Chapter 8 - Relaxation

Abstract

This chapter describes the molecular motions and processes for relaxation in liquids. Effective interaction between a precessing nuclear magnetization and its surroundings requires an oscillating magnetic field at the Larmor frequency of the nucleus. It is recognized that in liquids or gases the oscillating field can come from molecules that are in rapid, random motion, a process well known as Brownian motion. Although such motions are incoherent, they can be Fourier analyzed to obtain the “spectrum” of frequencies present in the random motions. The effectiveness of the fluctuating field in bringing about relaxation depends on the Fourier component of motion at the Larmor frequency, but it also depends on the square of the matrix element describing the interaction energy. Spin–rotation interaction can furnish a means for relaxation for molecules in gases or in highly mobile liquids. Paramagnetic relaxation, which involves magnetic dipolar interaction between a nuclear spin and an unpaired electron spin, is an efficient path for relaxation when even a small amount of paramagnetic material is present.