Wideline powder spectrum

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.

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. 3.3.5 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.

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]. 

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.

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... 

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... 

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