Figure 2. Energetic barriers between zigzag and helical
conformers [E (zigzag) - E (helical)] calculated for
perchloroalkanes according to their backbone length, where positive
values indicate helical stabilization and negative values indicate
zigzag stabilization.
All molecules have their helical geometries as the most stable ones
according to ΔE (T), but for different reasons. While both
perfluoro- and perchlorosilanes exhibit the same trend of
perfluoroalkanes, in which hyperconjugation stabilises helical
geometries, perchloroalkanes helical geometries are favoured by Lewis
energies [ΔE (L)] and hyperconjugation favours zigzag
geometries, as well as both steric and electrostatics. Steric zigzag
favouring is especially peculiar, once perchloroalkanes are steric
overcrowded because of short C-C bond lengths (~154 pm)
and large Cl van der Waals radii (~175 pm). These
results raise an intriguing situation: if hyperconjugation,
electrostatics and sterics are driving perchloroalkanes to zigzag
geometries, which component (steric or electrostatic) of the Lewis
energy [ΔE (L)] is the source of helical stabilisation? To
highlight the helical preference, we used the following criteria:
geometric parameters, detailed NBO analysis and by applying the QTAIM
method.
2.1 Geometric parameters, Electronic Distribution and Orbital
Hybridization. The analysed geometric parameters were 1,3-Cl,Cl
distances, C-C-C bond angles and the dihedral angles showed in Figure 1.
Perchloroalkane zigzag geometries have 1,3-Cl,Cl shorter distances than
helical ones: the average 1,3-Cl,Cl distance in zigzag perchloroicosane
is 306 pm versus 316 pm in helical geometry (Table S1 in the Supporting
Information). As 1,3-Cl,Cl distances are expected to represent the most
important repulsive Cl-Cl steric interactions, this result indicate that
zigzag geometries should experience higher sterical effects. Also,
zigzag geometries have higher C-C-C bonding angle values, closer to
120°, than helical ones (average angle is 118.8° in zigzag
perchloroicosane versus 116.4° in helical), suggesting a higher scharacter in σCC orbitals for zigzag geometries, which
in turn would indicate higher electronegativities for its C atoms.
Comparing electronic populations obtained from the Natural Electron
Configuration for s and p orbitals of zigzag and helical
perchloroicosane (Table S3), one can observe that s orbitals have
higher electronic population in the zigzag than in the helical
geometries. Orbital hybridizations in Tables S4 and S5 show that
σCCl orbitals have 22.2% s character in zigzag
geometries and 21.6% in helical geometries. Thus, both geometric and
electronic data point to higher s character in
σCCl bonding orbitals in the zigzag geometry, indicating
higher electronegativity to C atoms in this geometry.
Geometric and electronic evidence support that helical favouring is
essentially provided by steric effects, as it would be intuitively
expected for these perchloroalkanes. Moreover, short C-C bond lengths
and the large Cl atomic radii promotes geometrical deformation to
diminish steric repulsion between Cl orbitals. Such geometric
deformation makes C atoms more electronegative, thus in agreement with
Bent’s rule,[35] and withdraw electron density from Cl atoms. The
steric NBO energy [ΔE (NSA)] is calculated from the difference
between the Natural Localised Molecular Orbitals (NLMO) and the
correspondent Preorthogonal NLMO, which allows to estimate the effect of
Pauli exchange antisymmetry and spatial confinement on the molecular
energy.[27] As both NLMO and PNLMO energies are dependent on
electron density, the electron density withdrawing from Cl to C could
presumably be the responsible for NBO underestimation of steric effects
in zigzag geometries.
2.2 Steric Interactions. Despite the sum of all individual
steric interactions in perchloroalkanes results in a negative value
according to NBO (Table S6), indicating that zigzag geometries suffer
less steric effects, there are two interactions with large positive
values. The most important one is the 1,3-Cl-Cl repulsion, and these
local destabilising interactions are more intense in zigzag than in
helical geometries. These interactions would supposedly be capable to
dominate the zigzag/helical steric relative energies if they were not
underestimated by NBO, as a result of Cl electron density withdrawing by
C atoms. It is worth mentioning perfluoroalkanes and both perfluoro- and
perchlorosilanes do not show relevant repulsive interactions leading to
helical geometries, as expected by considering F smaller radii and Si-Si
longer bonds.
Because zigzag geometries showing overall smaller steric effects than
helical ones in perchloroalkanes are unexpected, they were recalculated
using the HF/6-31G** level (Table S7) in order to investigate possible
artifacts, as recommended by Weinhold.[27] Despite numerical
expected differences, there is just one significant qualitative
difference: steric interactions between LP of Cl atoms and
σCC orbitals become strongly zigzag favouring in the
HF/6-31G** level. However, the steric interactions between the LPs of Cl
atoms are more destabilising in zigzag geometries and remain helical
favouring.
2.3 Electrostatic Interactions. Figure 3 shows total charges
for C atoms obtained from the Natural Population Analysis[36] (NPA)
and its decomposition into its Lewis and non Lewis (NL) contributions
for perchloroalkanes, represented as the median value for all C atom for
each molecule/geometry.