Dong-Sheng Yang , University of Kentucky
In this reporting period, we published eight peer-reviewed papers on the spectroscopy, structures and energetics of metal clusters and metal-organic radicals. The metal clusters and complexes were produced in pulsed laser vaporization molecular beams 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, molecular conformations, adiabatic ionization energies, metal-ligand and ligand-based vibrational frequencies, and metal-ligand bond dissociation energies.
We have obtained for the first time the vibrationally resolved MATI spectra for several small lanthanum clusters: LaO, La2, LaO2, La2O2, La3O2, and La3O4, and determined the structures and electronic states for some of these clusters. Identification of the ground electronic states and molecular structures of transition metal clusters challenges experimentalists and theorists because of the high-density of low-lying spin states arising from the metal d electrons. Dilanthanum may be among the simplest homonuclear diatomics, since the ground electron configuration of the metal atom (6s25d1) has only one unpaired electron. In spite of this seemly simplicity, the ground electronic states have not yet been established for the neutral molecule, and low-lying electronic states have not been reported for the singly charged cation. The MATI spectrum exhibits three band systems originating from ionization of the neutral ground electronic state, and each system shows vibrational frequencies of the neutral molecule and singly charged cation. The three ionization processes are La2+ (2∑g+) ¬ La2 (1∑g+), La2+ (2Π1/2, u) ¬ La2 (1∑g+), and La2+ (2Π3/2, u) ¬ La2 (1∑g+). The singlet state is the ground state of the neutral molecule, the doublet states are the excited states of the cation. A quartet state was predicted by theory, but not observed by the experiment. The determination of a singlet ground state of La2 shows that lanthanum behaves differently from scandium and yttrium. LaO2 probed by our MATI experiment is in an excited state and the transition is 3B2 (LaO2+) ¬ 4B2 (LaO2). The ionization energy of the 4B2 state is 40134 cm-1, the La-O stretching frequency in the 3B2 state is 656 cm-1, and the O-La-O bending frequencies are 122 and 92 cm-1 in the ion and neutral states, respectively. LaO2 and LaO+ are both in a bent structure with C2v symmetry. For La3O4, the ionization energy of the 2A1 ground state is determined to be 28028 cm-1, two La-O stretching modes and two O-La-O bending modes of the 1A1 ion state are measured as 422/533 and 206/252 cm-1. The cluster has a C3v cage-like structure with alternating La-O-La bonds in both 2A1 and 1A1 states. Three of the four oxygen atoms are bonded to two lanthanum atoms, while the other oxygen is bonded to three lanthanum atoms. Each lanthanum forms three bonds with oxygens, consistent with the +3 formal oxidation state. The neutral cluster and cation have essentially the same structure, with only small differences in bond lengths and angles.
2. Metal-Aromatic Complexes
a) Electronic states and metal-ligand bonding of Gadolinium complexes of benzene and cyclo-octatetraene. The ZEKE spectra of Gd(C6H6) and Gd(C8H8) are rather similar and show a strong origin band and a major metal-ligand stretching vibration. The Gd(C6H6) complex prefers a higher electron spin state (11A2), with the metal-ligand bonding mainly from the covalent interaction, whereas Gd(C8H8) favors a lower spin state (9A2), with the major metal-ligand bonding from the charge-charge interaction. The covalent contribution to the Gd(C6H6) bonding results largely from the interaction between the metal 5d/6s and benzene p orbitals; the electrostatic contribution to the Gd(C8H8) bonding arises from the two-electron transfer from Gd to C8H8, which creates the charge-charge interaction between Gd2+ and COT2-. Although they play very little role in the formation of these complexes, Gd 4f orbitals contribute to the high spin electron multiplicities.
b) Conformational isomers and isomerization of group 6 (Cr, Mo, and W) metal-bis(toluene) sandwich complexes. For Cr-bis(toluene), four rotational conformers were identified with methyl-group dihedral angles of 0o, 60o, 120o, and 180o. The ground electronic states of these conformers are 1A1 (C2v, 0o), 1A (C2, 60o and120o), and 1Ag (C2h, 180o) in the neutral form and 2A1 (C2v, 0o), 2A (C2, 60o and 120o), and 2Ag (C2h, 180o) in the singly charged cationic form. For Mo- and W-bis(toluene), the four rotamers were resolved into three (0o, 60o/120o, and 180o) and two (0o and 60o/120o/180o) groups, respectively. For all three metal sandwiches, the most stable conformer is in the complete eclipsed configuration (0o) and has the highest ionization energy. The conversion from 60o/120o/180o to 0o rotamer was observed from helium to argon supersonic expansions and was more efficient for the heavier Mo and W species.
c) Preferential metal-binding sites and thermochemistry of lithium complexes of polycyclic aromatic hydrocarbons. Mechanisms of lithium (Li) storage in carbonaceous materials have received considerable attention because of their importance in developing high-density Li rechargeable batteries. Polycyclic aromatic hydrocarbon (PAH) molecules are model systems that can provide new insight into the precise Li binding site and thermochemistry of the carbon-based materials. The adiabatic ionization energies of the neutral complexes and frequencies of up to nine vibrational modes in the singly charged cations were determined from the ZEKE spectra. The metal-ligand bond energies of the neutral complexes were obtained from a thermodynamic cycle. Preferred Li/Li+ binding sites with the aromatic molecules were determined by comparing the measured spectra with theoretical calculations. Li and Li+ prefer the ring-over binding to the benzene ring with a higher p-electron content and aromaticity. Although the ionization energies of the Li complexes show no clear correlation with the size of the aromatic molecules, the metal-ligand bond energies increase with the extension of the p electron network up to perylene, then decrease from perylene to coronene.