Tensor-correlation experiment

An important application of spin diffusion in solid-state NMR is its use as a correlation mechanism in two-dimensional NMR spectra [13, 59]. The description of a two-dimensional tensor-correlation experiment using spin diffusion as a transport mechanism follows the general scheme for two-dimensional experiments (see Fig. 4.1) [7]. During the evolution time (tf), the polarization is labeled with the resonance frequency of spin k (fi ), during... 

Rotary resonance recoupling (R3) was used to measure chemical shift tensors,153 and the relative orientation of the shift and the dipolar tensors.154 It can also be used to conduct a 3D chemical shift correlation experiment to investigate molecular rearrangement.155 Another interesting application of the rotary resonance is to obtain an NMR spectrum with peaks at the principal values of the chemical shift tensor.156 The principle is that the spins in principal-value orientations experience an effective field of zero magnitude at the rotary resonance, hence their magnetization vectors do not oscillate while spins at other orientations may be dephased. 

Fig. 42. The form of the DQ-DQ correlation experiment.68 DQ coherence is excited and reconverted using a /-encoding pulse sequence such as BABA. In the experiment, conducted under rapid MAS, t is set equal to t2 and the reorientation of a homonuclear dipolar tensor is monitored through the DQ rotor-encoded spinning sidebands that emerge from Fourier transforming in the t — t — t2 time domain.

The preceding biaxial failure criteria suffer from various inadequacies in their representation of experimental data. One obvious way to improve the correlation between a criterion and experiment is to increase the number of terms in the prediction equation. This increase in curvefitting ability plus the added feature of representing the various strengths in tensor form was used by Tsai and Wu [2-26]. In the process, a new strength definition is required to represent the interaction between stresses in two directions. 

Figure 15 The model molecule used to demonstrate the possibilities of HOESY experiments in terms of carbon-proton distances and reorientational anisotropy. To a first approximation, the molecule is devoid of internal motions and its symmetry determines the principal axis of the rotation-diffusion tensor. Note that H, H,., H,-, H,/ are non-equivalent. The arrows indicate remote correlations.

The DD-CSA cross-correlated relaxation, namely that between 13C-1H dipole and 31P-CSA, can also be used to determine backbone a and C angles in RNA [65]. The experiment requires oligonucleotides that are 13C-labeled in the sugar moiety. First, 1H-coupled, / - DQ//Q-II CP spectra are measured. DQ and ZQ spectra are obtained by linear combinations of four subspectra recorded for each q-increment. Then, the cross-relaxation rates are calculated from the peak intensity ratios of the doublets in the DQ and ZQ spectra. The observed cross-correlation rates depend on the relative orientations of CH dipoles with respect to the components of the 31P chemical shift tensor. As the components of the 31P chemical shift tensor in RNA are not known, the barium salt of diethyl phosphate was used as a model compound with the principal components values of -76 ppm, -16 ppm and 103 ppm, respectively [106]. Since the measured cross-correlation rates are a function of the angles / and e as well, these angles need to be determined independently using 3/(H, P) and 3/(C, P) coupling constants. 

Throughout this summary we have neglected the effect of dispersion on the overall transport of mass and heat. This is due to the fact that if dispersion is included, dispersion tensors must be determined before the equation can be solved. This can be done by solving the appropriate transport equation within a unit cell. Because a unit cell cannot be defined in most reinforcements used in polymer matrix composites, however, dispersion tensors cannot be accurately determined, so we have left dispersion effects out of our equations. In general, we anticipate dispersion to play a minor role in the IP, AP, and RTM processes. This assumption can be checked, however, by evaluating the dispersion terms using an approach similar to [16] where experiments and correlations are used to determine the importance of dispersion. 

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