title: Temperature effects on prevalent structures of hydrated Fe⁺ complexes: infrared spectroscopy and DFT calculations of Fe⁺(H₂O)n (n = 3-8).
abstract: 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.
authors.affiliation: Department of Chemistry, Faculty of Sciences, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan.
authors.forename: Kazuhiko
authors.initials: K
authors.lastname: Ohashi
authors.affiliation: Department of Chemistry, Graduate School of Sciences, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan.
authors.forename: Jun
authors.initials: J
authors.lastname: Sasaki
authors.affiliation: Department of Chemistry, Graduate School of Sciences, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan.
authors.forename: Gun
authors.initials: G
authors.lastname: Yamamoto
authors.affiliation: Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan.
authors.forename: Ken
authors.initials: K
authors.lastname: Judai
authors.affiliation: Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan.
authors.forename: Nobuyuki
authors.initials: N
authors.lastname: Nishi
authors.affiliation: Department of Chemistry, Faculty of Sciences, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Sekiya