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

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 couplings from molecules in the first shell at 2.90 Å (Mn-H), two distinguishable groups of molecules in the second solvation shell at 4.37 and 5.05 Å, and a less-ordered third shell of protons at 7.5 Å. The structure in the second shell and the presence of the third shell of ordered water molecules are the first observations of this structure and are supported by previous molecular dynamics simulations of aquo Mn(II) ions in aqueous solutions [Sivaraja et al. J. Am. Chem. Soc. 1992, 114, 9600-9603]. DEEP spectroscopy holds promise for studies of solvation to other paramagnetic ions, including Gd(III) and those used as MRI contrast agents.