Monday, September 29, 2008
In the solid state, in the absence of magic angle spinning, the frequency of NMR lines depends on the orientation of the molecules with respect to the static magnetic field. For powdered samples, all orientations are represented in the sample and one obtains a broad envelope of peaks resulting from all possible orientations. Such broad resonance are called powder patterns and are said to be anisotropic. The frequency is not necessarily the only orientation dependant parameter. In some cases, the T1 relaxation time also depends on the orientation of the molecules with respect to the magnetic field. In such cases the T1 is said to be anisotropic. In contrast to the NMR resonances in solution which are characterized with a single T1, the powder pattern can be characterized with many different T1 relaxation times. Furthermore, the presence or absence of anisotropy in the T1 can help discriminate between certain types of molecular motion. An example of an anisotropic T1 is illustrated in the figure below for the wide line 2H inversion recovery spectra of acetone-d6 trapped in an organic inclusion compound. The line shape indicates that the acetone molecules undergo both fast methyl group rotation and fast two-fold flips about he carbonyl bond. One can see that the entire powder pattern does not have the same T1 as the line shapes are a function of the inversion recovery delay, tau. The T1 depends on the frequency within the powder pattern which in turn depends on the orientation of the molecules with respect to the magnetic field.