S-BAND ELECTRON-SPIN ECHO SPECTROSCOPY

In order to explore the advantages of multi-frequency ESE experiments, we have constructed a pulsed EPR spectrometer operating over a microwave frequency range of 2-4 GHz (S-band). Dramatic enhancement of electron spin echo envelope modulation (ESEEM) depth is observed for weakly coupled nuclei. Well-defined resonance peaks corresponding to nearly pure NQR frequencies are observed for N-14 in some nitroaromatic systems, at values of B(o) near "exact cancellation" and more distant from the cancellation condition than had been predicted. In the case of nitrogen modulation in a copper(II) complex with large g anisotropy, strong orientation selection is achieved at S-band as well as X-band; the S-band spectrum is different and complementary.