CHEM01200604004 Shri Sanyasinaidu Boddu - Homi Bhabha ...

solids tend to be broadened because of (a) magnetic interactions of nuclei with the
surrounding electron cloud (chemical shielding interaction), (b) magnetic dipole – dipole
interactions among nuclei and (c) interactions between electric quadrupole moment and
surrounding electric field gradient. Hence, it is difficult to get any meaningful information
from such patterns. However, by using suitable experimental strategies, such interactions can
be averaged out, thereby improving the information obtained from solid-state NMR patterns.
One such technique used to get high-resolution solid-state NMR pattern in solids is the magic
angle spinning nuclear magnetic resonance (MAS NMR) technique. In the following section
brief account of the MAS NMR technique has been given.
Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR): MAS NMR technique
involves rotating the powder samples at high speeds, at an angle of 54.7° (magic angle) with
respect to the applied magnetic field direction. When θ=54.7°, the term 3cos 2 θ becomes
unity. Since Hamiltonian for different anisotropic interactions have 3cos 2 θ – 1 term, these
anisotropic interactions get averaged out in time during fast spinning. This is schematically
shown in Fig.16 and explained below.
B 0
θ = 54.7°
θ 1
θ 2
Fig.16. Principle of MAS NMR experiment
At sufficiently fast spinning speeds, the NMR interaction tensor orientations with initial
angles of θ 1 and θ 2 relative to B 0 have orientaional averages of 54.7°, resulting in the
conversion of 3cos 2 θ – 1 term in expressions corresponding to various interactions to a very
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