Indirect dimension

The resulting spectrum, Cindirect/ is a symmetric matrix whose axes are defined by the indirect dimension, F1. Applying indirect covariance processing to a non-symmetric data matrix such as heteronuclear 1H-13C HSQC-TOCSY spectrum affords a symmetric homonuclear 13C-13C TOCSY spectrum analogous to that described originally by Turner.22... 

TS experiments are based on the simultaneous evolution of different frequencies in the indirect dimension of a multidimensional experiment. The concept has been described several years ago in order to obtain different spectra in a single NMR experiment. - These experiments... 

It proved helpful for the purpose of noise reduction to perform relaxation experiments in an interleaved fashion, as one pseudo-3D experiment, where the 2D planes in the F2 dimension correspond to various relaxation delays. The acquisition order (3-2-1) is selected so that cycling through various relaxation delays (in R1 or R2 experiments) or through NOE/NONOE 2D planes is performed prior to incrementing the evolution period in the indirect dimension (FI) (see e.g. Ref. [16]). The resulting pseudo-3D spectrum can be processed as a set of 2D spectra in tl and t3 dimensions, and then analyzed in the usual way. This procedure reduces the noise arising from switching from one 2D experiment to the other and helps minimize temperature variations between the spectra acquired... 

Therefore, it is now possible either to record the 13C-NMR signal and place the hydrogens in the undetected second dimension or to record the signal from the hydrogens and place the 13C resonances in the indirect dimension . 

The task of generating a display of heteronuclear X/Y-connectivities with optimum sensitivity can in principle be performed by recording a three-dimensional proton detected shift correlation in which the chemical shifts of both heteronuclei X and Y are each sampled in a separate indirect dimension. Three-dimensional fourier transformation of the data then generates a cube which is defined by three orthogonal axes representing the chemical shifts of the three nuclei 1H, X, Y, and the desired two-dimensional X/Y-correlation is readily obtained as a two-dimensional projection parallel to the axis... 

Figure 14.11 Basic 2D pulse sequence used for the measurement of residual dipolar local fields. After the excitation pulse the spins are allowed to evolve for some time tj (indirect dimension) under influence of the relevant spin interactions before crosspolarisation takes place. The direct detection during time t2 then takes place on the 13C side typically under proton dipolar decoupling (DD). The basic scheme can be extended by various spin manipulation techniques (not shown) during time tj. For instance, the heteronuclear dipolar contribution can be removed by a decoupling pulse...

We also have to think about the phase sensitive detection in both indirect dimensions. For example, if the phase is encoded in States mode in t and in echo-antiecho mode (using gradients) in t2, we have to acquire four FIDs for each combination of t and t2 delay values. States mode real or imaginary is selected by using an x or y phase for the pulse just before the t delay (lH SQC), and echo or antiecho mode is selected by using a negative gradient... 

It should be noted that the DQ-spectra presented here are obtained assuming ideal excitation. To accurately simulate the DQ projections from the HMQC experiments, real time rf-pulses should be included and the calculations should be performed for the indirect dimension of the HMQC sequence. 

As proved by the authors, the 2D CSA-amplified PASS has a number of favourable features. First, a gain in signal intensity by a factor of 2 is achieved over experiments that require either a storage period or quadrature detection in the indirect dimension. Second, in common with the XCS (chemical-shift modulation) and CSA amplification experiments, isotropic shifts do not occur in the ct>i dimension, leading to efficient sampling of the indirect dimension. Finally, spinning sideband intensities are the same as in conventional MAS spectra allowing routine analysis. 

This is confirmed by comparing ID double-quantum filtered experiments with increasing recouphng times. Extension of this to 2D SQ/DQ correlation identifies the nature of the dipolar coupled pair, since the couphng partner of the directly detected spin can be identified in the indirect dimension, where the chemical shift is the sum of the shift of the two protons involved. This spectrum (Fig. 4b) exhibits intense and well-resolved cross peaks between CD and PDMS. The dipolar couphng constant between the guest and host must therefore be of the order of, or exceed, a few himdred Hz, proving that whatever fast motion the polymer chain performs, it must be a local one. 

Several strategies have been developed to measure residual dipolar couplings in 13C-,15N-labelled proteins. Residual dipolar couplings can be measured in the frequency domain of multidimensional spectra of 13C- or 15N-labelled proteins by not decoupling in either the direct or indirect dimension.107154 Alternatively, residual dipolar couplings can be measured from the J-modulation of the cross-peak intensity in multidimensional NMR experiments.155156 Frequency-based experiments are the simplest to implement but sometimes suffer because of the complexity of the spectrum created by twice the number of cross-peaks. This latter drawback may be overcome by simple editing techniques.154-157-159 Intensity-based experiments are more prone to... 

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