Phase Dependence of Saturation Transfer EPR Signals and Estimated Rotational Correlation Times

Abstract

The second harmonic absorption out-of-phase signal (V₂′ mode) is usually observed in saturation transfer electron paramagnetic resonance spectroscopy to determine slow rotational correlation times. It is known that adjustment of 90° out-of-phase is critically important to detect accurate ST-EPR spectra. The digitized ST-EPR spectrometer using the Fourier transformation method developed by the authors has enabled determination of the phase dependence of ST-EPR signals simply by calculating V₂′ signals under the condition of various phase deviations from exactly 90° out-of-phase. The overall 90° out-of-phase accuracy of the present system was confirmed to be within 0.3°. Phase dependence of both the ST-EPR parameters and the correlation times estimated from them was quantitatively discussed. Conclusively, accuracy of phase-adjustment is required to be within ±0.5° for less than 10%, and ±2° or 3° for less than approximately 30% errors on the correlation times. For determination of correlation times the parameter L″/L provides better accuracy than the parameter C′/C, even if 90° out-of-phase is set with some phase deviation.