Spectroscopic Characterization of Lanthanum-Mediated Dehydrogenation and C–C Bond Coupling of Ethylene

La­(C₂H₂) and La­(C₄H₆) are observed from the reaction of laser-vaporized La atoms with ethylene molecules by photoionization time-of-flight mass spectrometry and characterized by mass-analyzed threshold ionization spectroscopy. La­(C₂H₂) is identified as a metallacyclopropene and La­(C₄H₆) as a metallacyclopentene. The three-membered ring is formed by concerted H₂ elimination and the five-membered cycle by dehydrogenation and C–C bond coupling. Both metallacycles prefer a doublet ground state with a La 6s-based unpaired electron. Ionization of the neutral doublet state of either complex produces a singlet ion state by removing the La-based electron. The ionization allows accurate measurements of the adiabatic ionization energy of the neutral doublet state and metal–ligand and ligand-based vibrational frequencies of the neutral and ionic states. Although the La atom is in a formal oxidation state of +2, the ionization energies of these metal–hydrocarbon cycles are lower than that of the neutral La atom. Deuteration has a small effect on the ionization energies of the two cyclic radicals but distinctive effects on their vibrational frequencies

Spectroscopic Characterization of Nonconcerted [4 + 2] Cycloaddition of 1,3-Butadiene with Lanthanacyclopropene To Form Lanthanum–Benzene in the Gas Phase

The reaction between La atoms and 1,3-butadiene is carried out in a laser-vaporization molecular beam source. Metal–hydrocarbon species with formulas La­(C_<i>n</i>H_<i>n</i>) (<i>n</i> = 2, 4, and 6) and La­(C_<i>m</i>H_<i>m</i>+2) (<i>m</i> = 4 and 6) are observed with time-of-flight mass spectrometry and characterized with mass-analyzed threshold ionization spectroscopy. A lanthanum–benzene complex [La­(C₆H₆)] is formed by 1,3-butadiene addition to lanthana­cyclopropene [La­(C₂H₂)] followed by molecular hydrogen elimination. Lanthana­cyclopropene is an intermediate generated by the primary reaction between La and 1,3-butadiene. Two other intermediates produced by the La + 1,3-butadiene reaction are La­[η⁴-(1-buten-3-yne)] [La­(C₄H₄)] and 1-lanthana­cyclopent-3-ene [La­(C₄H₆)]. The La­(benzene) complex exhibits distinctive metal–ligand bonding from that of the three intermediates as shown by the adiabatic ionization energies and ground electron configurations

Manifesting Direction-Specific Complexation in [HFIP_–H·H₂O₂]⁻: Exclusive Formation of a High-Lying Conformation

Size-selective, negative ion photoelectron spectroscopy in conjunction with quantum chemical calculations is employed to investigate the geometric and electronic structures of a protype system in catalytic olefin epoxidation research, that is, deprotonated hexafluoroisopropanol ([HFIP–H]−) complexed with hydrogen peroxide (H2O2). Spectral assignments and molecular electrostatic surface analyses unveil a surprising prevalent existence of a high-lying isomer with asymmetric dual hydrogen-bonding configuration that is preferably formed driven by influential direction-specific electrostatic interactions upon H2O2 approaching [HFIP–H]− anion. Subsequent inspections of molecular orbitals, charge, and spin density distributions indicate the occurrence of partial charge transfer from [HFIP–H]− to H2O2 upon hydrogen-bonding interactions. Accompanied with electron detachment, a proton transfer occurs to form the neutral complex of [HFIP·HOO•] structure. This work conspicuously illustrates the importance of directionality encoded in intermolecular interactions involving asymmetric and complex molecules, while the produced hydroperoxyl radical HOO• offers a possible new pathway in olefin epoxidation chemistry