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.