AB INITIO MOLECULAR DYNAMICS INVESTIGATION OF SINGLET C2H2LI2 - DETERMINATION OF THE GROUND STATE STRUCTURE AND OBSERVATION OF LIH INTERMEDIATES

摘要

The structural, electronic, and dynamic properties of the organolithium compound C2H2Li2 have been investigated via ab initio molecular dynamics simulations based on the Car-Parrinello method. Dynamical simulated annealing techniques applied to search for the low-energy configurations resulted in a structure that is not a dilithioethylene isomer as suggested by the stochiometric formula but an acetylenic derivative that can be visualized geometrically as a complex of lithioacetylene with lithium hydride (HC2LiHLi)-H-.. However, the ground state electronic structure is more suggestive of an ionic complex (H-C=C)(-.)(Li+H-Li+) in which the linear anion HCC- binds to the two lithium cations in the triangular complex Li+H-Li+. Several ethylene-like isomers were identified via high-temperature quenches, but these invariably turned out to lie at high energies (similar to > 30 kcal/mol). Analysis of the high-temperature dynamics indicated that ethylene-like isomers are always unstable toward an intramolecular hydrogen migration mediated via a lithium hydride unit. The direct observation of these intramolecular rearrangement reactions revealed the role of the lithium atoms as hydrogen transfer reagents and confirmed the importance of lithium hydride as an intermediate species. [References: 54]