Differentiation of models of rapid molecular motion by 2H NMR lineshape simulations

Abstract

Slow molecular motions in solids can, in principle, be thoroughly studied over a wide dynamic range by the analysis of 2H NMR lineshapes. However, for fast motions with correlation times less than 10 −7 s the lineshapes become insensitive to the time scale of the motion. In this limit, the NMR lineshapes only provide limited information about the geometry of the motion; in the case of an axially symmetric static electric field gradient tensor (C- 2H bond), one can only unambiguously discriminate between planar and nonplanar motions. As a consequence, the frequently observed near gh ∗ = 1 experimental lineshape is compatible with many models of motion. For example, it is impossible to distinguish between certain families of rapid jumps and families of rapid librations from 2H NMR lineshapes even when T 2 relaxation is considered. However, these motional mechanisms can be distinguished via lineshape distortions resulting from the anisotropic spin-lattice relaxation ( T 1 and T 1Q).