Nuclear energy absorption frequency

A graph that shows the characteristic energy absorption frequencies and intensities for a sample in a magnetic field is called a nuclear magnetic resonance (NMR) spectrum. 

If the oriented nuclei are now irradiated with electromagnetic radiation of the proper frequency, energy absorption occurs and the lower-energy state "spin-flips" to the higher-energy state. When this spin-flip occurs, the magnetic nuclei are said to be in resonance with the applied radiation—hence the name nuclear magnetic resonance. 

In order to dissipate the recoil energy Mossbauer was the first to use atoms in solid crystal lattices as emitters and also to cool both emitter and absorber. In this way it could be shown that the 7-ray emission from radioactive cobalt metal was absorbed by metallic iron. However, it was also found that if the iron sample were in any other chemical state, the different chemical surroundings of the iron nucleus produce a sufficient effect on the nuclear energy levels for absorption no longer to occur. To enable a search for the precisely required absorption frequency, a scan based on the Doppler effect was developed. It was noted that a velocity of 102 ms-1 produced an enormous Doppler shift and using the same equation (7) it follows that a readily attainable displacement of the source at a velocity of 1 cms-1 produces a shift of 108 Hz. This shift corresponds to about 100 line-widths and provides a reasonable scan width. 

The basic instrumentation used for spectrometric measurements has already been described in Chapter 7 (p. 277). The natures of sources, monochromators, detectors, and sample cells required for molecular absorption techniques are summarized in Table 9.1. The principal difference between instrumentation for atomic emission and molecular absorption spectrometry is in the need for a separate source of radiation for the latter. In the infrared, visible and ultraviolet regions, white sources are used, i.e. the energy or frequency range of the source covers most or all of the relevant portion of the spectrum. In contrast, nuclear magnetic resonance spectrometers employ a narrow waveband radio-frequency transmitter, a tuned detector and no monochromator. 

In addition to the above prescriptions, many other quantities such as solution phase ionization potentials (IPs) [15], nuclear magnetic resonance (NMR) chemical shifts and IR absorption frequencies [16-18], charge decompositions [19], lowest unoccupied molecular orbital (LUMO) energies [20-23], IPs [24], redox potentials [25], high-performance liquid chromatography (HPLC) [26], solid-state syntheses [27], Ke values [28], isoelectrophilic windows [29], and the harmonic oscillator models of the aromaticity (HOMA) index [30], have been proposed in the literature to understand the electrophilic and nucleophilic characteristics of chemical systems. 

The nuclear energy transitions that occur with the absorption of radio frequency light occur only in a strong magnetic field. 

Equation (8.23) is for gN positive for gN negative, a minus sign must be added.] Although there are 2/+1 nuclear-spin energy levels, they are equally spaced, and the selection rule allows only transitions between adjacent levels hence we get a single NMR absorption frequency. 

Application of B at the resonance frequency results in both energy absorption (+ nuclei become -and emission (— nuclei become + ). Because initially there are more + than -1 nuclei, the net effect is absorption. As B irradiation continues, however, the excess of +5 nuclei disappears, so that the rates of absorption and emission eventually become equal. Under these conditions, the sample is said to be approaching saturation. The situation is ameliorated, however, by natural mechanisms whereby nuclear spins move toward equilibrium from saturation. Any process that returns the z magnetization to its equilibrium condition with the excess of spins is called spin-lattice, or longitudinal, relaxation and is usually a first-order process with time constant T. For a return to equilibrium, relaxation also is necessary to destroy magnetization created in the xy plane. Any process that returns the X and y magnetizations to their equilibrium condition of zero is called spin-spin, or transverse, relaxation and is usually a first-order process with time constant T2. 

In common with optical spectroscopy, both classical mechanics and quantum mechanics are useful in explaining the NMR phenomenon, rho two ireai-monis yield identical relationships. Quantum mechanics. however, provides a useful relationship between absorption frequencies and nuclear energy states, w hereas classical mechanics yields a clear physical picture of the absorption process and how it is measureif. 

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