SOLVENT STRUCTURE AROUND CATIONS DETERMINED BY H-1 ENDOR SPECTROSCOPY AND MOLECULAR-DYNAMICS SIMULATION

Classical molecular dynamics simulations of metal ion solvation have been found to confirm the experimental H-1 ENDOR and EPR spectroscopic results on the coordination structure of water molecules surrounding paramagnetic Mn2+ ions in frozen aqueous solutions. The well-known simplification and intensification of the six-line Mn2+ EPR signal in frozen solutions upon reducing the pH was found to coincide with the appearance of three strong H-1 ENDOR resonances, reflecting symmetrization of the first two solvation shells. From the dipolar part of the hyperfine interaction, the Mn-H distances for the first and second shells were determined to be 2.87 and 4.8 angstrom, respectively. All protons in the first shell were located at the same distance. Classical molecular dynamics simulations confirmed the experimental distances to both shells using a three-point flexible water model which included electrostatic and van der Waals interactions to Mn2+. At 50 K, the structured second shell of the Mn-H radial distribution function is comprised of 18 water molecules, reflecting disorder which becomes dynamically averaged at 300 K as the second shell expands to encompass on average 22 H2O molecules at a mean distance of 5.0 angstrom. The ENDOR and dynamics results support the proposed model of second-shell solvent fluctuation of the Mn2+ ligand field splitting as the mechanism for electron spin relaxation of Mn2+. These results should encourage the use of classical molecular dynamics simulations to predict solvation structure of nonparamagnetic ions for which experimental data are not readily available.