Hydration energies of calcium hydroxide cation, CaOH+(H2O)x (x = 1–6), studied using guided ion beam tandem mass spectrometry

Hydration energies of CaOH⁺(H₂O)_x, x = 1–6, were obtained using threshold collision-induced dissociation (TCID) with xenon (Xe) as conducted with a guided ion beam tandem mass spectrometer (GIBMS). The primary reaction pathway observed for all complexes is the loss of one water ligand, followed by the loss of additional water molecules at higher collision energies for x > 1. The kinetic-energy-dependent cross sections for dissociation of CaOH⁺(H₂O)_x complexes were modeled to obtain 0 K binding energies after accounting for lifetime effects, energy distributions, and pressure effects. Except for x = 5, experimental threshold energies measured through TCID agree well with theoretical hydration energies determined using B3LYP/6-311+G(d,p) structure geometries followed by single point energies calculated at B3LYP, B3P86, M06, and MP2(full) levels of theory with a 6-311+G(2d,2p) basis set. B3LYP-GD3BJ and ωB97XD calculations were also used, with the former yielding results having the best agreement with the threshold energies extracted from the analysis of the TCID cross sections.