Wideline NMR

For t 0, a lineshape like the one depicted in Fig. 3.1.3(b) results. From the high-and low-frequency cut-offs and from the singularity, the principal values yy, and i zz of the coupling tensor can be obtained. At the same time, the largest principal value 5 and the asymmetry parameter ij can be determined by using (3.1.18) and (3.1.19). 

In general, the Hamilton operator H applicable to an experimentally observed resonance is the sum (3.1.1) of operators Hx of different interactions. The lineshape, therefore, is the result of all spin interactions. For observation of just one dominant interaction, special techniques need to be applied such as isotope enrichment, homonuclear multipulse and heteronuclear high-power decoupling. Nevertheless, in C NMR, for instance, despite high-power H decoupling an overlap of chemical shielding powder spectra centred at different chemical shifts is observed in most cases without site-specific isotope enrichment. 

Until MAS, multi-pulse, and Fourier techniques became available on a commercial basis, wideline NMR was the most frequently used method of solid-state NMR spectroscopy. 

---

Wideline NMR spectra were collected using a Bruker CXP 200 NMR spectrometer, operating at u>o/2n ( H) = 30.7 MHz. To obtain spectra void of spectrometer artifacts, the solid spin echo pulse sequence, n/2) -T-n/2) -x-echo, was used. Unless otherwise noted, the delay between pulses, X, was set at 30 Js. 

Vanadium and Pseudo-Boehmite Alumina. The 79 MHz wideline NMR spectra of these steam aged samples containing only 0.5 to 5.0% V (loaded using VO -Naphthenate) are shown in Figure 1. Two... 

The broad "type e" signal, whose contribution increases with increasing V content, shows NMR behavior that is consistent with the presence of bulk V Og (1) the wideline NMR pattern resembles that... 

Figure 5. Effects of pulse length on wideline NMR spectra of aluminosilicate gels containing (a) 5% V, 7/ s pulse length, resulting in exclusive detection of bulk Vp05 (b) 3% V, 7/ s pulse length, resulting in exclusive detection of amorphous V Oj-and (c) 3% V, 1/xs pulse length, resulting in a spectrum representative of all V+D species present.

Many methods for detennination of molecular order can only measure P2) [Mcbl, Warl]. Among them are measurements of the infrared dichroism and of the refractive index. P2) and P4) can be determined by Raman fluorescence depolarization and from wideline NMR spectra. However, X-ray diffraction and lineshape analysis of resonances from isolated NMR interactions can also provide higher moments. 

The wideline NMR spectrum 5(i ) can be written as a convolution of the resonance frequency with the orientational distribution P (cos/3) [Wefl],... 

The validity of (3.2.9) is restricted to the symmetries mentioned above, that is to cylindrical molecules, macroscopically uniaxial samples, and r] = 0. For many samples, these conditions are fulfilled when using NMR, because the quadrupole coupling tensor of aliphatic deuterons is often found to be axially symmetric. In wideline NMR, the anisotropy of the magnetic shielding is used. Here the angular resolution is lower, and the calculation has to be extended to include p > 0 [Henl]. In combination with MAS (cf. Section 3.3), the Legendre subspectral analysis has been used successfully for the determination of molecular order in partially ordered polymers [Harl]. 

Equation (3.2.14) is the starting point for numerical simulation of dynamic wideline NMR spectra. With the Jeener-Broekaert echo the imaginary part of the exponential depending on h is measured, and with the stimulated echo the real part is measured. The lineshapes of solid-echo spectra follow from (3.2.14) with t , = 0. 

The wideline NMR, IR, and Raman spectra of maleic hydrazide were determined and discussed in connection with its tautomeric forms. NMR techniques have been used to predict the structure of cyclization products from 3-carboxyacryloylhydrazines. The NMR spectra of JV-methyl-pyridazinium iodides and pyridazine iV-oxide were investigated, and the... 

Attempts have been made to correlate the outstanding electric properties (piezoelectricity, pyroelectricity) of PVDF with parameters obtained from wideline NMR experiments. Samples processed by different electric methods have been included in NMR experiments. Wideline NMR studies of irreversible effects induced by relatively high static electric fields have been reported by several authors. The results of wideline NMR ( H) have been used [53] in a combined investigation with a piezoelectric resonance method to find any effects of electric poling (0.78 MV/cm at 120°C) on structure and/or orientation. However, no indications of structural or orientational changes have been found. 

Several wideline NMR experiments were carried out on copolymers of vinylidene fluoride and trifluoroethylene (TrFE) [84,99-112]. Such copolymers are of special interest since TrFE in proportions greater than 10% induces the VDF component to preferentially crystallise in the )8 form. In contrast with neat PVDF, the copolymers exhibit a ferroelectric phase transition at a temperature well below melting. 

Legrand et al. [100] reported F wideline NMR measurements on a 70/30 mol% VDF-TrFE random copolymer, made in order to study molecular motion both below and above the ferroelectric transition temperature, Tc. The samples consisted of semicrystalline copolymer films of 0.51 mm thickness, with biaxial orientation of the crystalline axis. The samples were rolled (without poling) at 70°C, with a draw ratio of 300%. The F resonance was chosen, rather than the proton resonance, because the abundance ratio of F to nuclei is 1.4. In addition, the F free-induction decay (FID) lasts longer than that of the proton, which decreases the influence of spectrometer deadtime. FID analyses were made assuming a simple superposition of two... 

Consider the frequency domain again. The requirement for a sharp cutoff arises because we want to cut off as much noise as possible and the sharper the filter cutoff the closer it can be set to the spectral feature to be saved. It is clear that the filter cutoff needs to be only as sharp as the spectral feature. Therefore, a careful consideration of filter characteristics is most important for high resolution NMR, whereas the RC filter is tolerably good for very broad lines such as those in solids observed by normal NMR. But since the opportunity for some oversampling is much greater for high resolution NMR than in wideline NMR, the absolute necessity for an "ultrasharp" filter is not as great as it is supposed in either case. 

At low surface coverages molecular motion is more restricted, chemical exchange is slow, and the resonances are generally too broad to be observed by wideline NMR. Use of line-narrowing techniques such as MAS then becomes essential. N as the reporter isotope is generally preferable over C, because the spectra are simpler and the resonance shifts are larger for directly bonded atoms... 

In a series of V wideline NMR studies, Mastikhin and coworkers have explored the chemical nature of the catalytically active species 37 2]. While the spectra of industrial catalysts from various sources are found to be substantially different, these differences more or less disappear after exposure to the reaction mixture. This result confirms the previously held view that the catalytically active species forms under operating conditions. Figure 4 shows typical spectra recorded at a field strength of 7.0 T, at which the lineshape is dominated by the chemical shift anisotropy. The principal contribution to the spectrum in Fig. 4 arises from an axially symmetric powder pattern with approximate 81 and 8 values of — 300 and — 1300 ppm, respectively. Based on comparative studies of model preparations, Mastikhin et al. suggest that the key compound formed has the composition K3VO2SO4S2O7. The anisotropic chemical shift parameters of... 

Figure 4 79 MHz solid-state V wideline NMR spectra of mcxiel preparations and commercial SO2 oxidation catalysts (a) K, V02(S04), (b) K3VO2SO4S2O7, and (c) the commercial catalyst after exposure to the reaction mixture. (From Ref. 40.)...

Figure 5 121.5 MHz solid-state P wideline NMR spectra of V-O-P catalysts,...

Of the three amines to be discussed here, pyridine is the least basic, with pAf/, = 8.77. Gay and Liang 81 have recently used pyridine and substituted aromatic bases in conjunction with wideline NMR to probe reactivity differences in variously treated aluminas and mixed alumino-silicates. Difficulties were encountered with bases that bind tightly to the surface since magnetic dipolar effects then broaden the lines, causing overlap and loss of information. Pyridine gives a broad, ill-defined C spectrum, even at elevated temperatures and relatively high surface coverages. By contrast, Ellis and coworkers 82 have found that the ambient temperature C CP-MAS spectrum of pyridine at 0.05... 

Broadline NMR made its mark in the food industry [3-12] with substantial work done in Europe [13-16]. Originally, Chapman and others [17] applied wideline NMR to the determination of solid fat in oil. By heating a fat in oil sample, the fat is melted and the total intensity of the FID is measured. Then a second measurement is taken at a lower temperature where the fat has solidified. The ratio of the two measurements indicates the solid fat content. This method is quick and more precise than other methods such as differential scanning calorimetry (DSC). 

Figure 4.6 The trajectories of a benzene molecule in the AlMePO-a (upper) and -P (lower) pol Tnorphs obtained from a molecular dynamics (MD) simulation indicate that whereas benzene readily re-orients isotropically (freely) in the more cylindrical channels of the P polymorph, it undergoes more restricted motion in the more markedly triangular channels of the a polymorph. This is confirmed by wideline NMR of CgDg (see Section 7.2.3 for details).

The static (or slow MAS) spectra can be simulated assuming modes and frequencies for independent mechanisms of re-orientation, and simulations proceed iteratively to match the spectra. Several computer codes are available to perform the simulation, while some groups calculate spectra directly from analytical functions. wideline NMR studies have been applied to many adsorbate-microporous solid systems, including both physisorbed and chemisorbed species. The lineshape-matching process can sometimes be ambiguous, so that additional constraints on the possible mechanisms of motion, such as those provided by molecular dynamics or (time-averaged) by crystallography, are very helpful. 

Figure 7.7 Variable temperature wideline NMR of dg-benzene in AlMePO-P (left) and AlMePO-a (right). Making use of Molecular Dynamics simulations described in Section 4.5.3 (Figure 4.6), these spectra can be simulated by assuming isotropic motion of CgDg in AlMePO-P (left) and jt/3 flips around a axis in the more constrained channels of AlMePO-a (right). In each case the spectra are finally matched by varying the frequency of the re-orientations. [Reproduced from reference 73 with permission. Copyright 2005 American Chemical Society.]...

The NMR method makes use of measurements of the dependence of to Si cross polarisation efficiency as a function of CP contact time to determine the distances between different silicon atoms in the ordered complex to H atoms in /luru-xylene molecules in the pores. The experiment has to be performed where the molecules show little translational motion, and under conditions where any molecular re-orientation is well understood (from wideline NMR, for example see Section 7.2.3). To give additional information, the experiments were performed separately on selectively deuterated and dg- i-xylene molecules. Plots of the Si signal intensity from different crystallographic T-sites against the contact time show an increase to a maximum value. 

---