Spin Diffusion gd Orientation Dependence of Deuterium Tl in Solids

The study of spin diffusion in solids has been of recent interest. Spin diffusion occurs by flip-flop cross-relaxation due to the S$Sj_ + S!Sj, term of the dipolar couplings among the spins (I). When a large inhomogeneous interaction also exists for the S spin system, then the flip-flop transition may not conserve energy, i.e., the resonances of the coupled spins do not overlap and are separated by a frequency AR. In this case the rate depends on the possible mechanisms of cross-relaxation and spin diffusion. This situation is typical for spin systems with S = 1 in solids, e.g., *H, 14N, where the nuclear quadrupole interaction, HQ, disperses the resonances. In a single crystal sample, A52 can be adjusted very accurately using the orientation dependence of HQ. It was recently shown that when an S = 14N spin system is coupled to an extraneous dipolar reservoir having a large heat capacity provided by a proton (1) spin system, the cross-relaxation rate varies approximately as (An)-* = (IQo,il lQo,jl)-*, where Oo,i and Qo,j are quadrupole coupling offset frequencies of the two S nuclei (2). For a three-level system (S = 1) which is not coupled to an extraneous reservoir, a mechanism for cross-relaxation via double-quantum flip-flops has been proposed (3) and the transition rate depends on (AQ)-2 = (Qo,i Qo,,)-‘. Thus double-quantum cross-relaxation should be sensitive to the relative signs of Qo,i and Qo,j. It has also been shown that single-quantum spin diffusion can be dramatically enhanced by sample rotation which modulates Ano and causes an effective overlap of resonances that are dispersed by the quadrupole coupling (4). In this communication, we report measurements of the orientation dependence of T, for deuterium in a single crystal of para-dimethylterephthalate-& (99.3%-d), CD3C02@D4C02CD3. In this crystal the methyl groups have y N 1 set at room temperature due to rotation about their C, axes. This is a useful model sample since the aromatic deuterons are in rigid lattice sites and are expected to be coupled to the lattice only by spin diffusion to the rapidly relaxing methyl sites. Thus the c of the ring deuterons should have a marked orientation dependence.