Pulsed field ionization electron spectroscopy and molecular structure of aluminum uracil

Al-uracil (Al-C ₄H ₄N ₂O ₂) was synthesized in a laser-vaporization supersonic molecular beam source and studied with pulsed field ionization-zero electron kinetic energy (ZEKE) photoelectron spectroscopy and density functional theory (DFT). The DFT calculations predicted several low-energy Al-uracil isomers with Al binding to the diketo, keto-enol, and dienol tautomers of uracil. The ZEKE spectroscopic measurements of Al-uracil determined the ionization energy of 43 064(5) cm ^-1 [or 5.340(6) eV] and a vibrational mode of 51 cm ^-1 for the neutral complex and several vibrational modes of 51, 303, 614, and 739 cm ^-1 for the ionized species. Combination of the ZEEK spectrum with the DFT and Franck-Condon factor calculations determined the preferred isomeric structure and electronic states of the Al-uracil complex. This isomer is formed by Al binding to the 04 atom of the diketo tautomer of uracil and has a planar C _s symmetry. The ground electronic states of the neutral and ionized species are ²A″ and ¹A′, respectively. The ²A″ neutral state has a slightly shorter Al-O4 distance than the ¹A′ ion state. However, the ¹A′ ion state has stronger metal-ligand binding compared to the ²A″ state. The increased Al-O4 distance from the ²A″ state to the ¹A′ state is attributed to the loss of the π binding interaction between Al and O4 in the singlet ion state, whereas the increased metal-ligand binding strength is due to the additional charge-dipole interaction in the ion that surpasses the loss of the π orbital interaction.