NMR chemical shift anisotropy

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. 

Wolff R, Vogel C, Radeglia R (1993) IGLO calculations of Si NMR chemical shift anisotropies in silicate models. In Nuclear Magnetic Shieldings and Molecular Stracture, Tossell JA (ed), p 385-399... 

The conventional, and very convenient, index to describe the random motion associated with thermal processes is the correlation time, r. This index measures the time scale over which noticeable motion occurs. In the limit of fast motion, i.e., short correlation times, such as occur in normal motionally averaged liquids, the well known theory of Bloembergen, Purcell and Pound (BPP) allows calculation of the correlation time when a minimum is observed in a plot of relaxation time (inverse) temperature. However, the motions relevant to the region of a glass-to-rubber transition are definitely not of the fast or motionally averaged variety, so that BPP-type theories are not applicable. Recently, Lee and Tang developed an analytical theory for the slow orientational dynamic behavior of anisotropic ESR hyperfine and fine-structure centers. The theory holds for slow correlation times and is therefore applicable to the onset of polymer chain motions. Lee s theory was generalized to enable calculation of slow motion orientational correlation times from resolved NMR quadrupole spectra, as reported by Lee and Shet and it has now been expressed in terms of resolved NMR chemical shift anisotropy. It is this latter formulation of Lee s theory that shall be used to analyze our experimental results in what follows. The results of the theory are summarized below for the case of axially symmetric chemical shift anisotropy. 

An alternative interpretation of the previous solid-state NMR studies of the backbone motion of collagen has been reported. Based on the analysis of the solid-state NMR chemical shift anisotropy and quadrupolar line shapes for five different isotope labelled collagens, it has been shown that motional averaging of the NMR interactions occurs primarily via small-angle librations about internal bond directions. 

The difference in spectral lineshape between the Pj crystalline phase, which does not exhibit molecular motion, and P2 crystalline phase, is caused by a decrease in t q, and Ties. Clearly, the oxygen sites in both phases are similarly coordinated. The small decrease in tiq, cs> and t cs values between -60 to O C in the 2 crystalline phase may indicate the onset of the slow libration motion of PDES chains about a specific axis. From the static solid-state Si NMR result [47, 61,71], due to the onset of oscillations around the molecular chain axis and small conformational transition in the P2 crystalline phase, the Si NMR chemical shift anisotropy is reduced by 25% compared with the Pi crystalline phase. The static solid-state NMR result supports the solid-state Si NMR result. 

Pervushin K, Riek R, Wider G and Wuthrich K 1997 Attenuated T relaxation by mutual cancellation of dipole-dipole coupling and chemical shift anisotropy indicates an avenue to NMR structures of very... 

Tjandra N and Bax A 1997 Solution NMR measurement of amide proton chemical shift anisotropy in N-15-enriched proteins. Correlation with hydrogen bond length J. Am. Chem. Soc. 119 8076-82... 

Figure 3 Characteristic solid state NMR line shapes, dominated by the chemical shift anisotropy. The spatial distribution of the chemical shift is assumed to be spherically symmetric (a), axially symmetric (b), and completely asymmetric (c). The top trace shows theoretical line shapes, while the bottom trace shows rear spectra influenced by broadening effects due to dipole-dipole couplings.

From the NMR data of the polymers and low-molecular models, it was inferred that the central C—H carbons in the aliphatic chain in polymer A undergo motions which do not involve the OCH2 carbons to a great extent. At ambiet temperatures, the chemical shift anisotropy of the 0(CH2)4 carbons of polymer A are partially averaged by molecular motion and move between lattice positions at a rate which is fast compared to the methylene chemical shift interaction. 

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...

Conventional utilization of solution-phase NMR data acquisition techniques on solid samples yields broad, featureless spectra (Fig. 1A). The broad nature of the signal is due primarily to dipolar interactions, which do not average out to zero in the solid state, and chemical shift anisotropy (CSA), which again occurs because our compound of interest is in the solid state. Before one describes the two principal reasons for the broad, featureless spectra, it is important to understand the main interactions that a nucleus with a magnetic moment experiences when situated within a magnetic field in the solid state. In addition, manifestations of these interactions in the solid state NMR spectrum need to be discussed. 

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.)...

NMR spin relaxation is not a spontaneous process, it requires stimulation by a suitable fluctuating field to induce an appropriate spin transition to reestablish equilibrium magnetization. There are four main mechanisms for obtaining relaxation dipole-dipole (most significant relaxation mechanism for I = 1/2 nuclei), chemical shift anisotropy, spin rotation, and quadrupolar (most significant relaxation mechanism for I > 1/2 nuclei) (Claridge, 1999). 

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