Lanthanum-mediated dehydrogenation of butenes: Spectroscopy and formation of La(C₄H₆) isomers

La atom reactions with 1-butene, 2-butene, and isobutene are carried out in a laser-vaporization molecular beam source. The three reactions yield the same La-hydrocarbon products from the dehydrogenation and carbon-carbon bond cleavage and coupling of the butenes. The dehydrogenated species La(C₄H₆) is the major product, which is characterized with mass-analyzed threshold ionization (MATI) spectroscopy and quantum chemical computations. The MATI spectrum of La(C₄H₆) produced from the La+1-butene reaction exhibits two band systems, whereas the MATI spectra produced from the La+2-butene and isobutene reactions display only a single band system. Each of these spectra shows a strong origin band and several vibrational progressions. The two band systems from the spectrum of the 1-butene reaction are assigned to the ionization of two isomers: La[C(CH₂)₃] (Iso A) and La(CH₂CHCHCH₂) (Iso B), and the single band system from the spectra of the 2-butene and isobutene reactions is attributed to Iso B and Iso A, respectively. The ground electronic states are ²A₁ (C_3v) for Iso A and ²A′ (C_s) for Iso B. The ionization of the doublet state of each isomer removes a La 6s-based electron and leads to the ¹A₁ ion of Iso A and the ¹A′ ion of Iso B. The formation of both isomers consists of La addition to the C=C double bond, La insertion into two C(sp³) - H bonds, and H₂ elimination. In addition to these steps, the formation of Iso A from the La+1-butene reaction may involve the isomerization of 1-butene to isobutene prior to the C - H bond activation, whereas the formation of Iso B from the La+trans-2-butene reaction may include the trans- to cis-butene isomerization after the C - H bond activation.