Group 3 metal clusters are synthesized by laser vaporization in a pulsed cluster beam source and identified with laser ionization time-of-flight mass spectrometry. The adiabatic ionization energies and vibrational frequencies of these clusters are measured using mass-analyzed threshold ionization (MATI) spectroscopy. Their structures and electronic states are determined by combining the MATI spectra with quantum chemical calculations and spectral simulations. This dissertation focuses on the study of several small molecules, which include LaO2, La2, M2O2, M3O4, M3C2, and La3C2O, where M = Sc, Y, and La. Except for La2, these molecules exhibit strong ionic characters between the metal and oxygen or carbon atoms and can be described as [O-][La2+][O-], [M2+]2[O2-]2, [M8/3+]3[O2-]4, [M2+]3[C3-]2, and [La8/3+]3[C3-]2[O2-]. The interactions between the metal atoms form covalent bonds, which can be described by a triple bond in La2, a two-center two electron bond in M2O2, a three-center one electron bond in M3O4, and a three-center three electron bond in M3C2. In addition, the electron in the non-bonding highest occupied molecular orbital (HOMO) is localized in the La 6s orbital in LaO2 and La3C2O. The ground states of these molecules are all in low electron-spin states with the spin multiplicities of 1 or 2. Although the ground electronic state of LaO2 is a linear structure, the excited quartet state of the molecule is determined to be a bent structure. M2O2 and M3O4 have the planar rhombic and cage-like structures, respectively; whereas M3C2 has a trigonal bipyramid structure. La3C2O is formed by oxygen binding with two La atoms of La3C2. Ionization removes a metal-based (n+1)s electron in all neutral molecules, and the resultant ions have similar geometries to those of the corresponding neutral states. In the case of La2, additional ionization of a La 5d electron is also observed.

Metal-hydrocarbon complexes were prepared in a laser-vaporization molecular beam source and studied by single-photon zero electron kinetic energy (ZEKE) and mass-analyzed threshold ionization (MATI) spectroscopy. The ionization energies and vibrational frequencies of the metal complexes were measured from the ZEKE and MATI spectra. Metal-ligand bonding and low-lying electronic states of the neutral and ionized complexes were analyzed by combining the spectroscopic measurements with quantum chemical calculations and spectral simulations. In this dissertation, the metal complexes of mesitylene, aniline, cyclooctatetraene, benzene, ethene, and propene were studied. For each complex, different effects from metal coordination have been identified. Although metal-bis(mesitylene) sandwich complexes may adopt eclipsed and staggered conformations, the group VI metal-bis(mesitylene) complexes are determined to be in the eclipsed form. In this form, rotational conformers with the methyl group dihedral angles of 0 and 60° are identified for the Cr complex, whereas the 0° rotamer is observed for the Mo and W species. The unsuccessful observation of the 60° rotamer for the Mo and W complexes is the result of its complete conversion to the 0° rotamer in both He and He/Ar carriers. For group III metal aniline complexes, the ZEKE spectrum of each complex exhibits a strong origin band, a short M+-aniline stretching progression, and several low-frequency ligand based vibrational modes. The intensities of most of the transitions can be explained by the Franck-Condon (FC) principle within the harmonic approximation. However, the intensity of the low frequency out-of-plane ring deformation mode is greatly overestimated by the FC calculations and may be caused by the anharmonic nature of the mode. Although aniline offers two possible binding modes for the metal atoms, a п binding mode is identified with the metal atom over the phenyl ring. For Ce, Pr, and Nd(cyclooctatetraene) complexes multiple band systems are observed. This is assigned to the ionization of several low-lying electronic states of the neutral complex. This observation is different from the Gd(cyclooctatetraene) complex, for which a single band system is observed. The presence of the multiple low-energy electronic states is caused by the splitting of the partially filled lanthanide 4f orbitals in the ligand field. The ZEKE spectrum of the Gd(benzene) complex exhibits a strong origin band, whereas the spectrum of Lu(benzene) displays a weak one. The benzene ring is planar in the Gd complex, but bent in the Lu complex. Dehydrogenation and C-C coupling products are observed in the reaction of La atom and ethene/propene. For the La and ethene reaction, La(C2H2) and La(C4H6) complexes are identified. With propene, C-H bond activation leads to the formation of the La(C3H4) and H-La(C3H5) complexes, whereas the C-C coupling yields the La(trimethylenemethane) complex. In addition, the La(CHCCH3) and La(CHCHCH2) isomers of La(C3H4) are observed, which are produced by the 1,2- and 1,3-hydrogen elimination of propene.

Lanthanum-mediated bond activation reactions of small hydrocarbon molecules, including alkenes, alkynes, and alkadienes, were carried out in a laser vaporization metal cluster beam source. Time-of-flight mass spectrometry and mass-analyzed threshold ionization (MATI) spectroscopy, in combination with quantum chemical and multi-dimensional Franck-Condon factor calculations, were utilized to identify the reaction products and investigate their geometries, electronic structures, and formation mechanisms. La-hydrocarbon association was only observed in the reaction of La with isoprene. C-H bond activation was observed in all reactions, hydrogen elimination was observed as the prominent reaction for the alkenes (2-butene, isobutene, 1-pentene, and 2-pentene), alkynes (1-butyne, 2-butyne, and 1-pentyne), and 1,4-pentadiene, and C-C bond activation was observed for the five-membered hydrocarbons (1-pentene, 2-pentene, 1-pentyne, isoprene, and 1,4-pentadiene). The La-hydrocarbon radicals formed in these reactions had lanthanacyclic structures in various sizes, and each of the La-hydrocarbon complexes had a doublet ground state with a 6s1 La-based electron configuration. Ionization removed the 6s electron, and the resultant ion was in a singlet state. Formations of dehydrogenated products were either through a concerted hydrogen elimination process or the dehydrogenation after ligand isomerization. The C-C bond activation proceeded through La-assisted hydrogen migration, followed by C-C bond cleavage, or vice versa.

Lanthanum (La)-promoted hydrocarbon activation reactions were carried out in a laser vaporization metal cluster beam source. Reaction products were identified by time-of-flight mass spectrometry, and the approximate ionization thresholds of La-hydrocarbon complexes were located with photoionization efficiency spectroscopy. The accurate ionization energies and vibrational frequencies of the La complexes were measured using mass analyzed threshold ionization (MATI) spectroscopy. Their molecular structures and electronic states were investigated by combing the MATI spectroscopic measurements with quantum chemical and Franck-Condon factor calculations. In this dissertation, La-mediated C-H and C-C bond activation reactions were investigated for several small alkynes (acetylene, propyne) and alkenes (propene, 1,3-butadiene, 1-butene). The C-H bond activation was observed for both alkynes and alkenes and the C-C bond activation for alkenes. The metal-hydrocarbon intermediates formed by the C-H or C-C bond cleavage reacted further with one or more parent hydrocarbon molecules to produce larger species by C-C bond coupling reactions. Structural isomers of the intermediates and products were identified within an energy range of several kilocalories per mole. Reaction pathways for the intermediate and product formations were studied by theoretical calculations. The ground electron configuration of La atom is 4d16s2.Upon the hydrocarbon coordination, La atom is excited to a 4d26s1 configuration to facilitate the formation of two La-C bonds. After the metal-hydrocarbon complex formation, only one electron is left in the 6s orbital of the metal center. Therefore, the most stable electronic state of the La complexes studied in this work is in a doublet spin state. Ionization of the doublet state yields a preferred singlet ion state. Although La is in the formal oxidation state of +2, the ionization energies of the metal-complexes are significantly lower than that of the free atom. This observation suggests that the concept of the formal oxidation state widely used in chemistry textbooks is not useful in predicting the change of the ionization energy of a metal atom upon ligation. Moreover, ionization has a very small effect on the geometry of the hydrocarbon fragment in each complex but significantly reduces the La-C distances as a result of an additional charge interaction.