Accurate Gas-Phase Experimental Structures of Octasilsesquioxanes (Si₈O₁₂X₈; X = H, Me)

Abstract

The accurate structures of silsesquioxanes Si8O12H8 and Si8O12Me8 have been determined by gas-phase electron diffraction methods in order to obtain experimental data on single molecules unconstrained by a crystal lattice for comparison with data obtained by theoretical methods. For Si8O12H8 the experimentally determined structure shows ideal O h symmetry with Si−O distances and Si−O−Si angles of 161.41(3) pm and 147.9(2)° [r e, uncertainties (σ) in parentheses] compared with 162.9 pm and 147.8° for theoretical results from MP2/6-311++G(3df,3pd) calculations. In Si8O12Me8 a similar O h symmetry model gave experimental values of 161.74(5) pm and 148.9(2)° for the Si−O distances and Si−O−Si angles compared with calculated values of 163.2 pm and 148.6°, respectively.

Functionalized Bis(pentafluoroethyl)phosphanes: Improved Syntheses and Molecular Structures in the Gas Phase

“…On the one hand, it is well known that classical MD underestimates vibrational amplitudes and corrections for light atom pairs (see ref. [29,30] ), but corrections for C-C pairs (which have reduced masses even smaller than C-F) are in very good agreement with SHRINK. On the other hand, in our experience, SHRINK calculations with a cubic force field can sometimes produce unreasonably large corrections for bonded distances in molecules with low-frequency vibrational modes.…”

“…A novel approach involves calculating vibrational corrections by using molecular dynamics (MD). [29,30] The last two methods were applied in the two structure determinations in this contribution, and the MD method has been further developed in this context.…”

“…It is also interesting to compare the anharmonic vibrational corrections computed by two approaches, namely by molecular dynamics [29,30] and the SHRINK procedure, [31] by using potential energy third derivatives. The corrections (k = r a -r e ) for all the bonded distances in (C 2 F 5 ) 2 PH calculated by these two methods are available in the Supporting Information.…”

Predicting anisotropic displacement parameters using molecular dynamics: density functional theory plus dispersion modelling of thermal motion in benzophenone

“…However, the principal disadvantage of LD calculations is that they are nearly always performed in the harmonic limit. At high temperatures, attempting to fit an anharmonic potential with a harmonic one may lead to spurious results, as we have seen for an equivalent situation in the gas phase (Wann et al, 2008(Wann et al, , 2009.…”

Molecular Structure of 1,2-Bis(trifluoromethyl)-1,1,2,2-tetramethyldisilane in the Gas, Liquid, and Solid Phases: Unusual Conformational Changes between Phases

“…As harmonic force constants were calculated, the refinement type is denoted r h1 . 19 In addition, the SMD solvation model 20 as implemented in ORCA (version 3.0.2) 21 was used in conjunction with DFT calculations at the B3LYP-D3BJ/def2-TZVP level [22][23][24][25][26][27][28] to explore the potential-energy surface of 1,2-bis(trifluoromethyl)-1,1,2,2-tetramethyldisilane in solution with the aim of understanding the liquid-phase Raman spectroscopic data. The electron-scattering intensities were measured using an Epson Expression 1680 Pro flatbed scanner and converted to mean optical densities as a function of the scattering variable, s, using an established program.…”