화학공학소재연구정보센터
Journal of Physical Chemistry A, Vol.114, No.5, 2189-2200, 2010
First Principles Simulation of the Bonding, Vibrational, and Electronic Properties of the Hydration Shells of the High-Spin Fe3+ Ion in Aqueous Solutions
Results of parameter-free first principles simulations of a spin up 3d(5) Fe3+ ion hydrated in an aqueous Solution (64 waters, 30 ps, 300 K) are reported. The first hydration shell associated with the first maximum of the radial distribution function, g(FeO)(r), at d(Fe-O-1) = 2.11-2.15 angstrom, contains 6 waters with average d(OH) = 0.99 angstrom, in good agreement with observations. A second shell with average coordination number 13.3 can be identified with average shell radius of d(Fe-Ou) = 4.21-4.32 angstrom. The waters in this hydration shell are coordinated to the first shell via a trigonal H-bond network with d(O-I-O-II) = 2.7-2.9 angstrom, also in agreement with experimental measurements. The first shell tilt angle average is 33.4 degrees as compared to the reported value of 41 degrees. Wannier-Boys orbitals (WBO) show in interaction between the unoccupied 3d orbitals of the Fe3+ valence (Spill Lip, 3d(5)) and the occupied spin down lone pair orbitals of first shell waters. The effect of the spin ordering of the Fe 31 ion on the WBO is not observed beyond the first shell. From this local bond analysis and consistent with other observations, the electronic structure of waters in the second shell is similar to that of a bulk water even in this strongly interacting system. H-bond decomposition shows significant bulk-like structure within the second shell for Fe3+. The vibrational density of states shows a first shell red shift of 230 cm(-1) for the v(1),2(V2),V-3 overtone, in reasonable agreement with experimental estimates for trivalent cations (300 cm(-1)). No exchanges between first and second shell were observed. Waters in the second shell exchanged with bulk waters via dissociative and associative mechanisms. Results are compared with all AIMD study of Al3+ and 64 waters. For Fe3+ the average first shell tilt angle is larger and the tilt angle distribution wider. H-bond decomposition shows that second shell to second shell H-bonding is enhanced in Fe3+ suggesting an earlier onset of bulk-like water Structure.