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

Abstract

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 MP2/6-311ϩG* 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 Ϫ1 as computed at the MP2/6-311ϩG* 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. DALTON

The molecular structure of tetra-tert-butyldiphosphine: an extremely distorted, sterically crowded molecule

“…When the correction for basis set superposition was applied, the B3LYP/3-21G* calculations were shown to perform as well as those that employed the B3LYP functional with much larger basis sets as in the cases of tetramethyldiphosphine and tetrasilyldiphosphine. 6 Moreover, application of the B3LYP/3-21G* optimisations permits the calculations to be performed with reasonable computational economy for large systems such as the molecules of concern here. Single point energies of the halves of tetra-tert-butyl-diphosphine (PBu t 2 ) were also calculated at the corresponding levels of theory.…”

“…In addition, the bond energies, as equilibrium values, were calculated for vibrationless states at 0 K. The basis set superposition error (BSSE) was estimated by full counterpoise correction at all applied non-empirical theoretical levels. 19 To apply the model for dissociation, harmonic force constants of the P-P bond, f s , and for the substituents as a whole, f b , were calculated for tetra-tert-butyl-diphosphine as described previously 6,7 using molecular mechanics with the UFF force field, 20 in conjunction with HF and DFT(B3LYP) methods 18 and the 3-21G* basis set. The computed values are given in Table 1.…”

Structure by theory and experiment: One nationality, two languages

“…In this sense, the malleable R substituents in the solid-state structure act as springs in a molecular jack-in-thebox. This initial investigation lead to a more rigorous investigation of smaller P 2 R 4 / Ṗ R 2 (R ϭ Me, SiH 3 ) systems to gain further insight into the process of dissociation using theoretical methods [12].…”

PP-Rotation, P-Inversion and Metathesis in Diphosphines Studied by DFT Calculations: Comments on Some Literature Conflicts

“…Related to the above are conflicting conclusions that were also derived from several techniques employed in the investigation of another dynamic process of diphosphines in solution: rotational isomerisation about the P-P bond. Borisenko and Rankin [16] summarised the conflicting literature data for the case of tetramethyldiphosphine and their summary is given in Table 1. Further to their work on the homolytic cleavage of the phosphorus-phosphorus bond, they employed a variety of computational techniques (HF, MP2, B3LYP) to investigate rotational isomerisation about the P-P bond.…”

“…Borisenko and Rankin [16] summarised the conflicting literature data for the case of tetramethyldiphosphine and their summary is given in Table 1. Further to their work on the homolytic cleavage of the phosphorus-phosphorus bond, they employed a variety of computational techniques (HF, MP2, B3LYP) to investigate rotational isomerisation about the P-P bond.…”