Anisotropic chemical shift interactions

Figure 38. (left) Solid-echo 2H NMR spectra of glycerol-/ (7 = 189 K) [305]. A collapse of the solid-state spectrum is observed upon heating the corresponding time constants of the a-process are indicated, (right) Hahn-echo 31P NMR spectra of w-tricresyl phosphate (m-TCP, Tg = 210K) determined by the anisotropic chemical shift interaction [324]. 

An alternative means by which the isotropic and anisotropic chemical shift interactions can be separated is the 2D PASS (phase-adjusted spinning sidebands) experiment due to Levitt and co-workers [115]. By changing the timings of the application of five 71 pulses in the tj dimension, it is possible to separate the spinning sidebands by order. As a specific example, Fig. 9.24 shows the 2D PASS spectrum for the antibiotic, penidUin-V. [116] An analysis of this spectrum allowed the determination of the CSA prindpal values for aU the resonances. A distinct advantage of this approach is that only very few (typically 16) increments must be made in the indirect dimension. 

Figure 11 shows that Nitrogen-15 NMR spectrum of NC obtained at 25°C, well below the glass transition. The line shape is typical of the powder pattern expected from aU possible orientations of resonant sites subject to an anisotropic chemical shift interaction. The "divergence" corresponds to those sites where the external magnetic field and the principal axis of the chemical shift tensor are perpendicular the "shoulder" corresponds to parallel alignment of these two directions. The separation between these experimental features is measured to be,... 

In the simplest setup, the two strong field components may be set identical to Ci = Cs = C. The relatively large CIX or CSX term averages isotropic and anisotropic chemical shift effects as well as the heteronuclear dipolar coupling interaction between 15N or 13C and H. The difference of - or the sum of - the B coefficients selects the form of the recoupled heteronuclear dipole-dipole coupling interaction, as expressed in terms of the effective Hamiltonian in the interaction frame of the rf irradiation... 

The electrons modify the magnetic field experienced by the nucleus. Chemical shift is caused by simultaneous interactions of a nucleus with surrounding electrons and of the electrons with the static magnetic field B0. The latter induces, via electronic polarization and circulation, a secondary local magnetic field which opposes B0 and therefore shields the nucleus under observation. Considering the nature of distribution of electrons in molecules, particularly in double bonds, it is apparent that this shielding will be spatially anisotropic. This effect is known as chemical shift anisotropy. The chemical shift interaction is described by the Hamiltonian... 

A detailed investigation of the nuclear spin-lattice relation time, T l. in liquid [Ni(CO)J and [Fe(CO)s] as a function of temperature and resonance frequency has been carried out 212). It was concluded that relaxation occurs only by two mechanisms, i.e., spin-rotation interaction and anisotropic chemical shift. It was possible to obtain the anisotropic chemical shift difference of 440 ppm for [Ni(CO)4] and 408 ppm for [Fe(CO)s] and the spin-rotation constants. Apparent activation energies for diffusion of 1.0 kcal/mole for [Ni(CO)4] and 2.9 kcal/mole for [Fe(CO)5] were derived. 

Because F is an abundant nucleus in PTFE, both anisotropic chemical shift and dipole-dipole interactions are present in the sample. The homonuclear dipolar... 

As mentioned in Section 2, we basically focus on experiments with nuclei with 1= 1/2 (as, e.g., or under high power decoupling. In this case, the dominating interaction of the nucleus with its molecular surroundings consists of the anisotropic chemical shift. It is fully determined by three principal values an, 0 2, and cr33 which correspond to the diagonal values (7, (7, and of the chemical shift tensor in a magnetic tensor system M (Fig. 2) ... 

For our purposes, A designates the anisotropic chemical shift (CS), dipolar (D) interactions. The chemical shift implicitly contains the anisotropic molecular susceptibility which places a natural limit on the resolution of proton NMR of solids under conditions of high resolution as discussed by VanderHart [12]. 

In liquid-state NMR spectroscopy, the anisotropic interactions cannot be seen directly due to the rapid tumbling of the molecule. In soHd-state NMR, an ingenious technique called MAS averages out anisotropic interactions, leaving behind mainly isotropic interactions. Unless specified, we will only consider MAS experiments in this contribution. Due to the periodic motions induced by MAS, the NMR interactions can be expended into a Fourier series in units of the rotor frequency, using zeroth-, first-, and second-order terms. Addressing the chemical shift interaction, we can write the Flamiltonian as... 

In the liquid crystalline phase, the disks rotate around the column axis. A particularly simple way of characterizing such restricted molecular dynamics is provided by the dynamic order parameter 5, 0 < 5 < 1. It is defined as the ratio between the motionally averaged and the static anisotropic NMR interaction, e.g., dipole-dipole coupling, anisotropic chemical shift, or quadrupole coupling [14]. For the rotation of disks in a perfectly packed colunm, S = 0.5 for dipole-dipole coupling or quadrupole coupling, centered around the C—H (C—D)... 

For nuclei with I> 1 quadrupole relaxation usually dominates the magnetic relaxation behaviour. Contrary, for spin I-= 1/2 nuclei the three interactions, dipole-dipole, anisotropic chemical shift, and spin rotation may contribute simultaneously to an experimental relaxation rate... 

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