Binding sites and electronic states of group 3 metal-aniline complexes probed by high-resolution electron spectroscopy.

Group 3 metal-aniline complexes, M(aniline) (M = Sc, Y, and La), are produced in a pulsed laser-vaporization molecular beam source, identified by photoionization time-of-flight mass spectrometry, and investigated by pulsed-field ionization zero electron kinetic energy (ZEKE) spectroscopy and quantum chemical calculations. Adiabatic ionization energies and several low-frequency vibrational modes are measured for the first time from the ZEKE spectra. Metal binding sites and electronic states are determined by combining the ZEKE measurements with the theoretical calculations. The ionization energies of the complexes decrease down the metal group. An out-of-plane ring deformation mode coupled with an asymmetric metal-carbon stretch is considerably anharmonic. Although aniline has various possible sites for metal coordination, the preferred site is the phenyl ring. The metal binding with the phenyl ring yields syn and anti conformers with the metal atom and amino hydrogens on the same and opposite sides of the ring, respectively. The anti conformer is determined to be the spectral carrier. The ground electronic state of the anti conformer of each neutral complex is a doublet with a metal-based electron configuration of nd(2)(n + 1)s(1), and the ground electronic state of each ion is a singlet with a metal-based electron configuration of nd(2). The formation of the neutral complexes requires the nd(2)(n + 1)s(1) ← nd(1)(n + 1)s(2) electron excitation in the metal atoms.

High-spin electronic states of lanthanide-arene complexes: Nd(benzene) and Nd(naphthalene).

Neodymium (Nd) complexes of benzene and naphthalene were synthesized in a laser-ablation supersonic molecular beam source. High-resolution electron spectra of these complexes were obtained using pulsed-field ionization zero electron kinetic energy (ZEKE) spectroscopy. Second-order Møller-Plesset perturbation calculations were employed to aid spectral and electronic-state assignments. The adiabatic ionization energies were measured to be 38 081 (5) cm(-1) for Nd(benzene) and 37 815 (5) cm(-1) for Nd(naphthalene). For the Nd(benzene) complex, the observed frequencies of 831 and 286 cm(-1) were assigned to C-H out-of-plane bending and Nd(+)-C(6)H(6) stretching modes in the (6)A(1) ion state and 256 cm(-1) to the Nd-C(6)H(6) stretching mode in the (7)A(1) neutral state. To confirm these assignments, the ZEKE spectrum of the deuterated species was recorded, and the corresponding vibrational frequencies were measured to be 710 and 277 cm(-1) in the ion state and 236 cm(-1) in the neutral state. For the Nd(naphthalene) complex, the observed vibrational modes were C(10)H(8) bending (394 cm(-1)), Nd(+)-C(10)H(8) stretching (286 and 271 cm(-1)), Nd(+)-C(10)H(8) bending (80 cm(-1)), and C(10)H(8) twisting (105 cm(-1)) in the (6)A(') ion state and metal-ligand bending (60 cm(-1)) and ligand twisting (55 cm(-1)) in the (7)A(') neutral state. The formation of the ground state of the Nd(benzene) complex requires 4f → 5d and 6s → 5d electron excitation of the Nd atom, whereas the formation of the ground state of Nd(naphthalene) involves the 6s → 5d electron promotion.

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.

Clamshell structure of Sc(biphenyl) from high resolution photoelectron spectroscopy.

Sc(biphenyl) is produced in a laser vaporization molecular beam source and characterized by pulsed field ionization-zero electron kinetic energy (ZEKE) photoelectron spectroscopy and density functional theory. The theory predicts two low-energy isomers: a clamshell structure with Sc binding to both phenyl rings and a half-sandwich structure with Sc binding to single ring. The joint experimental and theoretical study shows that the Sc(biphenyl) complex prefers the clamshell structure with a twelve-fold binding mode. From the free ligand to the clamshell structure of the complex, phenyl rings first rotate to become coplanar, then bend toward the Sc atom, and finally are clamped in place by Sc binding.

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.

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.

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.

Pulsed-field ionization electron spectroscopy and conformation of copper-diammonia.

Copper-diammonia, Cu(NH3)2, and its deuterated species, Cu(ND3)2, are produced in supersonic molecular beams and studied by pulsed-field ionization zero electron kinetic energy photoelectron spectroscopy and ab initio calculations. Structural isomers with a copper atom binding to an ammonia dimer or two ammonia molecules are obtained by the calculations. By comparing the experimental measurements to the theoretical calculations, the neutral and ionic forms of copper-diammonia are determined to be in a doubly bound linear conformation in their ground electronic states. The adiabatic ionization potentials of Cu(NH3)2 and Cu(ND3)2 are measured as 29,532 (5) and 29,313 (5) cm(-1), respectively. The metal-ligand symmetric stretching frequencies are measured to be 436 cm(-1) for Cu+-(NH3)2 and 398 cm(-1) for Cu+-(ND3)2, and the metal-ligand bending frequencies 75,139 cm(-1) for CuCu+-(NH3)2 and 70125 cm(-1) for CuCu+-(ND3)2. Moreover, the dissociation energy of Cu(NH3)2-->CuNH3+NH3 is determined to be 11(3) kcal mol(-1) through a thermodynamic relationship.

Photoelectron spectroscopy and density functional theory of puckered ring structures of Group 13 metal-ethylenediamine.

The ethylenediamine (en) complexes of Al, Ga, and In atoms were prepared in laser-vaporization supersonic molecular beams and studied with pulsed field ionization zero electron kinetic energy photoelectron spectroscopy and density functional theory. Several conformers of each metal complex are obtained by B3LYP calculations, and a five-membered cyclic structure is identified by combining the experimental measurements and theoretical calculations. Adiabatic ionization potentials, vibrational frequencies, and bond dissociation energies are determined for the ring structure. The ionization potentials of the Al, Ga, and In species are measured to be 32 784 (5), 33 324 (5), and 33 637 (7) cm(-1), respectively, and metal-ligand dissociation energies of the ionic and neutral complexes are calculated to be 60.2/16.2 (Al(+)/Al), 55.5/13.0 (Ga(+)/Ga), and 50.0/11.4 (In(+)/In) kcal mol(-1). Metal-ligand stretch and bend as well as a number of ligand-based vibrations are measured. Harmonic frequencies and anharmonicities of the M(+)-N (M=Al,Ga,In) stretch are determined for all three M(+)-en ions and the C-C-N bend of Ga(+)-en and In(+)-en. In comparison to monodentate methylamine, the bidentate binding of ethylenediamine leads to a significantly lower ionization potential and higher metal-ligand bond strength of the metal complexes.