Dong-Sheng Yang

Pulsed-field ionization electron spectroscopy of group 6 metal (Cr, Mo, and W) bis(benzene) sandwich complexes.

Group 6 metal bis(benzene) sandwich complexes (M-bz(2): M=Cr, Mo, and W and bz=C(6)H(6)) were produced with laser vaporization molecular beam techniques and studied by pulsed-field ionization zero electron kinetic energy spectroscopy and density functional theory calculations. Each sandwich complex is in a D(6h) eclipsed configuration with (1)A(1g) and (2)A(1g) as the neutral and cationic ground electronic states, respectively. The adiabatic ionization energies for Cr-, Mo-, and W-bz(2) are measured to be 44,081(7), 44,581(10), and 43,634(7) cm(-1), respectively. The metal-benzene stretch and benzene torsion frequencies of the ion are measured to be 264, 277, and 370 cm(-1) and 11, 21, and 45 cm(-1) for Cr-, Mo-, and W-bz(2), respectively. In addition, a C-H out-of-plane bending mode is measured to be 787 cm(-1) for the Cr(+)-bz(2) complex, while a C-C in-plane bending mode is measured to be 614 cm(-1) for the W(+)-bz(2) complex. The unusual trend in the ionization energy and metal-benzene stretch frequency indicates strong relativistic effects on tungsten binding.

Pulsed-field ionization electron spectroscopy and binding energies of alkali metal-dimethyl ether and -dimethoxyethane complexes

Lithium and sodium complexes of dimethyl ether (DME) and dimethoxyethane (DXE) were produced by reactions of laser-vaporized metal atoms with organic vapors in a pulsed nozzle cluster source. The mono-ligand complexes were studied by photoionization and pulsed field ionization zero electron kinetic energy (ZEKE) spectroscopy. Vibrationally resolved ZEKE spectra were obtained for Li(DME), Na(DME) and Li(DXE) and a photoionization efficiency spectrum for Na(DXE). The ZEKE spectra were analyzed by comparing with the spectra of other metal-ether complexes and with electronic structure calculations and spectral simulations. Major vibrations measured for the M(DME) (M=Li,Na) ions were M-O and C-O stretches and M-O-C and C-O-C bends. These vibrations and additional O-Li-O and O-C-C-O bends were observed for the Li(DXE) ion. The M(DME) complexes were in C2v symmetry with the metal atom binding to oxygen, whereas Li(DXE) was in a C2 ring configuration with the Li atom attaching to both oxygen atoms. Moreover, the ionization energies of these complexes were measured from the ZEKE or photoionization spectra and bond dissociation energies were derived from a thermodynamic cycle.

Electron-spin multiplicities and molecular structures of neutral and ionic scandium-benzene complexes

Scandium-benzene complexes, Sc-(C6H6)1,2 are produced by interactions between the laser-vaporized scandium atoms and benzene vapor in pulsed molecular beams, and identified by photoionization time-of-flight mass spectrometry and photoionization efficiency spectroscopy. The electron-spin multiplicities and geometries of these complexes and their ions are determined by combining pulsed field-ionization zero electron kinetic-energy spectroscopy and density-functional theory calculations. For scandium-monobenzene, a short-range quartet ground state is determined for the neutral complex, and a low-energy triplet state is probed for the ion. For the dibenzene complex, the neutral ground state is a doublet, and two low-energy ion states are singlet and triplet. The quartet and triplet states of scandium-monobenzene and the triplet state of scandium-dibenzene possess sixfold symmetry, whereas the doublet and singlet of the dibenzene complex have twofold symmetry. Moreover, ionization energies and metal-ring stretching wavenumbers are measured for both complexes.

Electronic states of neutral and cationic bis(benzene) titanium and vanadium sandwich complexes studied by pulsed field ionization electron spectroscopy

Ti- and V-bz2 (bz=C6H6) sandwich complexes have been prepared in a laser-ablation cluster beam source and studied by pulsed field ionization-zero electron kinetic energy photoelectron spectroscopy and theoretical calculations. The ground electronic states of the neutral Ti- and V-bz2 complexes are determined to be 1A1g and 2A1g, and their ionization energies are measured to be 5.732+/-0.001 and 5.784+/-0.002 eV, respectively. These neutral complexes have eta6 binding and are in an eclipsed D6h configuration with flat benzene rings. Ionization of the 1A1g and 2A1g neutral states of Ti- and V-bz2 yields the 2B1g and 3B1g ion states, respectively, in a D2h point group with slightly puckered benzene rings. In addition, the binding and structures of these two complexes are compared with other first-row transition metal bis(benzene) sandwiches.

ZEKE spectroscopy of free transition metal clusters

Abstract Transition metal clusters are produced in molecular beams with laser vaporization of metal targets. Vibrationally and rotationally resolved electronic spectra of the metal clusters are obtained with zero electron kinetic energy (ZEKE) photoelectron spectroscopy. The resolution of the spectra is of the order of sub-millielectron volts. Experiments on bare vanadium and yttrium clusters and ligated yttrium, zirconium, and niobium clusters are presented. The bond length of vanadium dimer is determined from the rotationally resolved spectra. Electronic states and molecular geometries of larger clusters are determined by combining the vibrationally resolved spectra with density functional theory and Franck–Condon factor calculations.

A photoionization and photoelectron study of vibrational and electronic cooling in metal molecular beams

Abstract Threshold photoionization and pulsed-field-ionization zero-electron-kinetic-energy photoelectron spectra have been used to study the vibrational cooling of Cu–N(CH 3 ) 3 and the electronic cooling of Al–NH 3 in helium and argon supersonic jets. The vibrational temperatures of Cu–N(CH 3 ) 3 are estimated to be ∼40 K in argon and ∼120 K in helium, whereas the electronic temperatures of Al–NH 3 are about 20 and 80 K, respectively. Argon more efficiently cools the internal degrees of freedom of the metal systems, but helium provides additional spectroscopic information about the neutral molecules. The differences in the observed spectra have greatly facilitated spectral assignments for these complexes.

Zero kinetic energy spectroscopy of AlNH3 complex

Abstract Pulsed field ionization–zero kinetic energy photoelectron spectrum of the jet-cooled aluminum–ammonia complex, AlNH 3 , has been observed for the first time. The spectrum shows vibronic transitions from two spin–orbit levels of the neutral ground state, which allows the measurement of the following spectroscopic constants: ionization potential (39 746 cm −1 ), spin–orbit splitting (58 cm −1 ), intermolecular stretching vibrational frequencies ( ω 3 + =339 cm −1 , ω 3 + x 3 + =3.1 cm −1 , and ν 3 =227 cm −1 ), and intermolecular bending frequency ( ν 6 + =557 cm −1 ). The observation of the spin–orbit splitting confirms that the ground state of the neutral complex is the 2 E state.

Photoelectron spectra of metal-containing molecules with resolutions better than 1 meV☆

Abstract This article describes the applications of zero electron kinetic energy, ZEKE, photoelectron technique to metal-containing molecules. Examples include vanadium dimer, metal trimer compounds (Nb3O, Nb3C2, Nb3N2, Zr3O, and Y3C2), and metal atom-molecule complexes (InNH3, AlNH3, ZrO(CH3)2, and YO(CH3)2). The bond length of vanadium dimer cation has been determined from the rotationally resolved spectra with a resolution of 1.5 cm−1, or 0.19 meV. The geometric conformations and electronic states of the clusters and complexes have been identified by combining the vibrationally resolved spectra and Franck–Condon factor calculations. The vibrational spectra have linewidths of 4–8 cm−1.

Spectroscopy and calculations of weakly bound gallium complexes with ammonia and monomethylamine

The gallium complexes were produced in pulsed molecular beams and studied with zero electron kinetic energy (ZEKE) photoelectron spectroscopy. Intermolecular vibrational frequencies and adiabatic ionization potentials (IPs) were obtained from ZEKE spectra. Ground electronic states were identified by combining the ZEKE spectra with quantum chemical and Franck–Condon calculations. Ga–NH3 has an IP of 40 135 cm−1 and metal-ligand stretching frequencies of 270 cm−1 (ωs+) in the ion and 161 cm−1 (νs) in the neutral. The IP of Ga–NH2CH3 is 39 330 cm−1, and the vibrational frequencies are 93 cm−1 (νb) for the Ga–N–C bending, 124 cm−1 (ωb+) for the Ga+–N–C bending, and 299 cm−1 (ωs+) for the Ga+–N stretching. The strength of the gallium–methylamine binding is stronger than that of the gallium–ammonia. The ground state of Ga–NH3 is 2A′(Cs) and that of Ga+–NH3 is 1A1(C3v). In contrast, Ga–NH2CH3 has two doublets, 2A′ and 2A″(Cs), with virtually the same energies, whereas Ga+–NH2CH3 has a 1A′(Cs) ground state.

La-Activated Bicyclo-oligomerization of Acetylene to Naphthalene

We report the first example of metal-mediated acetylene bicyclopentamerization to form naphthalene in the gas phase. The bicyclic aromatic compound was observed in a complex with La. The La(naphthalene) complex was formed by the reaction of laser-ablated La atoms with acetylene molecules in a molecular beam source and was characterized by mass-analyzed threshold ionization spectroscopy. The bicyclo-oligomerization reaction occurs through sequential acetylene additions coupled with dehydrogenation. Three intermediates in the reaction have been identified: lanthanacyclopropene [La(C2H2)], La(cyclobut-1-en-3-yne) [La(C4H2)], and La(benzyne) [(La(C6H4)]. The metal-ligand bonding in the three intermediates is considerably different from that in the La(naphthalene) complex, as suggested by accurately measured adiabatic ionization energies.