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