Quadrupolar nuclear splitting

When the electric field gradient at the nucleus exerted by the electrons is nonzero, the nuclear levels will be split. The eigenvalues of the quadrupolar interaction Hamiltonian are given by... 

In case of a deuterated sample (spin 1 case), the spectra are usually dominated by the quadrupolar interaction, that is, the coupling of the nuclear quadrupole moment with the electric field gradient of the C-2H bond. For deuterons in C-2H bonds this can lead to a splitting of about 250 kHz. As in the case of dipolar interaction, a Pake spectrum is obtained for a powder sample. The z-principal axis of the quadrupolar interaction is oriented along the bond axis which makes deuteron NMR particularly useful for studies of segmental orientations and molecular dynamics (reorientation) [1],... 

In this section we have attempted to introduce the reader to the salient features of NMR spectroscopy nuclear spin / = 1, doublets whose splittings depend on the quadrupolar interaction at the position of the deuteron, and single crystal vs. powder spectra. We have not as yet addressed the pertinent question What about molecular motion How do the rates and classes of molecular motion affect NMR spectra The second subsection of Section C will discuss some features of NMR that make it a useful technique for probing molecular motions in membranes. 

When a molecule is partly oriented by dissolution in a liquid crystal solvent it is necessary to consider direct dipolar splittings, nuclear screening anisotropies, and possibly also very large quadrupolar splittings, in order to account for the spectrum. The ways in which this can affect multiple resonance experiments have been considered already. (33, 35-37, 46) In this section we discuss some applications. Naturally, proton decoupling is often used to simplify spectra of oriented molecules, although it is desirable to avoid complete decoupling of the solvent protons. 

To probe the potential influence of cation size differences on the distribution, Na[ Cs SEDOR experiments have also been conducted on mixed sodium cesium borate glasses containing 30 mol% alkali [57]. Being a nucleus (1=7/2) with a moderately small nuclear electric quadrupolar moment, the Cs nucleus features similar spectroscopic characteristics as Li. Usually sizeable first-order quadrupolar splittings reduce the inversion efficiency of the n-... 

Fig. 4. 62.8 MHz 207Pb H NMR spectrum of a solution of Li[Me3Pb-BH3] in tetra-hydrofuran, measured at various temperatures (extensive decomposition occurs above -30 °C).21 The signal shows splitting due to indirect nuclear 207Pb-uB spin-spin coupling as indicated (nB /= 3/2). At lower temperature, the lifetime of the "B spin states becomes increasingly shorter owing to efficient quadrupolar nB nuclear spin relaxation, and consequently the splitting of the 207Pb NMR signal starts to collapse.

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