Structure and thermodynamic properties of positive and negative cluster ions in saturated vapour over barium dichloride

Geometrical structure, vibration spectra, and enthalpies of dissociation have been investigated for the ions BaCl₃^?, Ba₂Cl₃⁺, Ba₃Cl₅⁺, and Ba₄Cl₇⁺ which were detected earlier in the saturated vapour over BaCl₂. Quantum chemical methods of density functional theory, the second and the fourth order Møller–Plesset perturbation theory have been applied. The effective core potential with cc-pVTZ basis set for barium atom and two full-electron basis sets including the diffuse and polarised basis functions for chlorine atom were used. The effect of the basis set size and the computation method on the results was analysed. According to the results, all the ions possess the compact shaped structure. The equilibrium geometrical structures were found as follows: the planar D_3h for BaCl₃^?, triple bridged bipyramidal D_3h for Ba₂Cl₃⁺, hexabridged D_3h for Ba₃Cl₅⁺, and septuple bridged C_2v for Ba₄Cl₇⁺. For positive ions, the different isomeric structures were considered, but no isomers for these ions have been found. The geometrical parameters and vibration frequencies were utilised for computing of thermodynamic functions of the ions, and then the thermodynamic functions were used for the treatment of the experimental mass spectrometric data. The enthalpies of formation Δ_fH°(0 K) of the ions were determined (in kJ/mol): ?994 ± 6 (BaCl₃^?), ?481 ± 10 (Ba₂Cl₃⁺), ?1276 ± 14 (Ba₃Cl₅⁺), ?2048 ± 35 (Ba₄Cl₇⁺).