Anisotropic behavior of the 13C nuclear spin dynamics in a single crystal of l-alanine

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Abstract
Strong anisotropic behavior of the 13C nuclear spin dynamics, namely, orientation dependent transient oscillation frequency of the spin locking cross polarization of the Cα carbon magnetization, the nuclear Overhauser enhancement, and the spin‐lattice relaxation times for the CH3 carbon nucleus were observed in a single crystal of l‐alanine. The spin locking cross‐polarization dynamics, including the transient oscillation effects, are explained by considering the dipolar interaction between the a‐carbon nucleus and its directly bonded proton, together with spin diffusion and spin‐lattice relaxation in the rotating frame, in the equation of motion of the density operator. We found that by assuming that the CH3 group undergoes random jumps between three equilibrium positions, the application of relaxation theory yields equations which provide a satisfactory interpretation of the experimental observations. Thus we conclude that the orientation dependence of the nuclear Overhauser enhancement, and the spin‐lattice relaxation time in the Zeeman frame of the CH3 carbon nucleus is caused by the anisotropic motion of the methyl group.