Hydration Energies of Iron Hydroxide Cation: A Guided Ion Beam and Theoretical Investigation

We present experimental collision-induced dissociation (CID) cross sections as a function of kinetic energy for FeOH⁺(H₂O) _n, where n = 1-4, with xenon (Xe) obtained using a guided ion beam tandem mass spectrometer. Complexes with n = 2-4 are observed to undergo water loss, followed by sequential water loss at higher collision energies. In addition, we find that loss of the neutral hydroxide group is competitive with the primary water loss for n = 1. Bond dissociation energies (BDEs) at 0 K are derived through modeling of the experimental cross sections after accounting for multiple collisions, kinetic shifts, and reactant internal and kinetic energy distributions. Quantum chemical calculations include geometry optimizations performed at the B3LYP/6-311+G(d,p) level of theory and then used for single point calculations at B3LYP, B3P86, MP2, and CCSD(T) levels with a 6-311+G(2d,2p) basis set. Additional geometry optimizations at the cam-B3LYP/def2-TZVP were also performed as well as empirical dispersion corrections at all levels. The various structures for the FeOH⁺(H₂O) _n complexes and their relative energies are discussed in detail. We also derive experimental BDEs for the OH loss from FeOH⁺(H₂O) _n, with n = 2-4, using the experimental BDE of n = 1 in combination with literature data for water loss from Fe⁺(H₂O) _n species. Measurements of BDEs for hydroxide and water loss from FeOH⁺(H₂O) _n ( n = 1-4) are the first such experimental measurements. Theoretically calculated BDEs are in reasonable agreement for water loss from both FeOH⁺(H₂O) _n and Fe⁺(H₂O) _n complexes and for D₀(Fe⁺-OH) but are too low for the loss of OH from the larger hydrated complexes.