Pulsed-field ionization electron spectroscopy and ab initio calculations of copper-diazine complexes

Copper complexes of pyrazine (1,4-C ₄H ₄N ₂), pyrimidine (1,3-C ₄H ₄N ₂), and pyridazine (1,2-C ₄H ₄N ₂) are produced in laser-vaporization supersonic molecular beams and studied by pulsed-field ionization zero electron kinetic energy (ZEKE) spectroscopy and second-order Møller-Plesset perturbation theory. Both σ and π complexes are considered by these ab initio calculations; only σ structures are identified in these experiments. Adiabatic ionization energies and metal-ligand vibrational frequencies of the σ complexes are measured from the ZEKE spectra. Metal-ligand bond dissociation energies of these complexes are obtained from a thermochemical cycle. The ionization energies follow the trend of Cu pyridazine (43 054 cm ^-1) < Cu pyrimidine (45 332 cm ^-1)<Cu pyrazine (46038 cm ^-1); the bond energies are in the order of Cu pyridazine (56.2 kJ mol ^-1) > Cu pyrazine (48.5 kJ mol ^-1) ∼Cu pyrimidine (46.4 kJ mol ^-1). The stronger binding of pyridazine is due to its larger electric dipole moment and possibly bidentate binding.