Chemical Shift Anisotropy Analysis

At low magnetic fields, it is often reasonable to ignore the contributions from chemical shift anisotropy (CSA). However, modern day investigators are using higher magnetic fields and this often requires an analysis that includes a CSA contribution as the CSA contribution increases with the square of the magnetic field. 

If one ignores even a small CSA contribution, the error introduced in the calculations may lead to considerable error in the calculation of rotational correlation times and their variation with temperature. In order to determine the contribution of each mechanism (dipolar and CSA) in C NMR relaxation studies, a mathematical approach has been devised [13-15] which involves combining several correlation functions followed by a series of iterative steps that lead to the correct rotational correlation times. This results in the determination of the separate dipolar and CSA contributions to the overall relaxation mechanism in C NMR relaxation studies. The following section outlines the necessary steps leading to the determination of both dipolar and CSA contributions as given in Eq. (4.4-10). 

---

Concerning the carbonyl groups in MT, the corresponding chemical shift anisotropy analysis leads to the consideration of fast oscillations of amplitude a about the para axis of the adjacent phenyl ring. 

Atalla RH, VanderHart DL. The role of solid-state carbon-13 NMR spectroscopy in studies of native celluloses. Solid State NMR 1999 15 1-19. Chen Y-Y, Luo S-Y, Hung S-C, Sunney I, Tzou D-LM. C solid-state NMR chemical shift anisotropy analysis of the anomeric carbon in carbohydrates. Carbohydr Res 2005 340 723-729. 

Chemical shift anisotropy 5.5,5 xxr yy zz Line-shape analysis MAS-sidebands Coordination symmetry... 

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...

Unlike the NMR study on PMMA, 13C- and 2H-labelled CMIM20 copolymers were not available, so investigation has been limited to analysis of the chemical shift anisotropy of the carboxyl group [75]. 

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... 

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,... 

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... 

Using time-dependent perturbation theory and taking full account of the symmetry and commutation relations for the high-order dipolar Hamiltonians, Hohwy et al.61 69 gave a systematic analysis of homonuclear decoupling under sample rotation and proposed a novel approach to the design of multiple-pulse experiments. Based on the theoretical analysis, they proposed a pulse sequence that can average dipolar interaction up to the fifth order. One example of these pulse sequences is shown at the top of Fig. 3. This sequence is sufficiently powerful that it is possible to obtain precise measurement of proton chemical shift anisotropies, as shown in Fig. 3. 

Another feature which can be conveniently exploited by P NMR spectroscopy of metal carbonyl derivatives is the large effect on the chemical shift anisotropy of P resonances caused by the bonding mode of P donor atoms. The chemical shift anisotropies obtained by SSB analysis of P CP/MAS spectra in a series of PPh (n=l, 2, 3) ligands bonded to transition metal carbonyl complexes are strongly dependent on the coordination arrangement of the phosphorus atoms as shown in Figure 21. 

Figure 21 Chemical shift anisotropies obtained by spinning sideband analysis of CP/MAS spectra in a series of PPh (n = 1,2,3) ligands bonded to transition metal carbonyl complexes...

---