparison with other chemical processes, 8, 152... 

The detection of Brpnsted acid sites, SiO(H)Al, is the most recent achievement of 170 NMR of zeolites [119-121]. High magnetic fields and double resonance techniques have allowed the observation of this important species in zeolite HY [120]. Chemical shifts of 21 and 24 ppm have been reported for zeolite HY for the Brpnsted sites in the supercage and sodalite cage, respectively [119]. Quadrupole interaction parameters are Cq = 6.0 and 6.2 MHz and r] = 1.0 and 0.9, respectively. Signal enhancement by 1H-170 cross-polarization has also permitted the detection of the acid sites in zeolite ZSM-5 [119], where they exist with lower abundance than in HY. 

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