Atomic nucleus magnetism

In this chapter, three methods for measuring the frequencies of the vibrations of chemical bonds between atoms in solids are discussed. Two of them, Fourier Transform Infrared Spectroscopy, FTIR, and Raman Spectroscopy, use infrared (IR) radiation as the probe. The third, High-Resolution Electron Enetgy-Loss Spectroscopy, HREELS, uses electron impact. The fourth technique. Nuclear Magnetic Resonance, NMR, is physically unrelated to the other three, involving transitions between different spin states of the atomic nucleus instead of bond vibrational states, but is included here because it provides somewhat similar information on the local bonding arrangement around an atom. 

There are two contributions to the magnetic dipole moment of an electron bound to an atomic nucleus, which, in semiclassical models, are attributed to orbital motion, represented by quantum number l, and spin, represented by quantum number, v. The orbital and spin components are linked, or coupled, on isolated atoms or ions to give an overall magnetic dipole moment for the atom. The total magnetic dipole moment of the atom is given by... 

Each atomic nucleus - as well as each particle - is characterised by a number of intrinsic parameters, including the spin I. This vector quantity, which has a preferential direction, is introduced in quantum mechanics and has no classical equivalent. It explains, among other things, the behaviour of atoms in media where there is a preferential direction. The spin of the nucleus can be related to the kinetic moment L using classical mechanics and it has the same dimensions (J - s). Thus an atom, placed in a magnetic field, will sense the orientation of the magnetic field. The spin is defined by the spin quantum number, /, which is dependent on the individual nucleus considered, and is always a positive multiple of 1/2, including zero. 

In brief, a magnetic atomic nucleus in a magnetic field may be either lined up with or opposed to the external field, and the transition from one state to the other corresponds to an amount of energy which can be provided by electromagnetic radiation in the radiofrequency range. 

The sensitivity of an atomic nucleus in the NMR experiment is related to its gyromagnetic ratio y. It determines the energy difference AE between the precession states in a magnetic field of flux density B0 (Figs. 2.1 and 2.35) ... 

The hyperfine structure (splitting) of energy levels is mainly caused by electric and magnetic multipole interactions between the atomic nucleus and electronic shells. From the known data on hyperfine structure we can determine the electric and magnetic multipole momenta of the nuclei, their spins and other parameters. 

Detection of magnetic moment associated with an odd number of protons in an atomic nucleus. 

NMR spectra result from the transition of an atomic nucleus in a magnetic field from a low-energy to a high-energy state [235], There are two aspects to the quantum-mechanical calculation of NMR spectra [301] calculation of shielding (chemical shifts) and calculation of splitting (coupling constants). Most of the... 

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. 

An important modern tool for the direct observation of spin configurations is neutron diffraction. Because the neutron is a neutral particle that carries a magnetic moment, it is primarily scattered by only the atomic nucleus and electrons with unpaired spins. Halpern and Johnson (247) have shown that the differential scattering cross section of an atom, including both nuclear and magnetic scattering, is... 

When we now speak of nuclear magnetic resonance, we are discussing the kind of NMR discovered by Bloch and Purcell, that is, nuclear magnetic resonance in bulk materials. The early work in NMR was concentrated on the elucidation of the basic phenomena (much of which we cover in Chapters 2 and 8) and on the accurate determination of nuclear magnetic moments, which were of interest in elucidating aspects of the structure of the atomic nucleus. 

The objective of the 1938 experiments was to measure the magnetic moments of the hydrogen and deuterium nuclei as accurately as possible. In 1938, however, a new sense of promise inspired the members of Rabi s group as they prepared to apply the new resonance method to the hydrogens and to measure the magnetic moments of the proton and the deuteron to a new level of precision. Eventually, this objective was accompHshed successfully, but not without surprises that led to new basic knowledge about the atomic nucleus. 

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