Challenges in studying steric interactions in highly strained perhalogenated alkanes and silanes

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 stabilized 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 behavior compared with QTAIM that indicates steric effects as the leading forces to helical geometry preference. Natural Bond Orbitals analysis and the Quantum theory of Atoms in Molecules were used to disclose the contribution of hyperconjugative, steric and electrostatic effects for the preferred helical geometry of perfluoro and perchloroalkanes and silanes.