Solid Parahydrogen Infrared Matrix Isolation and Computational Studies of Li_n-(C₂H₄)_m Complexes

Complexes of lithium atoms with ethylene have been identified as potential hydrogen storage materials. As a Li atom approaches an ethylene molecule, two distinct low-lying electronic states are established; one is the ²A₁ electronic state (for C_2v geometries) that is repulsive but supports a shallow van der Waals well and correlates with the Li 2s atomic state, and the second is a ²B₂ electronic state that correlates with the Li 2p atomic orbital and is a strongly bound charge-transfer state. Only the ²B₂ charge-transfer state would be advantageous for hydrogen storage because the strong electric dipole created in the Li-(C₂H₄) complex due to charge transfer can bind molecular hydrogen through dipole-induced dipole and dipole-quadrupole electrostatic interactions. Ab initio studies have produced conflicting results for which electronic state is the true ground state for the Li-(C₂H₄) complex. The most accurate ab initio calculations indicate that the ²A₁ van der Waals state is slightly more stable. In contrast, argon matrix isolation experiments have clearly identified the Li-(C₂H₄) complex exists in the ²B₂ state. Some have suggested that argon matrix effects shift the equilibrium toward the ²B₂ state. We report the low-temperature synthesis and IR characterization of Li_n-(C₂H₄)_m (n = 1, m = 1 and 2) complexes in solid parahydrogen which are observed using the C═C stretching vibration of ethylene in the complex. These results show that under cryogenic hydrogen storage conditions the Li-(C₂H₄) complex is more stable in the ²B₂ electronic state and thus constitutes a potential hydrogen storage material with desirable characteristics.