Coupling to quadrupolar nuclei

The resonances of a quadrupolar nucleus itself are also broad if its relaxation time is very short. The linewidths themselves may give useful chemical information, but obviously very broad bands will not show 

Calculated band shapes for spectra of a spin-1/2 nucleus ( H) coupled to a spin-1 nucleus ( N). The shape depends on the ratio of relaxation rate to the NH coupling. For very fast relaxation just a single line is observed, while for slow relaxation there are three lines of equal intensity. 

NMR spectrum I —912) of (a) PhsGeH and (b) PhGeHs with doublet and quartet splittings ( /oeH = 98 Hz in both cases). 

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Besides the opportunity to measure chemical shift anisotropies, MAS NMR also offers the possibility to measure the scalar couplings to other isotopes with magnetic moments. Examples to determine the spatial components of these couplings can be found in the literature48,49. Even couplings to quadrupolar nuclei not observable in solution can be deduced from the MAS spectra48. 2D techniques such as HETCOR are also applicable for solids49. 

The appearance of the spin-V2 nucleus spectrum is therefore also influenced by the factors described above which dictate the rate of quadrupolar rela.xation. Couplings to quadrupolar nuclei that exist in a highly symmetrical environment are likely to be seen because of the slower relaxation the nuclei experience. For this reason the proton spectrum of is an unusually sharp l l l triplet... 

In by far the majority of cases, couplings to quadrupolar nuclei are not observed, and it is usual to predict spectra by first assuming that all nuclei have spins of 1/2 or zero, and only after this to consider the possible effects of coupling to quadrupolar nuclei. Figure 4.31 shows calculated line shapes for a spin-1/2 nucleus coupled to a spin-1 nucleus such as N, for various values of the " N relaxation time. 

Two additional interactions experienced by the nucleus in the solid state are spin-spin couplings to other nuclei, and quadrupolar interactions, which involve nuclei of spin greater than 1/2. Spin-spin (Hx), or J, coupling,... 

Residual coupling effects due to quadrupolar nuclei in mas spectra. It was seen that for both dipolar and spin-spin coupling between nuclei the first-order anisotropic effects are proportional to P2(cos0) so that under MAS if a nucleus experiences both these effects, only the isotropic effects of J remain. However, the... 

Small scalar /-couplings between quadrupolar nuclei and spin-1/2 have been measured in inorganic solids using /-Resolved experiments. These were further used to acquire 2D /-HMQC heteronuclear correlation spectra, giving detailed insight into the chemical bonding scheme in the solid state. ... 

With a few exceptions, the listed spin couplings have been indirectly determined. As discussed in Section 1.5 there are essentially two methods available for determining scalar couplings to quadrupolar halogen nuclei. The first method is based upon the fact that the scalar contribution to the relaxation of a spin 1=1/2 nucleus (I) coupled to a quadrupolar halogen nucleus (S) is different for the two... 

The theory of residual dipolar coupling between quadrupolar nuclei has been developed and applied to B-chlorocatecholborane using and 35/37Cl [26]. MAS and DOR NMR were applied to extract infor-... 

An additional consequence of quadrupolar relaxation is that spin-spin coupling to other nuclei, which is readily observable if is present, is usually eliminated. Hence a valuable source of structural information is lost. 

Both of the magnetically active isotopes of boron are low-Q nuclei, and while the rate of quadrupolar relaxation is slow, it is still sufficient to adumbrate or totally vitiate spin-spin couplings to neighbouring nuclei. The quadrupolar relaxation rates observed are in the region of 50 s , sufficiently slow that other mechanisms can effectively compete for some of the action. Total relaxation rates are still sufficiently slow that excessive linewidth is not a major problem in boron spectroscopy. 

Si, N), the 7(E,X) can be obtained also from E NMR spectra by using Hahn-echo extended pulse sequences. In tin derivatives, in the case of couplings to quadrupolar X nuclei (X = N), it frequently... 

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