Quadrupole interaction effect

Besides NQR spectroscopy and the study of nuclear quadrupole interaction effects in broad-line NMR spectroscopy, paramagnetic electron resonance 6°1, Mossbauer spectroscopy, and the study of perturbed angular correlation of y-rays, are suitable methods for studying nuclear quadrupole interactions in solids. Indirect methods are also available for acquiring information about the nuclear quadrupole couplinjg constant from the liquid state (particularly NMR spectroscopy in liquids and in liquid crystals in some cases gives information about this constant). By microwave spectroscopy, the nuclear quadrupole interaction may be studied in the gaseous phase (see the paper by Zeil). We shall deal here only with the aspect of NQR spectroscopy in solids since this method has the broadest applicability to chemical problems in comparison with the other methods mentioned. 

Plotted on the ppm scale, spectral features arising from second order quadrupole interaction effects would increase by a factor of 1.9, for the 96 MHz spectrum and 3.4, for the 76 MHz spectrum, when compared with the 132 MHz spectrum. Inspection of the spectra in Figure lb indicates that this is not occurring, establishing that the spectral features are associated with separate sodium-23 MASNMR lines. This result can be more clearly observed for the partially exchanged, hydrated Y zeolites discussed below. 

The effect of electric quadrupole interaction for Fe is exemplified in Fig. 4.6. The ground state remains unsplit because of the lack of quadrupole moment for 7 = 1/2. The excited state with 7 = 3/2 splits into two doubly degenerate substates 3/2, 3/2) and 3/2, 1/2) due to the w/ dependence of the quadrupole energies ... 

In a conventional Fe Mossbauer experiment with a powder sample, one would observe a so-called quadrupole doublet with two resonance lines of equal intensities. The separation of the lines, as given by (4.36), represents the quadrupole splitting The parameter Afg is of immense importance for chemical applications of the Mossbauer effect. It provides information about bond properties and local symmetry of the iron site. Since the quadrupole interaction does not alter the mean energy of the nuclear ground and excited states, the isomer shift S can also be derived from the spectrum it is given by the shift of the center of the quadrupole spectrum from zero velocity. 

Pure nuclear magnetic hyperfine interaction without electric quadrupole interaction is rarely encountered in chemical applications of the Mossbauer effect. Metallic iron is an exception. Quite frequently, a nuclear state is perturbed simultaneously by... 

Angular Dependence of the Effect of Quadrupole Interaction in High-Field Spectra... 

On the other hand, internal magnetic fields at the iron nucleus arising from the magnetic moments of unpaired valence electrons can be much stronger than any applied field and their effect can easily exceed the quadrupole interaction. For instance, Mossbauer nuclei in magnetic materials such as metals or oxides may experience fields of 30-50 T even without applied field. Similarly, the typical... 

The effect of a positive quadrupole interaction on the ground and excited nuclear states of Ru is shown schematically in Fig. 7.33 as adapted from a publication by... 

Fig. 7.33 Effect of a positive quadrupole interaction on the ground and first excited states of Ru. The asymmetry parameter r) is assumed to be zero. The ratio of the quadrupole moments is taken to be Q3/tJQs/2 = 3. a = e qQsn and B = e qQ a (from [124])...

Dunlap, B.D. Mdssbauer effect data index. In Stevens, J.G., Stevens, V.E. (eds.) An Introduction to Electric Quadrupole Interactions in Mdssbauer Spectroscopy. Adam Hilger, London (1972)... 

A simple kind of symmetry which applies to a large number of crystals is axial symmetry that is, two directions are equivalent, such as x and y. When the quadrupole interaction is small compared to the energy separation between the 27 - - 1 energy levels of a nucleus in a magnetic field Ho, the effect of the quadrupole interaction is to destroy the equal spacing. For axial symmetry the effect may be described in terms of a single parameter (5, 90) eq, as follows ... 

The usefulness of quadrupolar effects on the nuclear magnetic resonance c I 7 yi nuclei in the defect solid state arises from the fact that point defects, dislocations, etc., give rise to electric field gradients, which in cubic ciystals produce a large effect on the nuclear resonance line. In noncubic crystals defects of course produce an effect, but it may be masked by the already present quadrupole interaction. Considerable experimental data have been obtained by Reif (96,97) on the NMR of nuclei in doped, cubic, polycrystalline solids. The effect of defect-producing impurities is quite... 

The importance of quadrupole interaction in zeolitic sorption has been pointed out by Barrer and Stuart (20). Such effects are clearly illustrated by the data for sorption of nitrogen and carbon dioxide in both H-chabazite and 5A zeolite. For these molecules, which have large quadrupole moments, the experimental values of K0 are much smaller than the theoretical values predicted from the idealized model suggesting either localized sorption at specific sites within the cavity or restricted rotational freedom. 

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