AB-INITIO CALCULATION OF VIBRATION FREQUENCIES, INFRARED INTENSITIES,AND STRUCTURES FOR H-4(-4(+), AND DEUTERATED ANALOGS(), LIH3+, LI2H2+, AND LI)

The six normal mode vibration frequencies and infrared intensities for H-4(+) and three tetra-atomic lithium-hydrogen ion clusters have been calculated using ab initio SCE MP2, and CISD methods. Vibration frequ encies are also reported for all possible deuterated analogs of the fo ur ions. The normal mode vibrational characteristics and structures of the ions are discussed and compared to each other. The H-4(+) ion can reasonably be regarded as a strongly bonded, triangular H-3(+) core w ith an additional H atom less strongly bound to one of the ring hydrog ens in a planar C-2v arrangement. The calculated normal mode vibration frequencies for H-4(+) and its deuterated analogs confirm this pictur e. The hydrogen-lithium clusters, LiH3+, Li2H2+, and Li-4(+), had stru ctures and normal mode frequencies which did not yield the same ring/l igand picture as H-4(+), Instead, these ions behaved like two interact ing diatomic fragments (e.g., H-2 and LiH+ subunits in LiH3+) oriented perpendicular to each other in a planar arrangement. The potential en ergy surfaces for the hydrogen-lithium cluster ions displayed two extr emely low frequency vibrational modes corresponding to angular motion of these diatomic subunits relative to each other.