Conformers of n-Si5Me12: A comparison of ab initio and molecular mechanicsmethods

Optimized geometries of the conformers of permethylated linear pentasilane, n-Si5Me12, were calculated by the HF/3-21G*, MM3, MM2, and MM+ methods, wh ich predict eight, nine, six, and six energetically distinct enantiomeric c onformer pairs, respectively, at geometries representing various combinatio ns of the anti (similar to 165 degrees), ortho (similar to 90 degrees), and gauche (similar to 55 degrees) SiSiSiSi dihedral angles in the backbone. T he results of the MM2 and MM+ methods, based on the same force field, diffe r insignificantly. The barriers between conformers appear to be exaggerated by the molecular mechanics methods, particularly MM2. Contour maps showing the groundstate energy as a function of the full range of two backbone SiS iSiSi dihedral angles, with all other geometrical variables optimized, comp uted by each of the methods (only a limited range of angles near the anti,a nti geometry in the case of HF/3-21G*) are compared with each other and wit h analogous results for a model compound, Si4Me10. Conformer interconversio n paths are discussed, and two meso transition states for enantiomer interc onversion have been located at the HF/3-21G* level of calculation. At the e ight HF/3-21G* optimized geometries, single-point energies (HF/6-31G* and M P2/6-31G*) and vibrational frequencies (HF/3-21G*) were computed. The predi cted IR and Raman spectra suggest that about half of the expected conformer s will be identifiable by vibrational spectroscopy under conditions of matr ix isolation. Relative conformer energies calculated by the MM2 and HF meth ods are similar and favor the anti dihedral angles over gauche and ortho, i n agreement with results of solution experiments. Those calculated by the h IM3 and HF methods are similar to each other and favor both anti and gauche dihedral angles nearly equally over ortho, in agreement with indications p rovided by gas-phase experiments. A rationalization of these solvent effect s is proposed. The energies of the conformers of Si4Me10 and Si3Me12 were u sed to set up a system of additive increments at the MM2, MM3, HF/3-21G*, H F/6-31G*, and MP2/6-31G* levels of calculation, which can be used to predic t conformational energies of longer permethylated oligosilanes. An intrinsi c 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 twis t sense is favored for adjacent aa, ag, oo, and gg pairs, whereas opposite twist sense is generally favored for adjacent so and go pairs. The MM3-deri ved set of increments has been tested against results computed for Si6Me14 and found to perform well.