Quadrupolar nuclei line broadening effect

When the observed nucleus is itself quadrupolar. Here the quadrupole interaction is dominant, the others being usually very minor in comparison. Normal MAS speeds are much smaller than typical quadrupole couplings (which lie in the MHz range), producing an enormous number of sidebands, except in the case of the central transition of half-integer spins. In this latter case distinctive line broadening effects appear which cannot be completely removed by MAS. Methods to overcome these difficulties will be reviewed. 

MHz relative to the protons in Mc4Si at exactly 100 MHz. The effect of quadrupole line broadening is attenuated to some extent by the magnitude of / (/= 9/2), and the line widths are among the narrowest of all quadrupolar nuclei studied to date. One would expect efficient relaxation via the quadrupole moment, giving very short T2 times and consequently broad NMR signals. Also if the electronic distribution around the nucleus is symmetrical, sharp resonances can be obtained. 

The reason here is that the l N nucleus has a small magnetic moment, which is hard to drive by r.f. fields, the difficulty being compounded in the case of frozen solution samples by line broadening due to the l N nuclear quadrupolar coupling. Echo envelope modulation effects are, on the other hand, quite easy to see for nuclei with small moments. For the simple case of an I = 1/2 nucleus weakly coupled to an S = 1/2 electron, it can be shown that the modulation depth is independent of the nuclear moment and depends only on the ratio between the Zeeman field and the local field at the nucleus due to the electron.15 Breadth of the shfs line is, moreover, not a serious obstacle to detection, provided that at least one modulation cycle can be seen in the echo envelope. 

As mentioned above, the alkali metal chlorides have cubic symmetry about the chlorine nucleus which requires that the EFG to be essentially zero in a perfect crystal. This results in CT NMR spectra with narrow lines that are free of quad-rupolar effects. As the environment aroimd the chlorine nucleus is transformed to lower symmetry, second-order quadrupolar effects begin to be observed, leading to broadened CT lines with quadrupolar line shapes. The broadening of the signals in chlorine CT NMR spectra as the quadrupolar effects become more significant is the most serious limitation to the types of materials which can be studied with typical SSNMR methods. 

Fig. 4.2. The NMR spectrum (94.1 MHz, room temp.) of the IfJ cation in HF solution. The signal is split into a sextet by coupling of the equivalent fluorines to (I = 5/2). The coupling constant Jjp = 2730 15 Hz. The lines in the sextet have equal intensities but unequal line widths and thus heights. This unequal broadening is caused by the quadrupolar relaxation of the iodine nucleus which effects the gp n states differently as predicted by theory [236]. Inverted...

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