The nuclear magnetic resonance phenomenon

The basis of nuclear magnetic resonance spectroseopy lies with the property of a nucleus known as its spin. The classical picture is that of a spherical atomic nucleus rotating about its nuclear axis. Quantum mechanies describes this angular momentum F as a quantized property defined by the nuclear spin quantum number I  

Structural Methods in Molecular Inorganic Chemistry, First Edition. David W. H. Rankin, Norbert W. Mitzel and Carole A. Morrison. 2013 John Wiley Sons, Ltd. Published 2013 by John Wiley Sons, Ltd. 

For every isotope of every element there is a nuclear ground state with a nuclear spin quantum number I. I must have a value of /2, where n is an integer. Isotopes having atomic and mass numbers that are both even (e.g. Si, Fe) have 1=0, and these nuclei have no magnetic moment and so do not give NMR 

When I is non-zero, the nucleus has a magnetic moment fi, given by 

Nuclei that have a nuclear electric quadrupole moment have energy levels that arc split in the presence of an electric field gradient. Such a gradient does not have to be applied externally, but is a molecular property, and is almost always non-zero, the exceptions being in perfectly octahedral or tetrahedral environments. Transitions between these energy levels are observed in nuclear quadrupole resonance (NQR) spectroscopy, described in the on-line supplement for Chapter 4 on NQR. 

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The nuclear magnetic resonance phenomenon was first observed through a continuous wave experiment [2-6]. Resonance between the two Mj levels of an / = V2 nucleus (without loss of generality) in a magnetic field Bo can be achieved by applying a magnetic field rotating with a frequency (see Eqs. (1.12) and (1.16)) such that... 

The nuclear magnetic resonance phenomenon occurs for certain nuclei that possess a magnetic moment g. When placed in a magnetic field, nuclear moments will align themselves in a discrete number of orientations. The number of orientations is determined by the nuclear spin I, which is a fundamental property of certain nuclei and has values of 0, 1/2, 1, 3/2, etc. The number of allowed orientations of a nucleus is given by 21 - - 1. The most commonly observed nuclei in... 

In order to appreciate its application to kinetics, a brief outline of the nuclear magnetic resonance phenomenon will be given. This treatment will be rather pic-... 

The nuclear magnetic resonance phenomenon occurs when nuclei aligned with an applied field are induced to absorb energy and change their spin orientation with respect to the applied field. Figure 3.5 illustrates this process for a hydrogen nucleus. 

Plate 32 Felix Bloch who was awarded the Nobel prize for physics in 1952 jointly with Edward Purcell. They led two independent research groups which, in 1945, first detected the nuclear magnetic resonance phenomenon in bulk matter. See Magnetic Resonance, Historical Perspective. Reproduced with permission from The Nobel Foundation. 

First experimental observations of the nuclear magnetic resonance (NMR) phenomenon were made in 1946 independently by groups working under Bloch (7) at Stanford and Purcell 112) at Harvard. For this discovery, these two physicists jointly were awarded the Nobel prize in physics in 1952. The experimental and theoretical aspects of nuclear magnetic resonance (NMR) have been developed rapidly so that today it is an indispensable technique for the investigation of a wide variety of chemical and physical phenomena. 

Today the most useful chemical instrument is probably the nuclear magnetic resonance (NMR) spectrometer. Magnetic resonance imaging (MRI), vital in modern medicine, is derived from NMR. In late 1945, a physics group at Stanford, led by Felix Bloch (1905-83) (with William W. Hanson [1909-49] and Martin W. Packard), and one at Harvard, led by Edward M. Purcell (1912-97) (with Henry C. Tbrrey [1911-99] and Robert V. Pound [1919- ]), independently discovered the phenomenon of nuclear magnetic resonance. In order to manifest NMR an atomic nucleus must have nonzero nuclear spin. Of the roughly 100 stable isotopes that have nonzero nuclear spin, H, present in the vast majority of... 

Porphyrin is a multi-detectable molecule, that is, a number of its properties are detectable by many physical methods. Not only the most popular nuclear magnetic resonance and light absorption and emission spectroscopic methods, but also the electron spin resonance method for paramagnetic metallopor-phyrins and Mossbauer spectroscopy for iron and tin porphyrins are frequently used to estimate the electronic structure of porphyrins. By using these multi-detectable properties of the porphyrins of CPOs, a novel physical phenomenon is expected to be found. In particular, the topology of the cyclic shape is an ideal one-dimensional state of the materials used in quantum physics [ 16]. The concept of aromaticity found in fuUerenes, spherical aromaticity, will be revised using TT-conjugated CPOs [17]. 

A nucleus under study by nuclear magnetic resonance techniques is affected by other nuclei in the same molecule. This phenomenon is known as spin-spin coupling. The effect arises (in adjacent nuclei) from the two electrons joining the nuclei in a covalent bond. Suppose the energy of states in which the electrons in the bond have opposing spins is lower than the state in which the electron spins are parallel. Then the AE between the two states (in this case a negative number) is called the coupling constant, J, expressed in frequency units, Hz. Internuclear... 

As the protons fall back to the lower energy state, the same radio frequency that was absorbed is emitted. This can be measured with a radio receiver. This phenomenon is known as proton nuclear magnetic resonance ( H NMR). 

The usefulness of the NMR technique in solid state physics stems from the fact that the widths, splittings, and shifts of the magnetic resonance of nuclei in solids often depend in a sensitive manner on the magnetic and electrical environment of the nucleus in the solid. In this sense the nucleus can be considered as a probe by which one may ascertain certain details of the nuclear and electronic structure of the solid under investigation. Considerable attention has been given by numerous authors to the theory of the magnetic resonance phenomenon, and it is considered to be in a satisfactory state at the present time. 

Nuclear magnetic resonance (NMR) is based on a phenomenon that nuclei which possess both magnetic and angular moments (i.e. have odd mass number or odd atomic number) interact with an applied magnetic field B0 yielding 21 + 1 (where 1 is the nuclear spin quantum number) energy levels with separation AE ... 

Figure 9-21 Schematic representation of the possible alignments of a magnetic nucleus (here hydrogen) in an applied magnetic field. Transitions between the two states constitute the phenomenon of nuclear magnetic resonance. The arrows through the nuclei represent the average component of their nuclear magnetic moment in the field direction.

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