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+, NaX2−, Na3X2+, and Na2X3− (X = Br, I). According to the calculations, Na2X+ and NaX2− 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 C2v symmetry, and bipyramidal with D3h symmetry. At a temperature of ∼1000 K, Na3X2+ and NaX3− 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, ΔfHo(0 K): 293±2 kJ/mol (Na2Br+), 354±2 kJ/mol (Na2I+), −536±2 kJ/mol (NaBr2−, −458±2 kJ/mol (NaI2−, 24±5 kJ/mol (Na3Br2+, 143±5 kJ/mol (Na3I2+, −810±5 kJ/mol (Na2Br3−, and −675±5 kJ/mol (Na2I3−.