Gaseous Rust: Thermochemistry of Neutral and Ionic Iron Oxides and Hydroxides in the Gas Phase

The experimental and theoretical thermochemistry of the gaseous neutral and ionic iron oxides and hydroxides FeO, FeOH, FeO2, OFeOH, and Fe(OH)(2) and of the related cationic water complexes Fe(H2O)(+), (H2O)FeOH+, and Fe(H2O)(2)(+) is analyzed comprehensively. A combination of data for the neutral species with those of the gaseous ions in conjunction with some additional measurements provides a refined and internally consistent compilation of thermochemical data for the neutral and ionic species. In terms of heats of formation at 0 K, the best estimates for the gaseous, mononuclear FeOmHn-/0/+/2+ species with m = 1, 2 and n = 0-4 are Delta H-f(FeO-) = (108 +/- 6) kJ/mol, Delta H-f(FeO) = (252 +/- 6) kJ/mol, Delta H-f(FeO+) = (1088 +/- 6) kJ/mol, Delta H-f(FeOH) = (129 +/- 15) kJ/mol, Delta H-f(FeOH+) = (870 +/- 15) kJ/mol, Delta H-f(FeO2-) = (-161 +/- 13) kJ/mol, Delta H-f(FeO2) = (67 +/- 12) kJ/mol, Delta H-f(FeO2+) = (1062 +/- 25) kJ/mol, Delta H-f(OFeOH) = (-84 +/- 17) kJ/mol, Delta H-f(OFeOH+) = (852 +/- 23) kJ/mol, Delta H-f(Fe(OH)(2)(-)) -431 kJ/mol, Delta H-f(Fe(OH)(2)) = (-322 +/- 2) kJ/mol, and Delta H-f(Fe(OH)(2)(+)) = (561 +/- 10) kJ/mol for the iron oxides and hydroxides as well as Delta H-f(Fe(H2O)(+)) = (809 +/- 5) kJ/mol, Delta H-f((H2O)FeOH+) = 405 kJ/mol, and Delta H-f(Fe(H2O)(2)(+)) = (406 +/- 6) kJ/mol for the cationic water complexes. In addition, charge-stripping data for several of several-iron-containing cations are re-evaluated due to changes in the calibration scheme which lead to Delta H-f(FeO2+) = (2795 +/- 28) kJ/mol, Delta H-f(FeOH2+) = (2447 +/- 30) kJ/mol, Delta H-f(Fe(H2O)(2)(+)) = (2129 +/- 29) kJ/mol, Delta H-f((H2O)FeOH2+) = 1864 kJ/mol, and Delta H-f(Fe(H2O)(2)(2+)) = (1570 +/- 29) kJ/mol, respectively. The present compilation thus provides an almost complete picture of the redox chemistry of mononuclear iron oxides and hydroxides in the gas phase, which serves as a foundation for further experimental studies and may be used as a benchmark database for theoretical studies.