Figure 4. Median values of a) QTAIM charges for C atoms and b) QTAIM electronic kinetic energy for chlorine atoms, according to their backbone length, comparing helical and zigzag geometries.
The electron delocalisation (Figure S3) results agree with such argument since C atoms in the zigzag geometry have lower electron donation than those in the helical geometry. Therefore, it is reasonable to assume that QTAIM will represent the steric interactions more closely with the expected by chemical intuition on this group of molecules, which will not be affected by the electron withdrawing from Cl to C.
Furthermore, according with QTAIM, Cl atomic kinetic energy is consistently higher in zigzag than in helical geometries (Figure 4b): the median difference between individual atoms reaches 0.01 a.u (~6 kcal mol-1) in perchloroicosane. The higher the atomic kinetic energy, the higher the atomic kinetic energy pressure, and hence, the higher the steric effects.[39], [40] As Cl atoms are more affected by steric effects and their kinetic energy is higher in zigzag geometries, this data indicates that steric effects are higher in the zigzag geometry. Thus, QTAIM results confirms the proposal in which steric effects are the driving force for helical geometry in perchloroalkanes.
Conclusions
Steric effects are the driving force to helical geometry in perchloroalkanes. Because steric effects are diminished for F smaller radii and longer Si-Si bonds, it was observed that perfluorosilanes and perchlorosilanes have their helical geometry stabilised by hyperconjugative interactions, following the same trend observed for perfluoroalkanes. Furthermore, we confirmed that NBO steric analysis can lead to incorrect conclusions if careless analysed. This warning was raised by Weinhold, who first indicated that superficial usage of the STERIC keyword in the NBO software output can be a two-edged sword.[27] It might be thought that these problems would mainly occur in molecules not highly sterically hindered, where any kind of deviation in calculations could lead to wrong conclusions. However, this case shows the opposite: electronic effects affect NSA results when interactions are so intense that they become hidden among other interactions. In this situation, it is necessary to have another QM tool for comparison. As QTAIM is based on the electron density and therefore has a different approach for describing electrons in molecules, it is and should be used as a useful tool in combination with NBO.