FIELD-DEPENDENT ISOTROPIC SHIFTS IN SOLID-STATE NUCLEAR-MAGNETIC-RESONANCE - A FLOQUET TREATMENT

The apparent field dependence observed for some isotropic shifts in hi gh resolution solid state NMR is reinvestigated using the Floquet form alism. For C-13-H-1 systems, second order dipolar shifts are derived t hat largely agree with the experimental observations made by VanderHar t and his theoretical treatment while resolving some difficulties in t he use of a secular approximation to handle an inherently time-depende nt problem. Additional second order terms are shown to give rise to bo th frequency shifts and line broadening which can be significant in so me spin pairs. This line broadening is found to arise from a second or der coupling of the rf field with the I-S dipolar interaction.