Chapter 4: Local Fields, Motional Averaging and Relaxation Limits Check Access

Motional averaging of secular spin interactions is an important phenomenon in NMR experiments in general. If it is incomplete, local fields arise due to residual couplings. The condition for motional averaging is specified and discussed for dipolar and quadrupolar spin interactions. Local fields are the reference for the high-field limit of spin relaxation, i.e. for the dominance of the external magnetic flux density. Residual secular spin interactions are also responsible for the so-called demagnetizing field of dipolar coupled spin systems at long distances. At low magnetic fields, the Redfield condition may limit interpretations based on perturbation-theoretical relaxation theories. At the end of the chapter, a figure summarizing the motional-averaging conditions and relaxation limits for restricted and unrestricted molecular dynamics is presented.