Lanthanum-mediated dehydrogenation of 1- and 2-butynes: Spectroscopy and formation of La(C 4 H 4 ) 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(C4 H4 ) is characterized with mass-analyzed threshold ionization (MATI) spectroscopy and quantum chemical computations. The MATI spectra of La(C4 H4 ) 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 -CH2 CCCH2 ) (Iso A) and La(η4 -CH2 CHCCH) (Iso B). The ground electronic states are 2 A1 (C2v ) for Iso A and 2 A (C1 ) for Iso B. The ionization of the doublet state of each isomer removes a La 6s-based electron and results in a 1 A1 ion of Iso A and a 1 A 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.