A theory for cross-polarization NMR of nonspinning and spinning samples

A theoretical model for the description of cross-polarization (CP) experiments on a spin system, consisting of a single spin S = 1/2 coupled to a set of N coupled homonuclear spins with I = 1/2, presented. The basic principles of the theoretical approach are first applied to a static spin system and then extended to a system that rotates at the magic angle. For the rotating spin system, the Floquet theory approach is utilized. In particular, the CP characteristics of fast rotating samples are discussed and a special effort is made to demonstrate the analogy between the CP spin dynamics for the nonspinning and the spinning case. The basic equations for the evaluation of the enhancement of CP signals are derived, and some experimental results previously presented in the literature are analyzed in terms of the model. No new experimental results are presented in this publication. The theory is designed to form the basis for the description of CPMAS experiments of spin systems with S > 1/2. (C) 1996 Academic Press, Inc.