Electron-diffraction study of the molecular structure of tetramethyldiphosphine

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Abstract
The molecular structure of tetramethyldiphosphine in the gas phase has been determined by electron diffraction. The study required a gas-nozzle temperature of 170°C, obtained by using hot compressed air. The molecule can be described by seven structural parameters from which all the 52 different interatomic distances can be calculated. All 52 distances were included in the full-matrix least-squares refinements which were performed. The final values of the seven independent parameters are C—H = 1.109±0.009 Å, C—P = 1.853±0.003 Å, P—P = 2.192±0.009 Å, ∠CPC = 99.6±1.0°, ∠PCH = 108.8±2.5°, ∠CPP = 101.1±0.7°, and ϕ, the dihedral angle expressing the relative orientation of the dimethylphosphino groups, is 164±23°, measured from the cis configuration. The parameters of the dimethylphosphino groups are very similar to the corresponding values for the methyl phosphines. The P—P bond length is at the low end of a range of literature values for this bond. The apparent deviation of 16° from the trans configuration is possibly due to a shrinkage effect involving a low frequency torsional oscillation about the P—P bond.

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