Lanthanum-mediated dehydrogenation of 1- and 2-butynes: Spectroscopy and formation of La(C₄H₄) isomers

La atom reactions with 1-butyne and 2-butyne are carried out in a laser-vaporization molecular beam source. Both reactions yield the same La-hydrocarbon products from the dehydrogenation and carbon-carbon bond cleavage and coupling of the butynes. The dehydrogenated species La(C₄H₄) is characterized with mass-analyzed threshold ionization (MATI) spectroscopy and quantum chemical computations. The MATI spectra of La(C₄H₄) produced from the two reactions exhibit two identical transitions, each consisting of a strong origin band and several vibrational intervals. The two transitions are assigned to the ionization of two isomers: La(η⁴-CH₂CCCH₂) (Iso A) and La(η⁴-CH₂CHCCH) (Iso B). The ground electronic states are ²A₁ (C_2v) for Iso A and ²A (C₁) for Iso B. The ionization of the doublet state of each isomer removes a La 6s-based electron and results in a ¹A₁ ion of Iso A and a ¹A ion of Iso B. The formation of Iso A from 2-butyne and Iso B from 1-butyne involves the addition of La to the C=C triple bond, the activation of two C(sp³)-H bonds, and concerted elimination of a H₂ molecule. The formation of Iso A from 1-butyne and Iso B from 2-butyne involves the isomerization of the two butynes to 1,2-butadiene in addition to the concerted H₂ elimination.