Wideline spectra

Despite the benefits of high resolution, measurements of wideline spectra of quadrupolar nuclei under static or MAS conditions are still commonly used in a variety of applications. For both integer and half-integer spins, simulations of quadrupolar lineshapes can yield full sets of NMR parameters associated with quadrupolar and chemical shift tensors and can be used for studying molecular dynamics. 

Fig. 10. The pulse sequence for the WISE experiment.21 This two-dimensional experiment separates H wideline spectra according to the isotropic l3C chemical shift of the 13C each H is bonded to. An initial H 90° pulse creates transverse H magnetization that is allowed to evolve in t. A short cross-polarization step then transfers the remaining H magnetization to the nearest 13C spin, i.e. the bonded one. The resulting 13C transverse magnetization is then allowed to evolve in ti under magic-angle spinning, where an FID is recorded.

If the orientation dependence of the resonance frequency of a spin 5 is determined by just one interaction, it can be exploited for use as a protractor to measure angles of molecular orientation. In powders and materials with partial molecular orientation, the orientation angles and, therefore, the resonance frequencies are distributed over a range of values. This leads to the so-called wideline spectra. From the lineshape, the orientational distribution function of the molecules can be obtained. These lineshapes need to be discriminated from temperature-dependent changes of the lineshape which result from slow molecular reorientation on the timescale of the inverse width of the wideline spectrum. The lineshapes of wideline spectra, therefore, provide information about molecular order as well as about the type and the timescale of slow molecular motion in solids [Sch9, Spil]. 

For reorientations with correlation times in the range of the inverse spectral width of the powder spectrum, temperature-dependent changes of the lineshape are observed which are characteristic of the motional process [Jell, Miill, Spil, Spi2]. As an example. Fig. 3.2.5 shows NMR spectra for different motional mechanisms and different correlation times [Miill]. However, such wideline spectra cannot be readily measured with single-pulse excitation, because the beginning of the FID will decay within the... 

Fig. 3.2.5 [Miill] Deuteron wideline spectra for different motional mechanisms and times tc-The angle between the axis of rotation and C—bond (principal axis Z of the quadfupole coupling tensor) is denoted by d. (a) Twofold jump with 9 — 60° (b) Twofold jump with 9 = 180° (flips of p phenylene). (c) Three-fold jump with 9 = 109° (rotation of a methyl group), (d) Rotational diffusion on a cone 9 = 109°. (e) Tetrahedral jump, (f) Isotropic rotational diffusion.

To bypass receiver deadtime effects, wideline spectra are derived by Fourier transformation of the decay of an echo. By use of the Hahn echo and the stimulated echo (Section 2.2.1), wideline spectra of and other spin-5 nuclei can be measured, for example, but not the spectra of dipolar coupled spins and of quadrupolar nuclei like H. The magnetization of nuclei with spin / = 1 can be refocused by the quadrupole echo or the solid echo, and by the Jeener-Broekaert echo or the alignment echo [Slil] (Fig. 3.2.6). 

Fig. 7.2.2 [Hanl] Solid-echo wideline spectra of the ring deuterons of bisphenyl-a polycarbonate-d at 253 K. The phenyl rings undergo a 180° flip motion with a wide distribution of motional correlation times, (a) Spectrum with signals from fast and slow flipping rings.

Fqpve 5.4 2D WISE NMR specrating H wideline spectra for different structural units according to their chemical shifts (a) conventional H wideUne spectrum of a blend of poly(styrene) (PS) and poly(vinylmethylether) (PVME (b) 2D H C WISE NMR spectrum indicating different mobilities of the two components [23]... 

Fig. 3.2.1 [Giin2] Energy level diagram and powder spectra of the deutron. The wideline spectrum consists of two superimposed powder spectra. The double-quantum resonance at frequency a)2Q appears independent of molecular orientation.

The orientational distribution fimction P (cos 0) enters the shape of the wideline spectrum 5(f2) in a slightly hidden way. The angular dependence of the resonance frequency is given by (3.1.23) via the orientation of the magnetic field in the principal axes system XYZ of the coupling tensor (cf. Fig. 3.1.2), while the orientational distribution function specifies the distribution of the preferential direction n in a molecule-fixed coordinate frame (Fig. 3.2.2(a)). Figure 3.2.3 shows the relationship between the different coordinate frames and the definition of the relative orientation angles. 

MAS spectrum, consisting of narrow lines separated by the rotor firequency cur. The maxima of the rotational echoes in (b) follow the evolution of the isotropic mean. If the MAS FID is sampled at the echo maxima only, no sidebands will appear in the MAS spectrum. A similar situation is encountered for fast spinning, when the rotational echoes are no longer resolved. The envelope of the sideband spectrum is the Fourier transform of the rotational echo decay (c) [Mar2]. For slow spinning speeds, it approaches the shape of the wideline spectrum (a) of the nonspinning sample. 

An application of the saturation-recovery filter to the suppression of signal from rigid components in bisphenol-apoly(carbonate) is shown in Fig. 7.2.2 [Hanl]. The wideline solid-echo spectrum of the phenyl deuterons exhibits a range of broad and narrow components (a) as a result of a distribution of motional correlation times. The mobile components are characterized by a shorter T than the more rigid components. Consequently the rigid components can be suppressed by partial saturation. After application of the saturation-recovery filter the shape of the wideline spectrum is dominated by the narrow signal in the centre from the mobile ring deuterons (b). 

The V NMR spectra under static conditions of K-doped catalyst with a K/V atomic ratio of 0.7, are shown in Figure 3. The wideline spectrum shows a small peak at c.a. -270 ppm, which suggests the minoritary presence of octahedral V [13-15], Two more components are observed at -530 ppm and -900 ppm. Under MAS conditions the main line appears at -583 ppm. This chemical shift value and the wideline spectrum with two components at -540 ppm and -900 ppm, probably corresponding to 82 and 83 respectively, suggest the presence of tetrahedral V of Q ... 

Figure 3. wideline spectrum of potassium doped V2O5/AI2O3 with a K/V atomic... 

From a so-called wideline spectrum, one may naively think that it is difficult to obtain the relaxation curve of a particular polymer in a blend. This is not true, because in many blends a short T2 is caused by the mobility of the one of the component polymers or that of side-chains. Thus, there is a chance to discriminate between polymers by their different T2 (mobility). For example, Segre et al. [146] observed two T2 decays for PS/PB. The fast-decaying component was attributed to rigid PS. The slow-decaying component shows the presence of two Ti relaxations. These were attributed to the interphase and pure rubbery PB (Model B). Parizel et al. [147] observed that the FID of polyurethane (PU) in a cross-linked PMMA consists of three... 

A second group of techniques may be called lineshape analysis. Simple methods entail the measurements of linewidths or second moments as a function of temperature. More sophisticated methods involve the analysis or the model fitting of spectral lineshapes. A prominent method is ID lineshape analysis for deuterium-labeled polymers, which is sensitive to motions in the frequency range of lO -lO s (149). The 2D wideline separation NMR (WISE) experiment permits correlation of the high resolution spectrum with the wideline spectrum, which provides dipolar information (11,150). The linewidth is a function of the frequency of the polymer motion relative to the time scale of dipolar couplings. 

Homonuclear dipolar-coupled, abundant-spin, or broad-line spectra may appear to be featureless and uninformative, but they contain a great deal of information about molecular structure and motion. For example, the wideline spectrum of polyethylene, which might look like the simulation depicted in Fig. 30, exhibits two overlapping resonances. The broader arises... 

Solid-state NMR methods allow the investigation of local orientation, dynamics and conformational order of polymer chain segments. Si chemical shifts contain information about molecular moieties and about the conformation of chain segments. The line width of a wideline spectrum characterizes the strength of the dipolar couplings among protons and, therefore, the molecular mobility. H spin diffusion, mediated by the homonuclear dipolar couplings, is a powerful technique to obtain information about the spatial proximity of molecular moieties. These concepts have recently been combined in a two-dimensional Wideline Separation Experiment (WISE-NMR spectroscopy)... 

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.

Fig. 3.1.3 Solid-state wideline speetra. (a) Powder spectrum as isotropic average for an axially symmetric coupling tensor (r/ = 0). The resonance frequency is related to the orientation of the coupling tensor hy (3.1.23) and can serve as a protractor for molecular orientations relative to the magnetic field, (b) Powder spectrum for t) = 2/3.

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

Fig. 3.3.5 H decoupled C spectra of isotactic polypropylene for different spinning frequencies o>r =l7tv and orientation angles i/r of the rotation axis, (a) Static sample. The wideline resonances of the different carbons overlap, (b) MAS spectrum with fast sample spinning. Narrow signals are observed at the isotropic chemical shifts only, (c) MAS spectrum with slow sample spinning. In addition to the centre line, sideband signals are observed at seperations naiR from centre lines, (d) OMAS spectrum with fast sample spinning. The orientation of the axis deviates from the magic angle. Each resonance forms a powder spectrum with reduced width, which can serve as a protractor (cf Fig. 3.1.3). Adapted from [Blu4] with permission from Wiley-VCH.

The fact that multiple quantum coherences can only be measured indirectly by their influence on the amplitude and phase of a subsequently acquired single quantum signal makes this technique a phase-encoding method. The necessity to acquire the spatial information in time-consuming extra dimensions is a penalty in all phase-encoding techniques. However, in the case of double quantum imaging of quadrupolar nuclei like, e.g., H, the wideline information of the single quantum spectrum can be utilized for contrast, because the quadrupolar interaction usually dominates all other spectral features and is a sensitive probe for molecular dynamics and orientations [80-83]. 

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... 

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... 

The variable offset cumulative spectra (VOCS) method has been a major step forward for the acquisition of broad wideline and ultra-wideline SSNMR powder patterns, representing a time-efficient way to overcome the excitation limitations of rectangular rf pulses [51,53,54]. VOCS involves acquiring the broad overall SSNMR spectrum by collecting a series of individual subspectra at evenly spaced transmitter frequencies. The spacing between subspectra is typically set to be equal to or less than the pulse excitation bandwidth in order to ensure proper spectral excitation in each experiment. The resulting series of subspectra are then added together in the frequency domain to yield the overall broad powder pattern. In this marmer, even extremely broad powder patterns can be collected without any... 

Zirconium ( Zr) (7=5/2). Microcoils and WURST pulses are used to acquire ultra-wideline (UW) NMR spectra for Zr and a spectrum comprised of both central and satellite transitions of Co. The efficiency of UW NMR spectroscopy improves for both microcoil and WURST pulse... 

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