Determination of Gas-Phase Ligand Binding Energies to Mg+ by FTMS Techniques

Article abstract of DOI:10.1021/ja00220a020

Relative and absolute gas-phase binding energies of 12 organic molecules to Mg⁺ have been determined by four different methods involving Fourier transform mass spectrometry (FTMS).Mg⁺ is generated in the FTMS cell by pulsed laser ionization and reacts with the organic compounds to give MgL⁺ either by direct condensation or by a fast two-step displacememt reaction.The equilibrium constant (K_eq) and the absolute rate constants (k_f and k_r) are obtained for the ligand exchange reaction: MgL⁺ + L' <*> MgL'⁺ + L.The equilibrium constants are converted to free-energy differences and relative binding energies are calculated to give the D^o(Mg⁺-L) scale.Entropy changes are assumed to be small and they are discussed in detail.The absolute gas-phase binding energies D^o(Mg⁺-MeOH) and D^o(Mg⁺-Me2CO) are assigned from equilibrium and photodissociation experiments to be 61 +/- 5 and 67 +/- 5 kcal/mol, respectively.The correlation of results from the different methods give a scale of relative and absolute bond energies to Mg⁺ for the 12 organic molecules examined.The results for Mg⁺ are compared with previously reported results on H⁺, CpNi⁺, Al⁺, Mn⁺, and Cu⁺.These comparisons show for this limited study on oxygen bases that Mg⁺ is a softer acid than H⁺ and Al⁺, very similar in acidity to Mn⁺ and Cu⁺, and a harder acid than CpNi⁺.