2H quadrupolar

Fig. 6 (a) SQ and (b) DQ rotor-synchronized 2H MAS NMR spectra of sodium tetrathionate dihydrate-d4 (solid lines). The dashed line in (a) represents the exact numerical simulation of the SQ spectrum for random molecular motion with the rate constant k given in the figure, (c) The corresponding experimental and simulated static 2H quadrupolar-echo spectra, (d) Simulated SQ (solid line) and DQ (dashed line) linewidths as functions of k. (Reproduced with permission from [88])... 

Quadrupolar interactions can offer direct information on the dynamics of organics within zeolite crystals. Eckman and Vega (304) studied the 2H quadrupolar echo decay in perdeuterated p-xylene adsorbed on zeolite ZSM-5. The deuterium quadrupolar interaction usually dominates the spin Hamiltonian, so that the powder pattern can be used as a test for models of molecular motion. At -75°C and 25°C typical rigid-lattice spectra were obtained. At 100°C however, the resonance arising from the aromatic deuterons was motionally narrowed, while the methyl resonance was not, The authors conclude that p-xylene molecules reorient about an axis which passes through the C3 axes of the methyl groups. 

Correlations between 2H quadrupole interaction parameters and hydrogen bond geometry have also been considered for situations other than 0-H---0 hydrogen bonds. For example, solid state 2H NMR spectra of 2H labelled amino acids, peptides and polypeptides were measured over a wide temperature range [74]. From spectral simulations based on dynamic 2H NMR theory, parameters such as the 2H quadrupolar coupling constant and asymmetry parameter were determined, and relationships between these NMR parameters and the hydrogen... 

In the solid, dynamics occurring within the kHz frequency scale can be examined by line-shape analysis of 2H or 13C (or 15N) NMR spectra by respective quadrupolar and CSA interactions, isotropic peaks16,59-62 or dipolar couplings based on dipolar chemical shift correlation experiments.63-65 In the former, tyrosine or phenylalanine dynamics of Leu-enkephalin are examined at frequencies of 103-104 Hz by 2H NMR of deuterated samples and at 1.3 x 102 Hz by 13C CPMAS, respectively.60-62 In the latter, dipolar interactions between the 1H-1H and 1H-13C (or 3H-15N) pairs are determined by a 2D-MAS SLF technique such as wide-line separation (WISE)63 and dipolar chemical shift separation (DIP-SHIFT)64,65 or Lee-Goldburg CP (LGCP) NMR,66 respectively. In the WISE experiment, the XH wide-line spectrum of the blend polymers consists of a rather featureless superposition of components with different dipolar widths which can be separated in the second frequency dimension and related to structural units according to their 13C chemical shifts.63... 

Solid-state NMR spectra of 2H, where the main concern is to refocus the reasonably small first-order broadening, are commonly obtained by measuring the quadrupolar echo on a static sample. In such a case, the o pq frequencies can be approximated... 

Static 2H NMR spectra suffer from low sensitivity, as resonances are spread continuously over the 200 kHz range. In samples with multiple sites, they also lack resolution. However, both shortcomings can be remedied by using MAS. As was shown in Sect. 2.3.3, in a rigid sample MAS averages out the inhomogeneous first-order quadrupolar broadening in the same way as it eliminates the CSA in... 

It has recently been demonstrated that the analysis of MAS sidebands patterns can be used to study molecular dynamics in the solid state [85-88]. Indeed, the line narrowing effect of MAS can be partly offset, or completely eliminated, if the 2H quadrupole tensor is reoriented due to motion on a time scale comparable to (first-order quadrupolar broadening, such motion-induced effects should be less evident in the DQMAS spectrum, as has indeed been observed by Wimperis and colleagues in several deuterated solids [87, 88]. For example, the simulation of the SQ spectrum of tetrathionate dihydrate-cfi yielded the same reorientational rate constant as the previously described quadrupolar echo approach (Fig. 6). 

Spin-spin (transversal) relaxation (relaxation time T2) is the second mechanism which is related to molecular mobility. It influences the half-height line widths w [tv = lj(n T2) in the frequency-domain NMR spectrum and is of some relevance in the NMR spectroscopy of quadrupolar nuclei such as, 2H, 14N, 170, 33S and others. In the context of this section, T2 is of very limited relevance. 

The conformations of flexible chain molecules incorporated in a nematic environment are investigated. Proton-proton and carbon-carbon dipolar coupling constant measurements are attempted for 1,2-dimethoxyethane and 1,2-diphenoxyethane, in addition to 2H NMR observations of quadrupolar splittings. These conformation-dependent properties are analyzed according to the RIS scheme. Studies are further extended to a mixture of 1,2-diphenoxyethane with a nematic liquid crystal, 4,4 -azoxyanisole. 

Proton and fluorine are the most frequently studied nuclei in the solid state NMR of polymers. However, useful information can be obtained from deuterium resonance, (2H I = 1). In the presence of nuclei with spin I > 1/2, which possess a quadrupolar moment, the most dominant interaction occurs between the quadrupolar moment and an electric field gradient tensor V, generated by the C—D bonding electrons. Consequently, the only dominant interaction, besides Hz (Eq. (1)), will be described by the quadrupolar Hamiltonian, which for a single spin has the form ... 

The potential of other nuclei for the study of surfaces is yet to be explored. Gottlieb and Luz (388) measured 2H spectra of a number of perdeuterated molecules adsorbed on active alumina and interpreted the results in terms of quadrupolar tensors. Yesinowski and Mobley (369) have shown that 19F MAS NMR can provide useful information about fluorinated surfaces of calcium hydroxyapatite, Cas(0H)(P04)3. In particular, l9F, 2H, and H MAS NMR may become powerful techniques for the study of interface systems in general. 

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],... 

The predominant interaction for a 2H spin system is the quadrupolar interaction, which couples the electric quadrupole moment of the 2H nucleus to its electronic surrounding. This interaction is a second-rank tensor Hq which lies approximately along the C-2H bond in organic molecules. Thus, in practice, 2H nuclei may be considered to be isolated. It shows that the 2H NMR formalism is similar to that of an isolated proton pair [8] ... 

The quadrupolar interaction constant Vq is large, of the order 250 kHz, which makes 2H NMR very sensitive to the degree of molecular mobility. 

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