Temperature effects on prevalent structures of hydrated Fe⁺ complexes: Infrared spectroscopy and DFT calculations of Fe⁺(H₂O)_n (n = 3-8)

Hydrated Fe⁺ ions are produced in a laser-vaporization cluster source of a triple quadrupole mass spectrometer. The Fe⁺(H₂O)_n (n = 3-8) complexes are mass-selected and probed with infrared (IR) photodissociation spectroscopy in the OH-stretch region. Density functional theory (DFT) calculations are also carried out for analyzing the experimental IR spectra and for evaluating thermodynamic quantities of low-lying isomers. Solvation through H-bonding instead of direct coordination to Fe⁺ is observed already at n = 3, indicating the completion of the first hydration shell with two H₂O molecules. Size dependent variations in the spectra for n = 5-7 provide evidence for the second-shell completion at n = 6, where a linearly coordinated Fe⁺(H₂O)₂ subunit is solvated with four H₂O molecules. Overall spectral features for n = 3-8 agree well with those predicted for 2-coordinated structures. DFT calculations predict that such 2-coordinated structures are lowest in energy for smaller n. However, 4-coordinated isomers are predicted to be more stable for n = 7 and 8; the energy ordering is in conflict with the IR spectroscopic observation. Examination of free energy as a function of temperature suggests that the ordering of the isomers at warmer temperatures can be different from the ordering near 0 K. For n = 7 and 8, the 4-coordinated isomers should be observed at low temperatures because they are lowest in enthalpy. Meanwhile, outer-shell waters in the 2-coordinated structures are bound less rigidly; their contribution to entropy is rather large. The 2-coordinated structures become abundant at warmer temperatures, owing to the entropy effect.