Ab Initio Molecular Dynamics Investigation of Singlet C2H2Li2: Determination of the Ground State Structure and Observation of LiH Intermediates

The structural, electronic, and dynamic properties of the organolithium compd. C2H2Li2 were studied via ab initio mol. dynamics simulations based on the Car-Parrinello method. Dynamic 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 stoichiometric formula but an acetylenic deriv. that can be visualized geometrically as a complex of lithioacetylene with Li hydride HC2Li.HLi. 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 Li cations in the triangular complex Li+H-Li+. Several ethylene-like isomers were identified via high-temp. quenches, but these invariably turned out to lie at high energies (.apprx.>30 kcal/mol). Anal. of the high-temp. dynamics indicated that ethylene-like isomers are always unstable toward an intramol. H migration mediated via a Li hydride unit. The direct observation of these intramol. rearrangement reactions revealed the role of the Li atoms as H transfer reagents and confirmed the importance of Li hydride as an intermediate species. [on SciFinder (R)]