Sideband pattern

For kaolinite the sample permeability was very low and the solution was poorly removed. The spectra (Figure 3C) are consequently complex, containing peaks for inner and outer sphere complexes, CsCl precipitate from resMual solution (near 200 ppm) and a complex spinning sideband pattern. Spectral resolution is poorer, but at 70% RH for instance, inner sphere complexes resonate near 16 ppm and outer sphere complexes near 31 ppm. Dynamical averaging of the inner and outer sphere complexes occurs at 70% RH, and at 100% RH even the CsCl precipitate is dissolved in the water film and averaged. 

Figure 3 shows 13c MAS spectra of acetone-2-13c on various materials. Two isotropic peaks at 231 and 227 ppm were observed for acetone on ZnCl2 powder, and appreciable chemical shift anisotropy was reflected in the sideband patterns at 193 K. The 231 ppm peak was in complete agreement with the shift observed for acetone diffused into ZnY zeolite. A much greater shift, 245 ppm, was observed on AICI3 powder. For comparison, acetone has chemical shifts of 205 ppm in CDCI3 solution, 244 ppm in concentrated H2SO4 and 249 ppm in superacid solutions. The resonance structures 5 for acetone on metal halide salts underscore the similarity of the acetone complex to carbenium ions. The relative contributions of the two canonical forms rationalizes the dependence of the observed isotropic 13c shift on the Lewis acidity of the metal halide. 

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). 

An investigation of lithium diisopropyl amide (LDA) by solid state NMR led to the observation of dramatic differences between the spectra of the solid polymer and the complex crystallized from THF. Li as well as "C and "N MAS spectra showed large sideband patterns in the former case and only a few sidebands in the latter. For both materials X-ray data are available and establish a helix structure for the polymeric material, which is insoluble in hydrocarbon or ethereal solvents, and a dimer structure of the THF complex (25, 26, Scheme 4). The obvious difference between both structures, apart from the solvent coordination in the THF complex, is the magnitude of the structural N-Li-N angle, which is close to 180° in the first case and close to 90° in the second (176° and 107°, respectively). Thus, a large difference for the electric field gradient around the Li cation is expected for the different bonding situations. 

The potential of x( Li) for structural studies is further demonstrated by results obtained for dianions of substituted ethenes. Reduction of (Z)-l,2-bis(trhnethylsilyl)-l -phenylethene with lithium in THF led to the formation of a dimer which in solution shows two distinct Li resonances at 0.37 and —1.20 ppm relative to 0.1 M LiBr. The strucmre derived from NMR studies in THF solution (39, Scheme 7) contains one lithium cation in close contact to the organic moiety and the other one, at high field, at a larger distance and presumably surrounded by solvent molecules. In the solid, two Li resonances separated by 1.85 ppm are found, which yield x( Li) values of 180 and 30 kHz for the low- and high-field MAS sideband pattern, respectively, thus supporting the CIP structural motif for the lithium cation that gives rise to the low-field resonance and fhe SSIP sfrucmral motif for that which resonates at higher field. 

A similar resull is obfained for fhe dianion of ( )-l,2-diphenyl-l,2-bis(lrimelhylsilyl)-ethene which in solution shows Li resonances af —0.8 and —3.8 ppm respectively. The Li MAS NMR spectrum of the solid displays the superposition of two sideband patterns separated by 2.5 ppm which yield quadrupole parameters x( Li) of 135 and 32 kHz with / ( Li) values of 0.27 and 0.80, respectively (Figure 19) . Here, the lithium... 

In Table 2, the A chemical shifts of the carbon atoms of alkoxy species attached to zeolite framework oxygen atoms are summarized. In general, the spins of surface alkoxy species are characterized by relatively long T times (2-5 s), an efficient CP, and broad spinning sideband patterns. The adsorption and... 

The H/D exchange between the methyl groups of adsorbed acetone molecules and the Bronsted acid sites of zeolite HZSM-5 was also observed upon adsorption of C-2-acetone on a deuterated catalyst (D,HZSM-5, nsi/ Ai = 21.5) at room temperature (Figs 20c and d). The " C MAS NMR spectrum of C-2-acetone adsorbed on zeolite D,HZSM-5 (Fig. 20e) consists of the carbonyl signal at 223 ppm with a featured sideband pattern and a methyl signal at 29 ppm. No significant... 

Fig. 42. Comparison of the experimental and calculated Hartmann-Hahn mismatch sideband patterns for polybutadienes with the average molecular weight between crosslinks M = 6810 (A), Mc = 2730 (B) and Mc = 1020 (adapted from Ref. 250>)...

The sideband patterns of the luminescence line narrowing spectra of [Rh(ppy)2bpy] + and [Rh(ppy)2en]+ also coincide, see Fig. 5b, but are distinctively different from the ones of the thpy- containing samples, thus verifying that the lowest energy excitations involve the ppy ligands [40,41], In... 

A similar experiment has been performed [68] but under MAS conditions. For a series of crosslinked natural rubber samples (A - FI), 13C edited 1H spinning sidebands have been extracted from the 2D spectrum. These sideband pattern are encoded by the residual dipolar couplings of the corresponding functional groups and are presented in Figure 14.12. 

Figure 14.12 Slices from a 2D experiment corresponding to the pulse sequence of Figure 14.11 performed on a series of crosslinked natural rubber samples A- FI (defined in [68]) under MAS. The slices reflecting proton sideband pattern for the different functional groups are encoded by the residual dipolar couplings. The distinct features of dipolar slices prove that the different functional groups may be considered as relatively isolated groups of spins on the time scale of the evolution and...

The other general way of resolving powder patterns from different chemical sites is to generate multidimensional NMR spectra in which the desired powder patterns (or magic-angle spinning sideband patterns) are resolved in one dimension, separated according to (for instance) isotropic chemical shift in another dimension. These techniques are discussed below in the relevant section for each type of nuclear spin interaction. 

Obtaining information from powder pattern lineshapes (or sideband patterns) always involves matching the experimental lineshapes to those obtained from simulation, and the simulations necessarily involve some model for the molecular motion. This model dependency is an inevitable limitation on this general method for studying molecular motion. The vast majority of studies assume some kind of Markov model for the motion,4 that is, it is assumed that the nucleus/molecule jumps between A discrete sites and that the time taken to... 

Another way of dealing with the resolution problem for powder lineshapes is to use multidimensional NMR techniques to separate powder pattern lineshapes (or magic-angle spinning sideband patterns) according to isotropic chemical shift, as mentioned previously. 

Fig. 14. The pulse sequence for recording the double-quantum 2H experiment.37 The entire experiment is conducted under magic-angle spinning. This two-dimensional experiment separates 2H spinning sideband patterns (or alternatively, static-like 2H quadrupole powder patterns) according to the 2H double-quantum chemical shift, so improving the resolution over a single-quantum experiment. In addition, the doublequantum transition frequency has no contribution from quadrupole coupling (to first order) so, the double-quantum spectrum is not complicated by spinning sidebands. Details of molecular motion are then extracted from the separated 2H spinning sideband patterns by simulation.37 All pulses in the sequence are 90° pulses with the phases shown (the first two pulses are phase cycled to select double-quantum coherence in q). The r delay is of the order 10 gs. The q period is usually rotor-synchronized.

An extensive analysis of the MQMAS sideband pattern (for both spin- and half-integer quadrupolar systems) was given by Griedrich et a/.141... 

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