No abstract

The stable backbone conformers of n-Si 6 Me 14 have been identified through geometry optimizations with the HF/3-21G(d), MM2, and MM3 methods. With the exception of the MM2 method, their relative potential energies, and also single-point energies calculated by the HF/6-31G(d) and MP2/6-31G(d) methods at HF/ 3-21G(d) optimized geometries (E calc ), agree with energies E incr 0 , obtained using additive increment sets previously derived from results for n-Si 4 Me 10 and n-Si 5 Me 12 , with mean deviations of 0.11-0.15 kcal/mol. The energy E incr 0 is a simple function of the number of gauche, ortho, and transoid Si backbone bond conformations and of SiSi adjacent bond interactions. With the MM2 method the deviations from additivity are larger; E incr 0 and E calc agree only with a mean deviation of 0.52 kcal/mol. Improved increment sets were obtained by a simultaneous least-squares treatment of data for n-Si 4 Me 10 , n-Si 5 Me 12 and n-Si 6 Me 14 , including a few increments for interaction between next-nearest bond conformations. This yields energies E′′ incr that reproduce E calc with mean deviations of 0.05, 0.03, 0.05, and 0.03 kcal/mol at the HF/3-21G(d), HF/6-31G-(d), MP2/6-31G(d), and MM3 levels of theory. With the MM2 method a similar agreement between E calc and E′′ incr was obtained only after inclusion of 12 increments for interaction between next-nearest bond conformations. The energies E calc of selected low-energy conformers of n-Si 7 Me 16 and n-Si 8 Me 18 obtained by geometry optimization at HF/3-21G(d) and MM3 levels are reproduced by E′′ incr with mean (maximum) deviations of 0.04 (0.11) and 0.07 (0.26) kcal/mol. We conclude that the HF/3-21G(d), HF/6-31G(d), MP2/ 6-31G(d), and MM3 potential energies of all stable conformations of permethylated oligosilane conformers of any length except those with folded chains can now be estimated accurately from a small increment set, and the MP2-based results represent the best current estimates for conformer energies in the gas phase. It is likely that future more accurate computed or measured energies of the three conformers of n-Si 4 Me 10 , the eight or nine conformers of n-Si 5 Me 12 , and approximately 10 specifically chosen conformers of n-Si 6 Me 14 will automatically provide improved increment sets and thus more accurate prediction of stable conformation energies for permethylated oligosilane chains of all lengths. Relative energies of conformers in solution are not predicted well by the MP2 calculations and will probably require an explicit consideration of solvent effects. In the meantime, they are best approximated by the HF calculations.

Photoelectron spectra and solution UV absorption and magnetic circular dichroism (MCD) of hexamethyldisilane (1), hexaethyldisilane (2), hexa-tert-butyldisilane (3), and the 1,(n+2)-disila[n.n.n]propellanes [n = 4 (4) and 5 (5)] were measured, as was the linear dichroism (LD) of 3 and 4 partially aligned in stretched polyethylene. The results support the assignment of the lowest energy electronic absorption band of the disilanes 1−5 to a doubly degenerate σSiSi(HOMO) → π*SiC(LUMO) transition and of the next band, observed in the solution spectra of 2−4 and in the gas-phase spectrum of 1, to a σSiSi → σ*SiSi transition. MP2/VTZ optimized geometries of 1−5 and ab initio molecular orbital energies (HF/VTZ//MP2/VTZ) and ionization potentials (ROVGF/VTZ//MP2/VTZ) of these disilanes reproduce the reported geometries and the trends observed in the photoelectron spectra, respectively. B3LYP/6-31G(d,p) calculations of the Kohn−Sham orbital energies and TD B3LYP/6-31G(d,p) calculations of transition energies and intensities of 1 as a function of Si−Si bond length suggest that many of the features of the UV absorption spectrum of 3, including the small energy difference between the two transitions observed and the large extinction coefficient of the band peaking at higher energy (σSiSi → σ*SiSi), are due to its very long Si−Si bond.