Isotropic magic angle spinning

Simulation of isotropic magic-angle spinning nuclear... 

Chemical shift (isotropic component) 5. ISO Magic-angle spinning Chemical bonding coordination number... 

Fig. 3 Solid state 31P NMR spectra of fosinopril sodium acquired under single pulse, high-power proton decoupling and various conditions of magic-angle spinning (A) static, (B) 2.5 kHz, (C) 4.0 kHz, (D) 5.0 kHz, and (E) 6.0 kHz. The isotropic chemical shift is designated by an asterisk. (From Ref. 15.)...

Frydman, L and Harwood, ).S. (1995) Isotropic spectra of half-integer quadrupolar spins from bidimensional magic-angle spinning NMR./. Am. Chem. Soc., 17, 5367-5368. 

The line broadening caused by partial motional narrowing can be distinguished from that due to isotropic reorientation at a reduced rate by appropriate magic angle spinning experiments. 

However, it is found that a combination of techniques, such as proton dipolar decoupling (removes the dipolar interactions), magic angle spinning (reduces the chemical shift tensor to the isotropic chemical shift value), and cross-polarization (increases the sensitivity of rare spins, like 13C) applied to a solid state material, results in sharp lines for 13C nuclei in the solid state10). Thus, the observation of narrow lines or high resolution NMR in the solid state is possible. 

Experimentally, the isopropyl cation was prepared by the low temperature reaction of 2-bromopropane-2-13C with frozen SbF5. The 13C spectrum was measured at 83 K using slow speed magic angle spinning. Analysis of the spectrum using the method of Herzfeld and Berger yielded tensor values of 8n = 497 ppm, 822 = 385 ppm and 833 = 77 ppm for the central carbon, which results in an isotropic chemical shift of 320 ppm. 

The other general way of resolving powder patterns from different chemical sites is to generate multidimensional NMR spectra in which the desired powder patterns (or magic-angle spinning sideband patterns) are resolved in one dimension, separated according to (for instance) isotropic chemical shift in another dimension. These techniques are discussed below in the relevant section for each type of nuclear spin interaction. 

Another way of dealing with the resolution problem for powder lineshapes is to use multidimensional NMR techniques to separate powder pattern lineshapes (or magic-angle spinning sideband patterns) according to isotropic chemical shift, as mentioned previously. 

Fig. 10. The pulse sequence for the WISE experiment.21 This two-dimensional experiment separates H wideline spectra according to the isotropic l3C chemical shift of the 13C each H is bonded to. An initial H 90° pulse creates transverse H magnetization that is allowed to evolve in t. A short cross-polarization step then transfers the remaining H magnetization to the nearest 13C spin, i.e. the bonded one. The resulting 13C transverse magnetization is then allowed to evolve in ti under magic-angle spinning, where an FID is recorded.

Fig. 15. The pulse sequence for the 13C 2H correlation experiment.38 This two-dimensional experiment, conducted under magic-angle spinning, separates 2H line-shapes according to the 13C isotropic chemical shift of nearby l3C spins, i.e. the bonded l3C in practice. The narrow black pulses are 90° pulses wide ones are 180° pulses. The 2H pulses are placed symmetrically within the rotor period.

---