Stability and structure of cluster ions: Halide ions with CO2

Abstract

Thermodynamic data, ΔH0n−1,n and ΔS0n−1,n, for clustering reactions of halide ions with CO2 were measured with a pulsed electron-beam high-pressure mass spectrometer. The large value of −ΔH00,1 (32.3 kcal/mol) and a sudden decrease of −ΔH01,2 (7.3 kcal/mol) for reaction F−(CO2)n−1+CO2=F−(CO2)n indicate the formation of the core ion, not F−⋅CO2, but FCO−2, i.e., FCO−2 is the fluoroformate ion. The measured binding energies −ΔH0n−1,n for the clusters F−(CO2)n were reproduced satisfactorily by the ab initio MO calculations. The FCO−2 ion is attacked successively and electrostatically by five CO2 molecules to form the first solvation shell. The two oxygen atoms in the core ion FCO−2 are able to accept four CO2 molecules as ligands. The bonding of CI−, Br−, and I− to CO2 is found to be mainly electrostatic.