Powder lineshape analysis

Another study used the VACSY experiment discussed in Section 2.2 to examine the molecular motion in the crystalline region on poly(e-caprolactone) via lineshape analysis of l3C chemical shift anisotropy powder patterns.79... 

Chemical shift relative to liquid CS2, some are peak positions not the isotropic shifts, see original references. Numbers in parentheses represent singularity positions of MAS powder patterns, t Estimated upper limit of the quadrupole effect parameter, except which is based on lineshape analysis. 

One-dimensional quadrupole echo NMR lineshape analysis of powder samples is particularly informative when fast, discrete jumps occur between sites of well-defined geometry as, for example, in a phenyl group undergoing two-site exchange. In this case, the characteristic Pake-pattern is transformed into an axially asymmetric lineshape with an apparent asymmetry parameter r] 9 0 (see Equation (6.2.3)) [1-8]. The asymmetric lineshapes, shown on the left in Fig. 6.2.2, can be derived by considering how the individual components of the principal EFG tensor become averaged by the discrete jumps. Within the molecular frame, and in units of as defined by Equation (6.2.2), the static axially symmetric tensor consists of the components = 1, = — 1/2, and V y = — 112. This traceless tensor satisfies the... 

In contrast to the SNMR method, it is crucial to measure an undistorted polycrystaUine static spectrum in order to have a reUable spectral analysis when the effects of finite RF pulses and detection bandwidth are ignored in the numerical simulation. Since Co static powder spectra in general have well-defined singularities and therefore are suitable for lineshape simulation, the issue of spectral distortion seems to be particularly important for Co NMR practitioners. In this section we will give a brief account of lineshape analysis, including experimental considerations and precautions for numerical simulations. 

If powder spectra of the type shown in Fig. 9.3 and 9.4 can be obtained experimentally, the principal values of the anisotropic interaction in question (though not the orientation of the PAS with respect to a fixed frame) can be obtained by a straightforward lineshape analysis. However, to obtain such spectra, it is necessary that there is only one distinct nucleus, and that one anisotropic interaction dominates. Usually, the static NMR lineshape is a broad featureless hump due to the overlapping of many powder patterns as well as the interplay of the different broadening mechanisms. As an example of this. Fig. 9.5 presents a NMR spectrum of a representative organic sohd, together with, for comparison, the corresponding solution-state spectrum. It is to be noted that the problem in such a case is not a lack of information, but rather there is essentially an overload, such that the net effect is the virtual loss of all information. In the remainder of this article, soHd-state NMR approaches by which this information can be recovered wiU be demonstrated. 

Aliev. A.E. Smart, S.P. Shannon. I.J. Harris. K.D.M. Structural and dynamic properties of the 1.10-dibromodec-ane/uiea inclusion compound, investigated by variable-temperature powder x-ray diffraction, solid-state "H NMR lineshape analysis and H NMR spin-lattice relaxation time measurements. J. Chem. Soc.. Faraday Trails 1996. 92. 2179. 

The difference between the Markov model lineshapes and those from the Smoluchowski model is particularly pronounced when the diffusion coefficient is of the order of the quadrupole coupling constant. In the limit of large diffusion coefficients, the two models converge, and in the limit of low diffusion coefficients, the spectra are dominated by small-amplitude oscillations within potential wells, which can be approximately modelled by a suitable Markov model. This work strongly suggests that there could well be cases where analysis of powder pattern lineshapes with a Markov model leads to a fit between experimental and simulated spectra but where the fit model does not necessarily describe the true dynamics in the system. 

For this reason, the study in question19 examined a sample of high-density polyethylene that was iso topically labelled with, 3C so as to produce isolated 13C spin pairs. Static 13C powder lineshapes were then observed as a function of temperature. Analysis of these by lineshape simulation shows that, indeed, the polyethylene chains do undergo 180° chain flips. The static lineshapes in this case result from the sum of chemical shift anisotropy and dipolar coupling. However, the chemical shift anisotropy is known and, as mentioned previously,... 

The principal values of the 15N chemical shift tensors for the amine and imine nitrogen atoms were derived from analysis of 15N lineshapes recorded for static powder samples under conditions of cross polarisation and H decou-... 

The peculiar temperature and composition dependence of the valence of chemically collapsed phases of SmS was first observed in the system Sm1 xGdxS (55). Recently a phase diagram in the (x, T)-plane has been proposed for this system (41) as well as for CeTh alloys (44). When cooled below about 200 K the chemically collapsed phases of SmS show a dramatic lattice expansion, in some cases with explosive character (55) and disintegration of the crystal into a black powder. An example of this transition towards a more divalent state of Sm on cooling in SmAs 18 S 82 and Sm 81 Y19S is shown in Fig. 22 and 23, where we show the data of Poliak et al (63). The temperature dependent configurational mixing of Sm ions which is directly visualized in the XPS data is the source of the anomalous temperature dependence of the lattice constant. No detailed analysis of the dependence of the Sm 4/lineshape on temperature... 

The use of NMR spectroscopy for probing molecules adsorbed on alumina dispersions is briefly cited. Soderlind and Blum [66] used deuterium NMR to probe the dynamics and structure of a surfactant (an alkylbenzenesulfonate, SHBS) adsorbed onto alumina. The phenyl ring was deuterium-labeled, and from the analysis of the lineshapes of H spectra, valuable information on the degree of motional freedom in the adsorbed layer was obtained. As deuterium possesses a quadrupole moment, it is sensitive to the orientation of the molecule with respect to the external magnetic field. This sensitivity generates a quadrupolar splitting, which is defined as the separation between the two allowed transitions for the deuterium nucleus. In liquid systems, the quadrupole interaction is averaged out, but for static systems powder... 

Fitting the powder hneshape, even with multi-field spectra, may not be efficient for extracting the tensor parameters with a good confidence level, because of the lack of site-specific information in the static powder lineshape. More precise values can be obtained from the analysis of the spinning sideband-manifolds in MQMAS or in DOR spectra. Flowever, due to the intrinsic insensitivity of MQMAS experiments, complete spinning sideband-manifolds are seldom visible in MQMAS spectra. Conversely, the DOR spinning-sideband manifold can be extremely usefiol for deducing the tensor parameters [79]. 

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