Magnetic properties, nuclei

Two parts of molecules have magnetic properties, nuclei and electrons. The magnetism of nuclei gives rise to nuclear magnetic resonance spectroscopy (NMR) although there is much similarity between the basic concepts of ESR and NMR, there are differences in the spectrometers used and their applications. 

The nuclei of many isotopes possess an angular momentum, called spin, whose magnitude is described by the spin quantum number / (also called the nuclear spin). This quantity, which is characteristic of the nucleus, may have integral or halfvalues thus / = 0, 5, 1, f,. . . The isotopes C and 0 both have / = 0 hence, they have no magnetic properties. H, C, F, and P are important nuclei having / = 5, whereas and N have / = 1. 

The NMR phenomenon is based on the magnetic properties of nuclei and their interactions with applied magnetic fields either from static fields or alfemaling RF fields. Quanfum mechanically subatomic particles (protons and neutrons) have spin. In some nuclei these spins are paired and cancel each other out so that the nucleus of the atom has no overall spin that is, when the number of protons and neutrons is equal. However, in many cases the sum of the number of protons and neutrons is an odd number, giving rise to... 

The magnetic properties used to probe aromaticity arise from the presence of a diatropic ring current which tends to push an aromatic molecule out of a magnetic field (calculated property magnetic susceptibility, /). and which exerts NMR shielding on a proton at or above the ring center (calculated property nucleus-independent chemical shift, NICS). NICS values are obtained from the... 

The following table lists the magnetic properties required most often for choosing the nuclei to be used in NMR experiments.114 Refer to several excellent texts and the literature for guidelines in nucleus selection. 

The first term is the Zeeman interaction depending upon the g(RS OW, q ) tensor, external magnetic field B0 and electron spin momentum operator S the second term is the hyperfine interaction of the th nucleus and the unpaired electron, defined in terms hyperfine tensor A (Rsklw, qj) and nuclear spin momentum operator n. The following terms do not affect directly the magnetic properties and account for probe-solvent [tfprobe—solvent (Rsiow, qJ)l ld solvent-solvent //solvent ( qj)] interactions. An explicit... 

Within a molecule, a nucleus is characterized by its magnetic properties and electronic environment and by the following consequent parameters associated with the corresponding NMR signals chemical shift, coupling constants, relaxation rates,15 and nuclear Overhauser enhancement (NOE).1617 All these values may be used to extract qualitative or quantitative information about the structure, the conformation, and the behavior of molecules in solution. 

NMR is a spectroscopic technique that relies on the magnetic properties of the atomic nucleus. When placed in a strong magnetic field, certain nuclei resonate at a characteristic frequency in the radio frequency range of the electromagnetic spectrum. Slight variations in this resonant frequency give us detailed information about the molecular structure in which the atom resides. 

NMR is a versatile spectroscopic technique for studying opaque heterogeneous samples, which has already been proven to have a number of useful applications in dairy research (Duce et al., 1995). The technique is also suitable for at-line and on/in-line process control. NMR is based on the magnetic properties of the nucleus of certain atoms, such as the nucleus of the hydrogen atom, lH, the nucleus of carbon-13, 13C, and the nucleus of phosphorus-31, 31P. It is convenient to divide the parameters obtained from NMR spectra into static and dynamic parameters. 

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