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.