Distance-Dependent Enhanced ENDOR Phase (DEEP) Spectroscopy and Its Application to Determination of Solvation Structure around Paramagnetic Ions in Disordered Solids: The Three Ordered Hydration Shells of Aquated Transition Ions

Abstract

We present a method that improves the spectral resolution and extends the distance range for detection of ordered solvent molecules surrounding a paramagnetic ion in frozen solutions using 1H electron−nuclear double-resonance (ENDOR) spectroscopy. This method is based on the R-3 distance dependence of the proton−electron spin hyperfine interaction and its effect on the electron−nuclear cross relaxation rate. This relaxation effect dramatically influences the ENDOR phase spectrum and appears through the radio-frequency (rf) phase angle dependence (using the conventional rf modulation detection scheme), and thus it is termed distance-dependent enhanced ENDOR phase (DEEP) spectroscopy. Applying DEEP spectroscopy to the conventional CW-ENDOR experiment, one is able to eliminate the large proton matrix peak from disordered solvent molecules. We observe a 4-fold increase in spectral resolution with DEEP spectroscopy for distant protons. When applied to solvation of aquo Mn(II) ions, we resolve 1H hyperfine coup...