Relaxation and dynamics of coupled spin systems subjected to continuous radio frequency fields

Abstract

A computational method for the prediction of spin trajectories in systems of two or more spins evolving under the influence of the static field, an applied radio frequency field, chemical shifts, spin–spin couplings, and magnetic dipolar relaxation has been described. The program has been tested to establish its correct operation by calculation of limiting cases with known solutions. It is shown that it is necessary to retain the nonsecular elements of the relaxation matrix in order to obtain correct results, primarily elements controlling cross relaxation between transverse components of magnetization of different spin states belonging to the same quantum number of the total magnetization along the axis of the static field. As a result of the retention of these terms, two unexpected features are predicted: frequency and phase shifts in spectra of two non-spin-coupled nuclei with relaxation rate comparable to frequency separation and unusually intense magnetizations in the locked state of two spins with the radio frequency applied at the center of the spectrum.