Lanthanum-Mediated Dehydrogenation of 1- and 2-Butynes: Spectroscopy and Formation of La(C\u3csub\u3e4\u3c/sub\u3eH\u3csub\u3e4\u3c/sub\u3e) 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 (C2v) 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