The smectic phase, an example of high-resolution NMR in solids

In smectic phase the directions of local fields are parallel, if this phase is produced by cooling the isotropic material in a strong magnetic field. Single molecules are rapidly fluctuating around these directions of preference with correlation times tausub(CL). For certain kinds of substances it is tausub(CL)<Δωsub(1/2)sup(-1) in these cases, the dipolar line width is determined in fact only by the components of the local fields in the direction of these axes. Any rotation of the specimen does not change the mutual orientation of the local regions for a long time (many hours) and hence the line width is Δω1sub(1/2) = x(3cos2THETA-1, THETA being the angle between H0 and the axis of preference. At a 'magic angle' of THETA = 54044' the dipolar line width must vanish. In this way, that means by mechanical rotation, an enhancement of Tsub(2eff) by a factor 103 to 104 could be achieved (H17C8-0-C6H4-CO-O-C6H4-NO2) and high-resolution signals of the protons were observed. In turn, T(2eff) was enhanced by pulse sequences attaining thus the same final value of about 0,13 sec. This value depends on the spin-lattice relaxation as demonstrated by other measurements. Other smectic compounds (such as H17C8-O-C6H4-N-NO-C6H4-O-C8H17) showed another rotational behaviour resulting in a minimum line width, which was much larger than that of the first sample and which was found at another angle than the magic one. From this experimental fact some considerations as to relaxation times are possible. (author)