Theory of ESR parallel-edge lines of slowly tumbling molecules

For each hyperfine component we have examined the sharp feature, called the ’’parallel-edge line,’’ which is observed at the limit of the ESR rigid powder and slow-tumbling spectra associated with those molecules in a liquid which have their symmetry axes oriented nearly parallel to that of the applied field. The parallel-edge linewidths are well determined in the second derivative ESR spectra. The motional broadening and shift of the parallel-edge lines which arise from the rotational modulation of the anisotropic molecular magnetic parameters can be separated from motionally narrowed inhomogeneous effects by taking the difference of parallel-edge linewidths and shifts of two hyperfine components. These differences in linewidths and shifts of two parallel-edge hyperfine lines are predicted to vary as τ2−1/2, where τ2 is the rotational correlation time. An analysis of the parallel-edge lines enables us to estimate slow tumbling times from ESR spectra without carrying out full-scale line shape simulations, which require an exact knowledge of motionally narrowed inhomogeneous linewidths.