Challenges in Studying Steric Interactions in Highly Strained
Perhalogenated Alkanes and Silanes.
Abstract
The decomposition of the molecular total energy in their
hyperconjugative, steric and electrostatic effects can lead to
interesting interpretations about the stereoelectronic effects that
govern their geometry and properties. In this work, we have studied
homologous series of perfluoroalkanes, perchloroalkanes,
perfluorosilanes and perchlorosilanes, and all molecules have preference
for helical geometries. According to Natural Bond Orbitals (NBO)
calculations, the silanes have their helical geometry stabilised by
hyperconjugative interactions, as well as the perfluoroalkanes. However,
it was surprisingly difficult to disclose that steric interactions are
ruling the helical geometry preference in perchloroalkanes by comparing
the NBO analysis and the Quantum Theory of Atoms ins Molecules (QTAIM).
Although perchloroalkanes have extremely intense steric interactions
between Cl lone pairs, some of them were underestimated by the NBO
analysis, which showed the opposite behaviour compared with QTAIM that
indicates steric effects as the leading forces to helical geometry
preference.