Electron spectroscopy, molecular structures, and binding energies of Al- and Cu-imidazole

Al- and Cu-imidazole are produced in laser-vaporization supersonic molecular beams and studied with pulsed field ionization-zero electron kinetic energy (ZEKE) spectroscopy and second-order Møller-Plesset (MP2) theory. The sigma and pi structures of these complexes are predicted by MP2 calculations, but only the sigma structures are identified by the experimental measurements. For these sigma structures, adiabatic ionization energies and several vibrational frequencies are measured from the ZEKE spectra, the ground electronic states of the neutral and ionized complexes are determined by comparing the observed and calculated spectra, and the metal-ligand bond dissociation energies of the neutral states are derived by using a thermochemical relation. The measured vibrational modes include the metal-ligand stretch and bend and ligand ring distortions. The metal-ligand stretch frequencies of these transient complexes are compared with those of coordinately saturated, stable metal compounds, and the ligand-based distortion frequencies are compared with those of the free ligand. Al-imidazole has a larger bond dissociation energy than Cu-imidazole, although the opposite order was previously found for the corresponding ions. The weaker bonding of the Cu complex is attributed to the antibonding interaction and the electron repulsion between the Cu 4s and N lone-pair electrons.