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

Abstract

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 ∼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.