Solids dipolar coupling

In normal liquids, most molecules move isotropically and dipolar couplings are averaged to 0. In static solids and in liquid crystals, molecular motion is non-existent or anisotropic and dipolar couplings are observed. In solids, dipolar couplings are obscured by chemical shift anisotropy but in liquid crystals, they are observed and can be large, up to the kHz range [423], The size of the direct dipolar coupling (Dy) between nuclei i and j is ... 

In addition to sample rotation, a particular solid state NMR experiment is further characterized by the pulse sequence used. As in solution NMR, a multitude of such sequences exist for solids many exploit through-space dipolar couplings for either signal enhancement, spectral assignment, interauclear distance determination or full correlation of the spectra of different nuclei. The most commonly applied solid state NMR experiments are concerned with the measurement of spectra in which intensities relate to the numbers of spins in different environments and the resonance frequencies are dominated by isotropic chemical shifts, much like NMR spectra of solutions. Even so, there is considerable room for useful elaboration the observed signal may be obtained by direct excitation, cross polarization from other nuclei or other means, and irradiation may be applied during observation or in echo periods prior to... 

Rotational-echo double-resonance (REDOR)(75,79) is a new solid-state NMR technique which is sensitive to through-space carbon-nitrogen interactions between selectively 13C and 15N-enriched sites separated by up to 5A (20-22). The parameter directly measured in a REDOR experiment is the heteronuclear dipolar coupling constant DCN, which is in itself proportional to the inverse third power of the intemuclear distance, rCN. It is this dependence on (icn)3 which accounts both for REDOR s ability to accurately measure short distances and its insensitivity to longer-range interactions. As a technique which can probe, in detail, intermolecular interactions over a distance range of 5A, REDOR is well suited to studying the distribution of small selectively-labeled molecules in polymer delivery systems. 

In the solid, dynamics occurring within the kHz frequency scale can be examined by line-shape analysis of 2H or 13C (or 15N) NMR spectra by respective quadrupolar and CSA interactions, isotropic peaks16,59-62 or dipolar couplings based on dipolar chemical shift correlation experiments.63-65 In the former, tyrosine or phenylalanine dynamics of Leu-enkephalin are examined at frequencies of 103-104 Hz by 2H NMR of deuterated samples and at 1.3 x 102 Hz by 13C CPMAS, respectively.60-62 In the latter, dipolar interactions between the 1H-1H and 1H-13C (or 3H-15N) pairs are determined by a 2D-MAS SLF technique such as wide-line separation (WISE)63 and dipolar chemical shift separation (DIP-SHIFT)64,65 or Lee-Goldburg CP (LGCP) NMR,66 respectively. In the WISE experiment, the XH wide-line spectrum of the blend polymers consists of a rather featureless superposition of components with different dipolar widths which can be separated in the second frequency dimension and related to structural units according to their 13C chemical shifts.63... 

Fig. 9 Examples of simplifying solid state NMR spectra by the TOSS and delayed decoupling pulse sequences. Shown is a comparison of the 31P CP/MAS NMR spectrum of fosinopril sodium utilizing the standard pulse sequence (A) and the TOSS routine (B). Also shown is the full 13C CP/MAS NMR spectrum of fosinopril sodium (C) and the nonprotonated carbon spectrum (D) obtained from the delayed decoupling pulse sequence utilizing a 80 /us delay time. Signals due to the methyl carbon resonances (0-30 ppm) are not completely eliminated due to the rapid methyl group rotation, which reduces the carbon-proton dipolar couplings.

Finally, a series of papers have been published on the solid state NMR spectra of a number of analgesic drugs. Jagannathan recorded the solid state 13C NMR spectrum of acetaminophen in bulk and dosage forms [60], From the solution-phase NMR spectrum, assignments of the solid state NMR resonances could be made in addition to explanations for the doublet structure of some resonances (dipolar coupling). Spectra of the dosage product from two sources indicated... 

The size and orientation of the 31P CSA tensor have been calibrated from single-crystal solid state NMR data of barium diethylphosphate [31]. The projection angles 6CHa22 and i between the 1H,13C-dipolar coupling and the components of the 31P-CSA-tensor de-... 

Fig. 8.2 Ori entations of an amide NH dipolar coupling bond-vector of the protein ubiquitin. Each cone of orientations is compatible with two different alignment directions adopted by the protein in two different alignment media. The central lines defining each cone correspond to the orientations obtained from the measured dipolar couplings. The outer lines include orientations that are possible if the dipolar coupling values are either increased or decreased by 1 Hz. The angle at which the two cones intersect is defined by ft. The solid dot at the cone intersection determines the orientation of the dipolar coupling vector. (Reproduced with permission from B. E. Ramirez and A. Bax, J. Am. Chem. Soc. 1998, 720, 9106-9107.)...

The dipolar-coupling Hamiltonian (TTy) describes the through-space coupling between two nuclear spins l and Ij. The dipolar coupling has an rk- dependence, and is key to the determination of internuclear distances in both solid-state and solution-state NMR. The high-field truncated form of the dipolar Hamiltonian is given by... 

Fig. 11.2 Solid-state 13C NMR spectra of a powder sample of U-13C-15N-glycine illustrating the broad NMR resonances in the static sample (a) and the effects of magic-angle spinning at 5 kHz (b) and 10 kHz (c). Both the chemicalshielding tensors and the homonuclear dipolar coupling...

Composite-pulse decoupling schemes like WALTZ [36, 37], DIPSI [38], or GARP [39], which are used in solution-state NMR, have failed to offer any significant improvements in the solid state compared to CW decoupling. The residual line width in CW-decoupled spectra is dominated by a cross term between the chemical-shielding tensor of the protons and the heteronuclear dipolar-coupling tensor [40, 41]. 

Solid-state analogs of the HMQC [103, 104] and HSQC [105] experiment, MAS-J-HMQC and MAS-J-HSQC [106, 107], have been proposed. They rely on the suppression of the large homonuclear dipolar couplings by FSLG irradiation of the protons. In contrast to the liquid-state implementations, both experiments use low-y detection in the solid state. 

At moderate MAS frequencies, solid-state NMR spectra of uniformly labeled material have somewhat broader lines than their unlabeled counterparts. This has been attributed to only partial averaging of the dipolar-coupling between the low-y nuclei by MAS and... 

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