Nonselective relaxation experiments

As for nonselective-relaxation experiments, Eq. 15 can only be used for a three-spin system, because, for j>3, the problem is underdetermined. 

It also is possible to determine py values from a single, nonselective experiment for more than three spin systems, by combining nonselective relaxation-rates with n.O.e. experiments. According to Noggle and Shir-mer, Eq. 5 can be solved under conditions in which, at time f = 0, spin i is observed with simultaneous saturation of spin j, resulting in the expression... 

The fact that the -SiO(CH3)2- chain units that were grafted to the silica surface and those in the middle portion of the chain show the same chemical shift in the and Si spectra precluded the use of chemically selective NMR relaxation techniques for characterizing the PDMS-silica inteifacial layer. We therefore performed nonselective low resolution NMR relaxation experiments to characterize the structure of the PDMS grafted layer. 

The second separation method involves n.O.e. experiments in combination with non-selective relaxation-rate measurements. One example concerns the orientation of the anomeric hydroxyl group of molecule 2 in Me2SO solution. By measuring nonselective spin-lattice relaxation-rat s and n.0.e. values for OH-1, H-1, H-2, H-3, and H-4, and solving the system of Eq. 13, the various py values were calculated. Using these and the correlation time, t, obtained by C relaxation measurements, the various interproton distances were calculated. The distances between the ring protons of 2, as well as the computer-simulated values for the H-l,OH and H-2,OH distances was commensurate with a dihedral angle of 60 30° for the H-l-C-l-OH array, as had also been deduced by the deuterium-substitution method mentioned earlier. 

On the other hand when more than two polarization transfer steps are combined (ID analogs of 4D experiments) one can make a decision, without affecting the end result, as to whether the second, the third or both these steps should be selective. In order to avoid possible losses of magnetization during the selective pulses, due to either relaxation and/or nonperfect excitation profiles, it is usually possible to make one of these steps nonselective. 

Figure 5 Inversion-recovery curves for slow exchange between two sites. Lines A and B are the results from one experiment. They show the recovery after a nonselective inversion of both sites, showing that the two sites have slightly different T, values. C and D are obtained from a different experiment. Line C shows the recovery of a site that has been selectively inverted, and line D shows the behaviour of the line that was not perturbed in this experiment. The inverted line relaxes faster, due to a combination of spin-lattice, relaxation and exchange, and the unperturbed line shows the characteristic transient.

It is also possible to use T, measurements to derive distance data. The procedure for this entails a combination of nonselective, semiselective, and selective relaxation time measurements. The nonselective T, measurement is one in which the 180° pulse in the usual inversion-recovery experiment (James, 1975) is applied to all of the interacting nuclei. Only one nucleus is subjected to the inverting 180° pulse in the selective T, measurement and only two nuclei in the biselective experiment. The measured values may vary in these three experiments and may be shown to give (Solomon, 1955 Nicollai and Tiezzi, 1979)... 

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