Volume 16 1, number 3 CHEMICAL PHYSICS LEI-fERS 15 September I989 THE STRUCTURE AND BONDING OF Li,H ION-PAIR STATES J.A. MONTGOMERY Jr., H.H. MICHELS zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHG United Technologies Research Center, East Harlford, CT 06 108, USA O.F. GUNER and K. LAMMERTSMA Department of Chemistry, University ofAlabama at Birmingham, Birmingham, AL 35.294, USA Received 20 June I989 Ab initio calculations on L&H ion-pair states and a topological analysis of the charge density of the resulting optimized struc- tures are reported. The global LiSH minimum is a planar CZvstructure with H--Li$ ion-pair character. The pyramidal Csv structure, which is 21.5 kcal/mol higher in energy (MP2/6-3 I1 + tGs*), is found to have ionic H-Li bonds, but no Li-Li bonds. Although vibrational analysis indicates this structure to be a true minimum on the potential energy surface, inclusion of the vibrational zero-point energy makes the thermodynamic stability of this structure questionable. I. IntrolIuction The structure and bonding of electron-deficient clusters, composed of the light elements hydrogen, lithium, beryllium, and boron, is of significant the- oretical interest [ l-5 1. Their inherent high energy content also makes these systems of potential prac- tical value for energy storage. Nicolaides has recently suggested that ion-pair states may produce stable high energy molecules [ 61. However, a detailed analysis of the H- +H3+ inter- action in Csv symmetry [ 71 indicates that the ground state is an unstable saddle-point structure. As tri- gonal H3 is unstable, it may be argued that the in- stability of trigonal pyramidal H4 is caused by the destabilizing transfer of charge from H- to HZ when the Cjv symmetry is broken: This limitation may be overcome in the corresponding Li isomer, because both Li$ and Li3 are stable species [ 4,s 1. Therefore H-+Li: ion pairs may constitute stable energetic Li,H molecules. In this study we report ab initio calculations on potential Li3H bound ion-pair states and evaluate their energetic and electronic properties. A compar- ison is made with Talbi and Saxon’s recent MCSCF/ CI study of L&H [ 91, which was undertaken while our work was in progress. 2. Calculations and results Ab initio SCF and MP2 calculations on L&H were performed with the GAUSSIAN 86/88 [lo] and CADPAC [ 111 electronic structure codes using Pople’s double and triple split-valence basis sets, augmented with polarization and diffuse functions (6-3IG*, 6-3 zyxwvutsrqponmlkjihgfedcbaZYXW 1+ + G**, 6-3 1lG**, and 6- 3 11 f +G**). At each level of theory considered, the geometries were optimized within the indicated symmetries and harmonic vibrational frequencies were calculated to characterize the structures as min- ima (all real frequencies ) or saddle points (one im- aginary frequency). Addition of H- to Li; may occur at a vertex, edge, or face of the equilateral triangular Li: structure. The geometry optimization of these configurations revealed two minima shown as 1 and 2 in fig. 1. The planar edge complexed structure zyxwvutsrqponmlkjihgfedc 1 (C,, ) is the global minimum at all levels of theory (tables 1 and 2) in agreement with previous work [ 3,4]. At our highest level of theory (MP2/6-311+ +G**) the trigonal pyramidal structure 2 (C,,) is 21.5 kcal/mol higher in energy than the ground state isomer 1. This energy difference is rather insensitive to the level of theory employed (see tables 1 and 2) and is in good agree- ment with the 20.3 kcal/mol MCSCF/SOCI result of Talbi and Saxon. 0 009-26 14/89/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division ) 291