ELECTRONIC-STRUCTURE OF LAMBDA-5-PHOSPHAACETYLENES AND CORRESPONDING TRIPLET METHYLENES

The electronic structure and geometry of H2PCSiH3 (1) and (NH2)2PCSiH3 (3) were investigated by extended basis set (DZP) ab initio molecular orbital theory. Both singlet and triplet structures for 1 and 3 are considered. The calculated structure for (1)1 (ground state) is best characterized as a P-C multiply bonded species with an extremely short (159.5 pm) bond. The phosphorus center is planar in the ground state, and the P-C-Si angle is 137.9-degrees. Contrary to expectations based on a singlet carbene model for the ground state, the P-C bond lengthens upon protonation of the carbon center. The lowest triplet state of 1 is only 5.6 kcal/mol above the ground-state structure. The structure of the triplet is remarkably different from that of the ground state, with a P-C bond length of 180.0 pm and a pyramidal phosphorus center. The P-C-Si angle remains large (144.1-degrees), and the triplet state is best characterized as a triplet methylene. Replacement of the two hydrogens on phosphorus in 1 by two NH2 groups to form 3 has little effect on the geometry of the triplet but has a pronounced effect on the singlet. The P-C bond in (1)3 shortens to 152.5 pm, and the P-C-Si angle becomes almost linear. The singlet-triplet splitting increases by a factor of 2.3 to 13.9 kcal/mol.