Determination of the molecular structure of tris(trimethylsilyl)phosphine in the gas phase by electron diffraction, supported by molecular mechanics calculations

Abstract The molecular structure of tris(trimethylsilyl)phosphine in the gas phase has been determined by electron diffraction. Results of three refinements are reported. In the first refinement, overall C 3 symmetry and local C 3 symmetry for the SiMe 3 groups were assumed, but these groups were allowed to tilt away from one another so that their C 3 local axes no longer coincided with the PSi bonds. The major geometrical parameters were r a (PSi) 225.9 (1), r (SiC) 188.2(1) pm, ∠ SiPSi 105.1(2) and ∠ CSiC 107.9(1)°. The tilt angle was 5.9(3)°, and the SiMe 3 groups were twisted 17.2(2)° away from the staggered position, at which the overall molecular symmetry was C 3v ; the PSiC angles were therefore 106.8, 109.3 and 116.7°. In the other two refinements the restriction of C 3 symmetry within the SiMe 3 groups was removed, and the differences between SiC distances and between CSiC angles were first fixed at values calculated by molecular mechanics (MM2), and then these new restrictions were partially relaxed. Neither of these refinements fitted the experimental data quite as well as the first one.