Relative and absolute bond dissociation energies of sodium cation complexes determined using competitive collision-induced dissociation experiments

Absolute L₁Na⁺-L₂ and relative Na⁺-L bond dissociation energies are determined experimentally by competitive collision-induced dissociation of L₁Na⁺L₂ complexes with xenon in a guided ion beam mass spectrometer. The ligands examined include H₂O, C₆H₆, CH₃OH, CH₃OCH₃, NH₃, and C₂H₅OH, which cover a range in Na⁺ affinities of only 20 kJ/mol. Dissociation cross sections for formation of Na⁺L₁ + L₂ and Na⁺L₂ + L₁ are simultaneously analyzed with a model that uses statistical theory to predict the energy dependent branching ratio. The use of independent and common scaling factors for each channel in this analysis is evaluated and discussed, as is the importance of properly handling ligand internal rotors. The cross section thresholds thus determined are interpreted to yield the 0 K L₁Na⁺-L₂ bond dissociation energies and the relative 0 K Na⁺-L binding affinities. The relative binding affinities are converted to absolute 0 K Na⁺-L binding energies by using the absolute bond energy for Na⁺-NH₃, determined previously in our laboratory, as an anchor value. Comparisons are made to previous experimental and theoretical Na⁺-L thermochemistry from several sources. The absolute L₁Na⁺-L₂ bond dissociation energies were also calculated using ab initio theory at the MP2(full)/6-311+G(2d, 2p)//MP2(full)/6-31G* level (corrected for zero-point energies and basis set superposition errors) and are in good agreement with the experimentally determined values.