2 research outputs found
Spectroscopic Characterization of Lanthanum-Mediated Dehydrogenation and C–C Bond Coupling of Ethylene
No full textLaÂ(C<sub>2</sub>H<sub>2</sub>) and LaÂ(C<sub>4</sub>H<sub>6</sub>) 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.
LaÂ(C<sub>2</sub>H<sub>2</sub>) is identified as a metallacyclopropene
and LaÂ(C<sub>4</sub>H<sub>6</sub>) as a metallacyclopentene. The three-membered
ring is formed by concerted H<sub>2</sub> 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
Lanthanum-Mediated C–H Bond Activation of Propyne and Identification of La(C<sub>3</sub>H<sub>2</sub>) Isomers
No full textη<sup>2</sup>-Propadienylidenelanthanum
[LaÂ(η<sup>2</sup>-CCCH<sub>2</sub>)] and deprotioÂlanthanaÂcyclobutadiene
[LaÂ(HCCCH)] of LaÂ(C<sub>3</sub>H<sub>2</sub>) are identified from
the reaction mixture of neutral La atom activation of propyne in the
gas phase. The two isomers are characterized with mass-analyzed threshold
ionization spectroscopy combined with electronic structure calculations
and spectral simulations. LaÂ(η<sup>2</sup>-CCCH<sub>2</sub>)
and LaÂ(HCCCH) are formed by concerted 1,3- and 3,3-dehydrogenation,
respectively. Both isomers prefer a doublet ground state with a La
6s-based unpaired electron, and LaÂ(η<sup>2</sup>-CCCH<sub>2</sub>) is slightly more stable than LaÂ(HCCCH). Ionization of the neutral
doublet state of either isomer produces a singlet ion state by removing
the La-based electron. The geometry change upon ionization results
in the excitation of a symmetric metal–hydrocarbon stretching
mode in the ionic state, whereas thermal excitation leads to the observation
of the same stretching mode in the neutral state. Although the La
atom is in a formal oxidation state of +2, the ionization energies
of these metal–hydrocarbon radicals are lower than that of
the neutral La atom. Deuteration has a very small effect on the ionization
energies of the two isomers and the metal–hydrocarbon stretching
mode of LaÂ(η<sup>2</sup>-CCCH<sub>2</sub>), but it reduces considerably
the metal–ligand stretching frequencies of LaÂ(HCCCH)
