OBSERVATION OF TWO La(C$_3$H$_2$) ISOMERS FORMED BY DEHYDROGENATION OF PROPYNEOBSERVATION OF TWO La(C$_3$H$_2$) ISOMERS FORMED BY DEHYDROGENATION OF PROPYNE

Abstract

C-H bond activation of small hydrocarbons is of importance in chemistry and industrial applications. La(C$_3$H$_2$) was formed by the reaction of laser-ablated La atoms and propyne (C$_3$H$_4$) in supersonic molecular beams. Two isomers of La(C$_3$H$_2$) were detected for the first time by mass-analyzed threshold ionization (MATI) spectroscopy. From the MATI spectra, the two isomers exhibit origin bands at 42953(5) and 43609(5) cm$^{-1}$ and vibrational intervals of 425 and 535 cm$^{-1}$, respectively. They were identified as La(CCCH$_2$) formed from 1,3-dehydrogantion and La(HCCCH) formed by 3,3-dehydrogenation and were confirmed by measurements with deuterium substituted propyne (C$_3$D$_4$) as the precursor. The 1,3-dehydrogenated complex shows a higher ionization energy and larger metal-ligand stretching frequencies than the 3,3-dehydrogenated species. Based on DFT/B3LYP calculations, the electronic transitions responsible for the observed MATI spectrum of La(HCCCH)isomer is $^1$A $\leftarrow$ $^2$A,and that of La(CCCH$_2$)isomer is $^1$A$^{\prime}$ $\leftarrow$ $^2$A$^{\prime}$. C-H bond activation of small hydrocarbons is of importance in chemistry and industrial applications. La(C$_3$H$_2$) was formed by the reaction of laser-ablated La atoms and propyne (C$_3$H$_4$) in supersonic molecular beams. Two isomers of La(C$_3$H$_2$) were detected for the first time by mass-analyzed threshold ionization (MATI) spectroscopy. From the MATI spectra, the two isomers exhibit origin bands at 42959(5) and 43614(5) cm$^{-1}$ and vibrational intervals of 468/540 and 407/448 cm$^{-1}$, respectively. They were identified as La(CCCH$_2$) formed from 1,3-dehydrogantion and La(HCCCH) formed by 3,3-dehydrogenation and were confirmed by measurements with deuterium substituted propyne (C$_3$D$_4$) as the precursor. The 1,3-dehydrogenated complex shows a higher ionization energy and larger metal-ligand stretching frequencies than the 3,3-dehydrogenated species. The electronic transitions responsible for the observed MATI spectra of both isomers are $^1$A $\leftarrow$ $^2$A and will be discussed with aid of B3LYP/SDD calculations and Franck-Condon vibrational analysis.