Typical NMR Experiments

After data acquisition has been completed, it is a good habit to save the NMR data in the operator s disk space. In this way, the original data are safe (especially after long accumulations such as overnight or overweekend runs) from a power failure, accidental deletion, or simply forgetting to save the data after a workup (it happens). 

---

The relative population of each spin state is determined by the Boltzmann distribution, Eq. (2.8). Under conditions of a typical NMR experiment the ratio of spin state populations is near unity, differing only by a few parts per million. 

The sensitivity of typical NMR experiments to the type and time scale of various motions is illustrated in Fig. 7. For simplicity, we have chosen equal correlation times for all motions, namely = 1 x 10 s, = 1 x 10 s and Xg = 5 x 10 s. The powder spectra refer to quadrupole echo sequences [57], and characterize two-site jumps (a, b), three-site jumps (c), planar rotational diffusion (d), tetrahedral jumps (e) and isotropic spherical diffusion (f), respectively. The significant differences of the lineshapes arise from the different motional anisotropies. Evidently, quadrupole echo spectra [57] contain valuable information on the type of motion [10,48,49, 58]. 

Figure 3.153 In a typical NMR experiment, when an ensemble of protons is placed in an external magnetic field Bo, a net magnetization along Bo is caused. After applying a 90° RF pulse, the net magnetization is flipped to the xy plane. The longitudinal relaxation process T )...

In the absence of a magnetic field, the energies of the magnetic quantum states of a nucleus are identical. Consequently, a large collection of protons contains an identical number of nuclei with magnetic quantum numbers m = +112 and m = —1/2. When placed in a magnetic field, however, the nuclei tend to orient themselves so that the lower energy state (m = +112) predominates. It is instructive to calculate the extent of this predominance in a typical NMR experiment. For this purpose, the Boltzmann equation (Equation 8-1) can be written in the form... 

FIGURE 15.25 The two molecules obtained by substituting one of the two hydrogens are enantiomers therefore the hydrogens are enantiotopic. Enantiotopic hydrogens are equivalent in typical NMR experiments. 

---