Conformers of n-Si₅Me₁₂:  A Comparison of ab Initio and Molecular Mechanics Methods

Abstract

Optimized geometries of the conformers of permethylated linear pentasilane, n-Si5Me12, were calculated by the HF/3-21G*, MM3, MM2, and MM+ methods, which predict eight, nine, six, and six energetically distinct enantiomeric conformer pairs, respectively, at geometries representing various combinations of the anti (∼165°), ortho (∼90°), and gauche (∼55°) SiSiSiSi dihedral angles in the backbone. The results of the MM2 and MM+ methods, based on the same force field, differ insignificantly. The barriers between conformers appear to be exaggerated by the molecular mechanics methods, particularly MM2. Contour maps showing the ground-state energy as a function of the full range of two backbone SiSiSiSi dihedral angles, with all other geometrical variables optimized, computed by each of the methods (only a limited range of angles near the anti,anti geometry in the case of HF/3-21G*) are compared with each other and with analogous results for a model compound, Si4Me10. Conformer interconversion paths are discussed, and two meso transition states for enantiomer interconversion have been located at the HF/3-21G* level of calculation. At the eight HF/3-21G* optimized geometries, single-point energies (HF/6-31G* and MP2/6-31G*) and vibrational frequencies (HF/3-21G*) were computed. The predicted IR and Raman spectra suggest that about half of the expected conformers will be identifiable by vibrational spectroscopy under conditions of matrix isolation. Relative conformer energies calculated by the MM2 and HF methods are similar and favor the anti dihedral angles over gauche and ortho, in agreement with results of solution experiments. Those calculated by the MM3 and MP2 methods are similar to each other and favor both anti and gauche dihedral angles nearly equally over ortho, in agreement with indications provided by gas-phase experiments. A rationalization of these solvent effects is proposed. The energies of the conformers of Si4Me10 and Si5Me12 were used to set up a system of additive increments at the MM2, MM3, HF/3-21G*, HF/6-31G*, and MP2/6-31G* levels of calculation, which can be used to predict conformational energies of longer permethylated oligosilanes. An intrinsic energy value is assigned to each of the a, o, and g dihedral angles, and interaction energy values are assigned to each combination of two dihedral angles. The interaction values follow the expected rules in that equal twist sense is favored for adjacent aa, ag, oo, and gg pairs, whereas opposite twist sense is generally favored for adjacent ao and go pairs. The MM3-derived set of increments has been tested against results computed for Si6Me14 and found to perform well.

Polysilane Dendrimers

“…Steric effects are also responsible for the observation of three conformational families of dihedral angles: gauche (G, ω ~ 60°), ortho (O, ω ~ 90°) and anti (A, ω ~ 180°) dominating in linear polysilanes [96][97][98]. Even more conformers are possible in branched and dendritic polysilanes due to the redundancy of silicon-silicon chains combined with strongly repulsive interactions between substituents.…”

Conformational equilibria in dihaloheptasilanes X2Si[SiMe(SiMe3)2]2 with X = F, Cl, Br, I: A Raman spectroscopic and quantum chemical DFT study

“…21 They are populated negligibly (Ä1%) at room temperature. 21 They are populated negligibly (Ä1%) at room temperature.…”

Conformation Control of Oligosilanes by Trimethylsilyl Groups: Dodecamethyl-, Undecamethyl-2-trimethylsilyl- and Decamethyl-2, 4-bis(trimethylsilyl)-n-pentasilane Studied by Raman Spectroscopy and Quantum Chemical Calculations

“…Extensive high quality ab initio calculations of the conformational properties of n-Si 5 Me 12 , which also include MP2/VTDZ//MP2/VTDZ values for solution phase free [12,13]. The calculations predict seven energetically distinct conformers which the authors denote as t + t + , t + g + , t + o -, g + g + , t + g -, o + o + and o + g -, respectively.…”

“…Here, t, g and o stand for transoid, gauche and ortho and refer to the SiSiSiSi dihedral angles in the range ±160-175°(t + , t -), ±75-105°(o + , o -), and ±50-70°( g + , g -). Vibrational frequencies as well as Raman and IR intensities for the conformers have been calculated at the HF/3-21G* level [12]. From these calculations it is clear that only the range of the SiSi stretching vibrations can be used to distinguish the various conformers, which can also be understood by applying the concept of group vibrations.…”

“…All values are taken from refs [12]. and[13] Values have been converted from kcalmol -1 and rounded to two decimals; frequencies from HF calculations and scaled by 0.91…”