Chemical shift anisotropy lineshapes

In the review period there have been many studies of molecular motion using analysis of chemical shift anisotropy lineshapes. One that nicely illustrates what is currently possible concerns the motion of 13CO intercalated in C o.9 This is a particularly interesting example as both the CO and C6o molecules undergo reorientation, with the onset of motion occurring at different temperatures for the two species. Furthermore, the work uses a prior calculation of the potential... 

Selective inversion recovery experiments i.e. only select frequencies within the powder pattern are excited, have also been performed on 2H for the purposes of studying molecular motion. Initial experiments were performed on deuterated dimethylsulfone (DMS) to demonstrate the utility of the experiment.46 Selective inversion recovery curves were fitted to a suitable motional model, a two-site jump model in the case of DMS, to yield the motional rates as a function of temperature. A significant feature of this work is that the activation energy for the motion so obtained differs markedly from that obtained from earlier 13C chemical shift anisotropy lineshape studies. 

One potential problem with chemical shift anisotropy lineshape analysis (or indeed analysis of lineshapes arising from any nuclear spin interaction) is that the analysis results in a description of the angular reorientation of the chemical-shielding tensor during the motion, not the molecule. To convert this information into details of how the molecule moves, we need to know how the chemical-shielding tensor (or other interaction tensor) is oriented in the molecular frame. A further possible complication with the analysis is that it may not be possible to achieve an experiment temperature at which the motion is completely quenched, and thus it may not be possible to directly measure the principal values of the interaction tensor, i.e. anisotropy, asymmetry and isotropic component. If the motion is complex, lack of certainty about the input tensor parameters leads to an ambiguous lineshape analysis, with several (or even many) possible fits to the experimental data. 

Several methods have been developed to determine the chemical shift anisotropies in the presence of small and large quadrupolar broadenings, including lineshape analysis of CT or CT plus ST spectra measured under static, MAS, or high-resolution conditions [206-210]. These methods allow for determination of the quadrupolar parameters (Cq, i)q) and chemical shift parameters (dcs, //cs> <5CT), as well as the relative orientation of the quadrupolar and chemical shift tensors. In this context, the MQMAS experiment can be useful, as it scales the CSA by a factor of p in the isotropic dimension, allowing for determination of chemical shift parameters from the spinning sideband manifold [211],... 

Fig. 2 Experimental analysis of the chemical shift anisotropy of high-silica ZSM-5 zeolite, (a) 29Si MAS NMR and (b) extracted CSA lineshapes from a two-dimensional CSA recoupling sequence dashed lines are simulated lineshapes. Adapted with permission from [79]. Copyright 2008 American Chemical Society...

Fig. 2 Schematic representation of the 13C NMR signal of a single crystal containing the functional group AB, oriented (A) perpendicular to the applied field, and (B) parallel to the applied field. The lineshape in (C) represents the NMR signal of a polycrystalline sample with a random distribution of orientations yielding the chemical shift anisotropy pattern displayed. (From Ref. 15.)...

The magnitude of the chemical shift anisotropy depends on the bonding situation and the nucleus gyromagnetic ratio. Since the bonds formed by lithium in organolithium compounds or other lithiated systems are mainly ionic, the anisotropy of the lithium chemical shift is generally small. It is more pronounced for Li than for Li. Li spectra are dominated by the quadrupolar effect and the CSA contribution to the Li lineshape is often negligible. Exceptions are compounds with poly-hapto bound lithium, such as... 

As a consequence of the small quadrupole moment of Li, the quadrupolar interaction in solid state NMR spectra is much smaller for Li than for Li. This has been used to advantage for the determination of the Li chemical shift anisotropy from the Li static solid state powder spectrum of 2,4,6-tris(isopropyl)phenyllithium (see below) . Applying MAS up to 10 kHz, the CSA contributions to the lineshape can be completely ehminated in most Li spectra of organolithium compounds. If the measurement of the quadrupolar... 

Fig. 3. Example spectra from the one-dimensional dipolar-shift experiment taken from reference 7. (a) (Top) Experimental l3C chemical shift anisotropy powder pattern for Ru(C5H5)2 and (below) for comparison, the dipolar shift l3C spectrum for the same compound, (b) Calculated dipolar-shift lineshapes for different angles (indicated) between the lH-13C dipolar and chemical shift anisotropy tensor principal z-axes.

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 effect of chemical shift anisotropy on 2H powder lineshapes has also been discussed.35 Another study emphasizes the importance of recording 2H... 

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... 

In C NMR spectroscopy, deviations from a Lorentzian lineshape, which is usually obtained in liquids, can be caused by a chemical shift anisotropy (CSA). If a CSA is present, the position of the resonance line depends on the relative orientation of the molecule with respect to the direction of the magnetic field applied (27,22). The superposition of the individual resonance lines results in typical lineshape patterns that can be described by two parameters the chemical shift anisotropy, AS, and the asymmetry parameter, Tj, respectively. In the case of an axially symmetric CSA tensor, i.e., 17 = 0, the relation between the resonance frequency, w, and the orientation of the molecule is given by... 

In some cases, when a spin-1/2 nucleus such as P, or is coupled to a quadrupolar nucleus, the simulation by a computer program of the unusual lineshape of the spin-1/2 may provide information about the chemical shift anisotropy, the quadrupole coupling constant and the indirect scalar spin-spin coupling constant involving the metal atom. This method has been applied to the P spectra of phosphines bound to cobalt in heteronuclear clusters [17]. 

MAT) to resolve powder lineshapes arising from chemical shift anisotropy.5 The MAT experiment was developed by Gan6 and is a two-dimensional experiment, which resolves chemical shift anisotropy powder patterns in f2 according to their isotropic chemical shifts in /j. The whole experiment is conducted under very slow MAS. Under very slow MAS, the spectrum approximates to that of a static (non-spinning) experiment, and it is this feature that produces static-like powder patterns in the f2 dimension of the MAT experiment. 

Fig. 2. The MAT experiment applied to poly(2-hydroxypropyl ether of bisphenol A)5 (top) to examine the 180° ring flips affecting 13C 4 and 5. (a) The complete two-dimensional MAT spectrum.5 The projection in f2 is effectively the lineshape that would be recorded for a powder sample. As this spectrum clearly shows, the chemical shift anisotropy powder patterns from the nine 13C sites in this polymer are extensively overlapped and would not be resolved without the aid of this MAT experiment, (b) The powder lineshapes for each 13C site taken from the two-dimensional spectrum in (a).5 Those for carbons 4 and 5 show distortions of the lineshape shoulders typical of motional averaging, in this case from 180° phenyl ring flips.

The use of standard NMR spectroscopy without any selective averaging techniques has generally had little importance in the field of catalysis. An exception is high-field V NMR, which yields characteristic lineshapes in the solid state that are easily interpreted in terms of the chemical shift anisotropy ] 11 ]. Generally, we can distinguish the three situations illustrated in Fig. 1 The spectrum in Fig. Ic is observed for compounds with asymmetric coordination environments. It shows three distinct features, which can be identified with the three cartesian chemical shift components 8, 8yy, 8 in the molecular axis system. Figure lb corresponds to the case of cylindrical symmetry, where 8 = 8yy 8, and hence two distinct lineshape components appear. Finally, for chemical environments with spherical symmetry, the chemical shift is the same in all three directions. The solid-state NMR spectrum then contains only a single symmetric peak (Fig. la). 

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... 

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