Figure 1. Calculated
central C-C-C-C or Si-Si-Si-Si- dihedral angles for each molecule at the
B3LYP/6-31G* level (in degrees). At the right side, a
generic example of the central dihedral angle (highlighted). In the case
of the molecules with odd number of atoms in the backbone, it was
considered the average value between the two central dihedral angles.
Total Energetic barriers ΔE (T) were calculated at the
B3LYP/6-31G** level and were decomposed using the NBO analysis:
ΔE (L) represents the Lewis energy[31], [32] of the
molecule when hyperconjugation is not taken into account, ΔE (NL)
is the non Lewis energy that represents the stabilisation by
hyperconjugation, ΔE (NCE) stands for Natural Coulomb
Electrostatic analysis [27] and ΔE (NSA) represents the steric
energies obtained from the Natural Steric Analysis.[33], [34]
Energetic barriers were calculated using the equation\({}_{\text{barrier}}=E_{\text{zigzag}}-E_{\text{helical}}\) and their
results are showed in Figure 2 for perchloroalkanes and Figure S1 in the
Supporting Information for perfluorosilanes and perchlorosilanes.
Positive values represent helical and negative values represent zigzag
favouring interactions.