Theoretical studies of diphosphene and diphosphinylidene in their closed-shell states, low-lying open-shell singlet and triplet states, and transition states. Search for a stable bridged structure

The authors have undertaken a systematic study of the various low-lying electronic states of the H/sub 2/P/sub 2/ system, about which little is known experimentally, using the methods of ab initio molecular electronic structure theory. For each state they have predicted its molecular geometry, energy, dipole moment, vibrational frequencies, and their associated normal modes. The ground state is trans-HP=PH, 3.5 kcal/mol below cis-HP=PH. The H/sub 2/P=P isomer lies at a relative energy of 28 kcal/mol above the ground state. For the ground state the highest occupied molecular orbital (HOMO) is n/sub +/, a symmetric lone-pair combination; the second HOMO is the P-P ..pi.. bond. In H/sub 2/P=P a lone pair n/sub y/ is the HOMO, and the P-P ..pi.. bond is the second HOMO. For both isomers the lowest unoccupied molecular orbital (LUMO) is the antibonding P-P ..pi..* orbital. A variety of states involving n-..pi..* and ..pi..-..pi..* excitations have been studied for various molecular geometries. The lowest energy open-shell singlet is a pyramidal H/sub 2/PP /sup 1/A'' state (out-of-plane angle 77/sup 0/) at a relative energy of 46 kcal/mol. A skewed HPPH /sup 1/B state (torsion angle 109/sup 0/) is at 64 kcal/mol. Highly distorted geometries are also characteristic of the triplet states. A pyramidal H/sub 2/PP /sup 3/A'' state (out-of-plane angle 79/sup 0/) has a relative energy of only 21 kcal/mol, and thus it is predicted to lie below the closed-shell planar H/sub 2/PP state. A skewed HPPH /sup 3/B state (torsion angle 90/sup 0/) is at 29 kcal/mol. An unsuccessful search has been made for a stable doubly bridged PH/sub 2/P structure, analogous to SiH/sub 2/Si.