Amplitude modulation and relaxation due to diffusion in NMR experiments with a rotating sample

The simultaneous effects of diffusion and coherent averaging by magic angle sample spinning (MASS) are investigated both theoretically and experimenta lly for spins diffusing in a spatially varying susceptibility field. The di ffusion introduces a periodic modulation and a relaxation of the magnetizat ion amplitude. The attenuation exponent for the nth echo at an early time i s given by D(0)gamma (2)g(2)pin(4 omega (3)(r)), where omega (r) is the rot or frequency, D-0 the diffusion coefficient, gamma is the gyromagnetic fact or, and a factor g(2) which is related to, but different from, the volume a verage of the gradient squared. (C) 2000 Elsevier Science B.V. All rights r eserved.