Conformers of n-Si6Me14: Ab Initio, Molecular Mechanics, and Additive Increment Methods

Abstract

The stable backbone conformers of n-Si6Me14 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 (Ecalc), agree with energies , obtained using additive increment sets previously derived from results for n-Si4Me10 and n-Si5Me12, with mean deviations of 0.11−0.15 kcal/mol. The energy 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; and Ecalc 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-Si4Me10, n-Si5Me12 and n-Si6Me14, including a few increments for interaction between next-nearest bond conformations. This yields energies that ...