P-P bond cleavage;: energetics and structural changes in tetramethyldiphosphine and tetrasilyldiphosphine from ab initio MO calculations

The molecular structures and conformations of tetramethyldiphosphine and tetrasilyldiphosphine and their corresponding dimethylphosphido and disilylphosphido radicals were computed by high-level ab initio molecular orbital calculations utilising a range of methods (HF, MP2, B3LYP) and different basis sets. The thermodynamic properties of the homolytic dissociation reaction were also calculated at the G2 level and compared with those computed by other theoretical methods. The results indicate that although the MP2/6-311+G* calculations are superior in reproducing experimental structural and high-level theoretical thermodynamic data, the thermodynamic properties computed using the B3LYP method with effective core potentials for Si and P or even with a small (3-21G*) basis set are on a par with calculations employing larger basis sets and more elaborate treatment of electron correlation at the MP2 level. This offers the possibility of quick reasonable estimation of thermodynamic properties of large dissociating systems. An estimation of bond energies based on the energetics of structural changes upon dissociation of the diphosphines gives values in agreement with the previously estimated P-P bond energies in organic compounds. The current ab initio calculations demonstrated the existence of two conformers of tetramethyldiphosphine, gauche and anti, with the anti form being more stable by 6.1 kJ mol(-1) as computed at the MP2/6-311+G* level, in disagreement with the previous results of the electron diffraction structure investigation. This disagreement in the conformational composition and the large difference in computed and experimental values of the P-P-C angles indicate that the structure of tetramethyldiphosphine in the gas phase may need re-determination.