Residual quadrupolar couplings

Instead of measuring only the time-dependent dipolar interaction via NOE, it is also possible to determine dipolar couplings directly if the solute molecule is partially aligned in so-called alignment media. The most important resulting anisotropic parameters are RDCs, but residual quadrupolar couplings (RQCs), residual chemical shift anisotropy (RCSA) and pseudo-contact shifts (PCSs) can also be used for structure determination if applicable. 

A spectrum of perdeuterated potassium laurate in oriented soap-water multilayers (5) is presented in Figure 2. The measured quadrupolar coupling constants are much smaller than those of a static C-D bond, which are about 167 kHz (6) the residual quadrupolar splittings... 

Deuterium nuclei in water molecules have negligible asymmetry parameters and residual anisotropies. After correcting for the paramagnetic impurities present, that affect relaxation rates somewhat (35), one obtains from the experimental data a value of the longitudinal relaxation rate for the water deuterons in the bound state of 650 s 1. This value incorporates the quadrupolar coupling constant (above determined) and the correlation time for bound waters. Using the standard expression for quadrupolar relaxation (29,35) yields a value for t 1.6 ns. 

The generalization of the residual dipolar couplings for arbitrary quadrupolar/Zeeman ratios has been presented.The predictions of the essentially numerical treatment compare well with previously derived high-field analytical models, as well as with experimental solid state NMR spectra observed in a borane compound possessing a spin pair. An alter-... 

It has been long appreciated that a chiral environment may differentiate any physical property of enantiomeric molecules. NMR spectroscopy is a sensitive probe for the occurrence of interactions between chiral molecules [4]. NMR spectra of enantiomers in an achiral medium are identical because enantiotopic groups display the same values of NMR parameters. Enantiodifferentiation of the spectral parameters (chemical shifts, spin-spin coupling constants, relaxation rates) requires the use of a chiral medium, such as CyDs, that converts the mixture of enantiomers into a mixture of diastereomeric complexes. Other types of chiral systems used in NMR spectroscopy include chiral lanthanide chemical shift reagents [61, 62] and chiral liquid crystals [63, 64). These approaches can be combined. For example, CyD as a chiral solvating medium was used for chiral recognition in the analysis of residual quadrupolar splittings in an achiral lyotropic liquid crystal [65]. 

In the absence of a significant Cq, the indirect spin-spin interaction between quadrupolar and spin-1/2 nuclei manifests itself in the spectrum of the latter as a multiplet consisting of 2/-I-1 evenly spaced peaks. However, if Uq is a significant fraction of vq, the spin states for the quadrupolar nucleus are not quantized precisely in the direction of Bo and hence MAS does not completely average the dipolar interaction between the nuclei. The effect, referred to as residual dipolar coupling, manifests itself as distortions in the multiplet structure in the NMR... 

Even in cases where the quadrupolar parameters can be determined from direct observation of the quadrupolar nuclei, the residual dipolar coupling may be used to determine important molecular properties. For example, based on the magnitude of the residual dipolar coupling between and determined from a simulation of the... 

The efficient relaxation of quadrupolar nuclei in solution generally results in self-decoupling, such that quadrupolar effects are generally not observed in NMR spectra of spin-1/2 nuclei. Self-decoupling may also be a factor in NMR spectra of solid samples this often manifests itself as an asymmetric line shape. By decreasing the temperature, the Ti relaxation time of the quadrupolar nucleus may increase sufficiently to allow acquisition of NMR spectra where the effects of residual dipolar coupling are sufficiently resolved. ... 

The residual dipolar coupling also manifests itself in NMR spectra of dipolar-coupled quadrupolar spin pairs.Wu and Yamada also observed residual dipolar coupling attributed to the N- B dipolar interaction in the NMR spectra of MAS samples of triethanolamine borate. More recently, the effect of the N- B, "B- B, and Mn- Mn residual dipolar interactions on the B and Mn NMR line shapes acquired through MQMAS were also demonstrated. ... 

Gly-Tyr was synthesized as both the [ C]-amido (90 atom % enrichment and amido-[ C, N] (90 atom % enrichment, respectively) isotopomers by known methods. CPA, purchased from Sigma as a suspension in toluene, was crosslinked and soaked with Gly-Tyr as described previously, and the washed, dried crystals examined by solid-state NMR. The C CPMAS NMR spectrum of the C-enriched Gly-Tyr (Fig. 12) displays residual C- N quadrupolar coupling (8 = 176 ppm). However, the C CPMAS difference spectrum of CPA and the CPA/Gly-Tyr complex (Fig. 12) displays a single resonance (5 = 178 ppm, LW /2 60 Hz) revealing that, under the experimental conditions, the peptide bond of this slow reacting substrate has been cleaved to the extent of > 90%. 

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