Solid Parahydrogen Infrared Matrix Isolation and Computational Studies of Li n -(C 2 H 4 ) 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 the2 A1 electronic state (for C2v geometries) that is repulsive but supports a shallow van der Waals well and correlates with the Li 2s atomic state, and the second is a2 B2 electronic state that correlates with the Li 2p atomic orbital and is a strongly bound charge-transfer state. Only the2 B2 charge-transfer state would be advantageous for hydrogen storage because the strong electric dipole created in the Li-(C2 H4 ) 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-(C2 H4 ) complex. The most accurate ab initio calculations indicate that the2 A1 van der Waals state is slightly more stable. In contrast, argon matrix isolation experiments have clearly identified the Li-(C2 H4 ) complex exists in the2 B2 state. Some have suggested that argon matrix effects shift the equilibrium toward the2 B2 state. We report the low-temperature synthesis and IR characterization of Lin -(C2 H4 )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-(C2 H4 ) complex is more stable in the2 B2 electronic state and thus constitutes a potential hydrogen storage material with desirable characteristics.