Dipolar interactions and chemical shifts

Rapid isotropic tumbling of molecules is restrained for the network polymers in the gel state. A proton dipolar broadening of many kilohertz is observed in an NMR spectrum due to the strong dipolar-dipolar interaction and chemical shift anisotropy as a result of the restraint on the molecular motion. One method used for the removal of proton dipolar broadening is to employ a high-power proton decoupling field [5]. The scalar couplings are... 

A rigid solid represents the slow-exchange limit in the analogy between T2 and exchange-broadening, and in general any resonance will be broadened or split by dipolar interactions and chemical shift anisotropy (CSA). Nuclei of spin >1/2 will also be affected by interactions of their electric quadrupoles with local electric field gradients. 

The anisotropic nature of the dipolar, quadrupolar and chemical shift anisotropy interactions requires that the isotropy of molecular orientation relative to the applied magnetic field be broken in order to allow their direct observation in terms of shifts in the frequencies of resonances.20,32,38 40 For high resolution NMR studies this has meant, thus far, that some degree of alignment of the molecule needs to be established. Almost all molecules will align to a small extent due to the anisotropy of their magnetic... 

Fig. 6. Simulated 2D PISEMA spectra of powder samples. Correlation of dipolar coupling and chemical shift interactions associated with an amide N-H bond (A) and the histidine side chain Njt-H bond (B). For the amide N-H bond, the simulations were performed using CSA principal values of 33 = 64, 22 = 77, and Sii = 217 ppm, an N-H bond length of 1.07 A, and the Euler angles ( = () and yg=17°) to define the relative orientations of the chemical shift and dipolar coupling tensors. For the histidine side chain Nrt-H bond, the following parameters were used in the simulations CSA principal values of 533 = 77, 522 = 203, and 5 =260 ppm,...

Among the massive array of experimental methods, based on many different physical phenomena, able to afford structural information, NMR spectroscopy plays a central role. Indeed, parameters such as chemical shifts (5iso), chemical shift tensors (5n,522,533), homonuclear and heteronnclar dipolar interactions (D), chemical shift anisotropy (CSA), relaxation times (Ti, T2, Tip), and lineshape (Am) allow the investigation of short range structural environments (HB and nuclear proximities) as well as the evaluation of slow and fast dynamic processes. 

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