57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

In this reporting period, we published six peer-reviewed papers on the spectroscopy, structures and energetics of metal clusters and metal-organic radicals. The metal clusters and complexes were produced in a pulsed laser vaporization molecular beam source and studied by pulsed field ionization-zero electron kinetic energy (ZEKE), mass-analyzed threshold ionization (MATI), and infrared-ultraviolet (IR-UV) ionization spectroscopy and electronic structure calculations.  These studies determined electronic states, structural arrangements, accurate ionization energies, metal-ligand and ligand-based vibrational frequencies, and metal-ligand bond dissociation energies.  The new knowledge obtained from this work is of relevance to cluster and surface sciences, organometallic chemistry, and chemical catalysis. The spectroscopic data provide benchmarks to test new or improved computation chemistry methods. Moreover, the project provided training of students in laser spectroscopy, mass spectrometry, and computational chemistry and in the design, operation, and interpretation of scientific experiments. The ACS-PRF grant provided us with an opportunity to explore a new research direction and had a significantly positive impact on our research program.

In spite of extensive studies, the current knowledge about the electronic states and molecular structures of metal clusters and oxides is largely derived from theoretical predictions. However, a reliable prediction of the electronic states and geometric structures is often complicated by the presence of many low energy structural isomers and a high density of low-lying electronic states of each isomer and generally requires confirmation by spectroscopic measurements. We investigated structures and electronic states of MO₂, M₂O₂, and M₃O₄, (M = Sc, Y, La, Ce, Pr, Pr, Ho, Tm, or Lu). Spectral analysis for some of these systems is still in progress, the results of MO₂, M₂O₂, and M₃O₄ (M = Sc, Y, and La) are summarized below.

For a MO₂ molecule, its structure may be linear or bent. A linear structure could have a D_∞h symmetry (OMO, dioxide) or a C_∞v symmetry (MOO, superoxide), whereas a bent structure could have a C_2v symmetry (OMO, dioxide; M(O₂), peroxide) or a C_s symmetry (MOO, superoxide). The MATI spectrum of LaO₂ exhibits a strong origin band, La⁺-O stretching and O-La⁺-O bending progressions, and thermal excitation of the O-La-O bending mode. Our combined experimental and computational analysis shows that the spectrum involves the ³B₂ ← ⁴B₂ transition of the bent lanthanum dioxide. ⁴B₂ is an excited state of the neutral molecule (~ 3.40 eV above the ground state), is formed by transferring two La electrons to oxygen atoms, and has a valence electron configuration consisting of oxygen 2p- and lanthanum 6s-based orbitals. ³B₂ is an excited state of the cation and formed by the removal of the non-bonding La 6s electron, and has a molecular geometry similar to the ⁴B₂ neutral state. This is the first example where a single-photon MATI spectrum is observed from an excited initial electronic state lying several electronvolts above the ground electronic state.

For M₂O₂ (M = Sc, Y, and La), four structural isomers were considered and a planar D_2h structure was identified with alternating M-O-M bonds. While the ground states (²A_g) of the M₂O₂⁺ ions are the same for the three metals, the ground state of the Sc₂O₂ (¹A_g) neutral molecule appears to be different from those of Y₂O₂ and La₂O₂ (³B₁). To form the M₂O₂ clusters, each M atom gives up two electrons [nd and (n+1)s] and the electron spin multiplicity of the ground state depends on strength of the interaction between the remaining (n+1)s electrons on the M atoms.

For M₃O₄ (M = Sc, Y, and La), five possible isomers were considered and a cage-like structure in C_3v point group was identified as the most stable one. The cage-like structure of M₃O₄ contains alternative M-O-M-O bonds and is formed by joining three M₂O₂ fragments together, each sharing two O-M bonds with others. One of the four oxygen atoms is bonded to three metal atoms and the other three are bonded to two metals, whereas each metal atom forms at least three bonds with oxygen atoms. Molecular orbital analysis shows that the three metal atoms in the ²A₁ neutral ground state of M₃O₄ lose a total of eight electrons in forming the cluster and each metal has a +2.67 oxidation state. Ionization of the ²A₁ state yields the ¹A₁ ion by removing a metal-based s electron. For the ¹A₁ sate, five vibrational modes are measured for all three clusters, and additional two modes are observed for Sc₃O₄.