![]() By adjusting the length and power of the RF pulse it is possible to flip M 0 vector into the x′– y′ plane this is equivalent to equalizing the populations of the quantum mechanical energy levels and is known as a 90° pulse. M 0 will precess around B 1 with a frequency ω 1 equivalently to eqn (see Figure 2). With the introduction of RF at this frequency, right-hand circularly polarized along the z-axis, the equivalent magnetic field in this frame, B 1, becomes oriented in the x′– y′ plane and has an amplitude equal to the amplitude of the B-field component of the RF. To illustrate this it is commonplace to define a set of axes rotating about B 0 at the Larmor frequency. Manipulation of the orientation of M 0 relative to B 0 using circularly polarized radio frequency (RF) at this resonant frequency leads to the generation of the NMR signal. The absorption and irradiation of energy associated with transitions between these levels constitute the physical phenomena observed in an experiment of magnetic resonance. The Zeeman interaction between the nuclear magnetic dipole and an external static magnetic field gives rise to a manifold of energy levels for the nucleus depending on its orientation with respect to the axis defined by the magnetic field. The discussion about the details of the interaction between the nuclear electric quadrupole moment of quadrupolar nuclei and local electric field gradients will be deferred to another section. In this section one is concerned with this magnetic interaction, which constitutes the basis of all NMR experiments. The basic interaction necessary to understand NMR is the so-called Zeeman interaction, occurring between a magnetic dipole moment and the magnetic fields (applied plus local fields) existing at the nuclear site. deAzevedo, in NMR Quantum Information Processing, 2007 2.2 INTERACTION WITH STATIC MAGNETIC FIELDSĪtomic nuclei with non-zero total angular momentum interact with the electromagnetic fields present in their environment through the nuclear magnetic dipole moment and, in the case of nuclei with I > 1/2, the nuclear electric quadrupole moment. ![]() ![]() Less frequently employed are spectroscopic techniques such as optical absorption and emission, and nuclear magnetic resonance (NMR). The principle technique used for the measurement of isotope ratios is mass spectrometry (MS) using various combinations of ion source and mass analyzer for the analysis of many organic and inorganic materials. Today, a range of isotopes has been discovered or made for every known element, the vast majority being made by nuclear reactions using man-made sources of nucleons. Thompson, who discovered two stable isotopes of neon (mass 20 and 22) using one of the earliest mass spectrometers. The term ‘isotope’ was first used by Soddy in 1913, the same year in which the existence of naturally occurring isotopes was first reported by J.J. Isotopes are atomic species of the same element that differ in atomic mass that is, nuclei with a given number of protons but differing numbers of neutrons. Nuclides are most commonly referred to by the elemental symbol preceded by the mass number ( A), e.g., 12C, 238U. As the number of protons in the nucleus (the atomic number Z) increases more neutrons are required to prevent the nucleus from breaking apart under the strain of proton–proton repulsion. Sleeman, in Encyclopedia of Analytical Science (Second Edition), 2005 IntroductionĪtomic nuclei (nuclides) consist of positively charged protons and uncharged neutrons particles collectively known as nucleons, which interact through a short-range attractive force that holds the nucleus together. We refer the reader who wishes to learn more about the subject to the brief review by Lucken 〈63PMH(2)89〉. ![]() There is still, however, a great paucity of data on NQR spectra of heterocyclic molecules, and the great majority of the authors of the specialist reviews in this volume have been obliged to omit mention of the technique altogether. Chlorine as a substituent has been far more fully studied, and the resonances have been used as a probe of electronic interactions between the chlorine atom and the heterocyclic nucleus, in a number of cases. Unfortunately, its quadrupole moment is rather low, and spectra are difficult to obtain, even in the most favourable cases. The nucleus of greatest interest to the heterocyclic chemist is that of nitrogen. McKillop, in Comprehensive Heterocyclic Chemistry, 1984 2.01.4.2 Nuclear Quadrupole ResonanceĪtomic nuclei which possess spin quantum numbers greater 1 2 than have quadrupole moments also, and direct transitions between nuclear quadrupolar energy levels can be observed under favourable conditions.
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