Bond Energies of Copper Ion−Ligand L Complexes CuL2+ Determined in the Gas Phase by Ion−Ligand Exchange Equilibria Measurements

The free energy changes, Δ G 2, for the reaction CuL 2 + = Cu + + 2L were obtained in the gas phase for some 23 different ligands L. These results were based on the determination of ligand exchange equilibria of the type CuA 2 + 2B = CuB 2 + 2A. The equilibria were observed in a gas-phase ion−molecule reaction chamber sampled with a mass spectrometer. The entropy changes were determined by evaluation of the entropies S °, of the reactants from vibrational frequencies and moments of inertia obtained with HF/3-21G* basis sets. Combining and one obtained also the values. The range of values extended from 72 kcal/mol for the most weakly bonded ligand ethyl chloride to 135 kcal/mol for the most strongly bonding one, 1-methylimidazole. The results provide a partial confirmation of the Hard and Soft Acids and Bases (HSAB) principle. Comparison with available data for M + = Li + and Ag + show that soft bases such as Me 2 S bond relatively more strongly to soft acids such as Cu + and Ag + . However the actual bond energies are affected also by other interactions such as electrostatic contributions due to ion−ligand dipole attractive forces. Several of the ligands used correspond quite closely to the functional groups present on peptide residues. Therefore, a partial scale of peptide residue bonding to Cu + can be established. Histidine is found to be the most strongly bonding residue.