Structures and relative stability of hydrated ferrous ion clusters and temperature effects

Iron plays a key role in the evolution of living systems and so it is an essential element in a wide range of biological phenomena, energy transduction mechanisms, and oxygen carriers. The accurate determination of the ionic radius, hydration free energy or hydration enthalpy of the ferrous ion as a function of temperature requires careful determination of the structures of the complex Fe2+(H2O)(n) and their relative stability as a function of temperature. We then investigated the structures of hydrated ferrous ion clusters at the singlet and quintet spin states in the gas phase and their relative stability in a wide range of temperature (0 K and 25-400 K). All the calculations were performed at the MP2/6-31++G(d,p) level of theory. New stability rules were proposed for the various isomers investigated. The hydrated ferrous ion in the singlet and quintet spin states is hexa-coordinated, irrespective of the temperature. This result contrasts a little bit with the previously reported finding in which the CN is similar to 6 around 215 K and less at a higher temperature of 305 K. Furthermore, quintet spin state structures dominate exclusively lower spin state structures at all temperatures in such a way that no natural spin-crossover is possible between the quintet and singlet spin states. This confirms the well known experimental result which suggests that hexa-aqua-iron(ii) is in the high spin state. Finally, the saturated binding electronic energy, binding enthalpy and binding free energy per water molecule of the solvated Fe2+ ion are respectively -124.5, -128.6 and -89.7 kJ mol(-1) in water. The corresponding values in ammonia are -142.9, -144.3 and -110.9 kJ mol(-1), respectively. As far as clustering energies per solvent molecule at saturation are concerned, we reported -6.5, -6.8 and -3.2 kJ mol(-1) respectively for the saturated clustering electronic energy, clustering enthalpy and clustering free energy per water molecule of the solvated Fe2+ ion in water. The corresponding values in ammonia are -5.4, -5.5 and -2.5 kJ mol(-1), respectively. The saturation occurs for n >= 25.