Structural and thermodynamic characteristics of ionic associates in vapors over sodium bromide and iodide

Abstract

Nonempirical methods are used to calculate the geometric parameters, the frequencies of normal vibrations, and thermochemical characteristics of ions existing in saturated vapors over sodium bromide and iodide: Na₂X⁺, NaX ⁻₂ , Na₃X ⁺₂ , and Na₂X ⁻₃ (X = Br, I). According to the calculations, Na₂X⁺ and NaX ⁻₂ triatomic ions have a linear equilibrium configuration of D _∞h symmetry. Pentaatomic ions can exist in the form of three isomers: linear with D _∞h symmetry, planar cyclic with C _2v symmetry, and bipyramidal with D _3h symmetry. At a temperature of ∼1000 K, Na₃X ⁺₂ and NaX ⁻₃ pentaatomic ions are shown to be present in vapor mainly in the form of linear isomers. The energies and enthalpies of ion molecular reactions with the participation of the above ions are calculated, and the formation enthalpies of the ions are determined, Δ _f H ^o(0 K): 293±2 kJ/mol (Na₂Br⁺), 354±2 kJ/mol (Na₂I⁺), −536±2 kJ/mol (NaBr ⁻₂ , −458±2 kJ/mol (NaI ⁻₂ , 24±5 kJ/mol (Na₃Br ⁺₂ , 143±5 kJ/mol (Na₃I ⁺₂ , −810±5 kJ/mol (Na₂Br ⁻₃ , and −675±5 kJ/mol (Na₂I ⁻₃ .