Introduction
Halogenated polymers are widespread molecules in our daily routine:
polytetrafluorethylene and polyvinylchloride are two of the most
well-known carbon-based polymers,[1] while fluorosilicon polymers
have recently became leading motifs of rubbery
materials.[2]–[4] Fluoroalkanes have proved to be interesting
molecules even beyond their economic importance, but also concerning
debates about the dominant stereoelectronic effects on their
geometry.[5]–[9] Recent advances have been recently achieved by
our group, relating stereoelectronic effects and geometry on
perfluoroalkanes[10] and multivicinalfluoroalkanes.[11] The
systematic study of perchlorinated molecules have been raised since
early 70’s[12] and had reached significant advances with the arising
of computational chemistry.[13]–[20] Despite the experimental
limitations on synthesizing and isolating perchloroalkanes with more
than 4 carbon atoms,[21], [22] theoretical studies about them
are particularly interesting once they allow to analyse stereoelectronic
effects in sterically overcrowded molecules. Moreover, perfluoro and
perchlorosilanes are the perfect analogues to be studied with perfluoro
and perchloroalkanes, once their larger bond length in the silicon
backbone is expected to minimize steric effects, reducing overcrowding
and allowing other stereoelectronic effects to arise as leading effects.
However, for the best of our knowledge, there is a lack of advances in
this topic after the seminal study of Neumann,[15] which presented a
model to understand and predict geometric preferences in small
halogenated molecules for the first time. In order to bring advances to
this important topic, this study compares homologous series of perfluoro
and perchloro alkanes and silanes using modern quantum mechanic methods
as the Natural Bond Orbital (NBO)[23] and the Quantum Theory of
Atoms in Molecules (QTAIM)[24], in order to understand their
conformational preferences and to rationalize the stereoelectronic
effects that rule such preferences.
Computational Details
Perchloroalkanes, perfluorosilanes and perchlorosilanes with backbone
containing n=4 to 20 C or Si atoms were fully optimised at the
B3LYP/6-31G** level of theory for zigzag and helical geometries.
Frequency calculations were performed on all optimised geometries at the
same theoretical level to ensure only positive eigenvalues for all
helical geometries. Relative energies between zigzag and helical
geometries for compounds with backbone containing n=4 to 10 C or Si
atoms were compared with single point calculations from gold
standard [25], [26] DLPNO-CCSD(T)/cc-pVTZ with cc-pVTZ/C as
auxiliary basis set. Figure S5 show the comparison between theoretical
levels. The B3LYP/6-31G** level of theory showed consistent results in
comparison with DLPNO-CCSD(T)/cc-pVTZ and was used to all remaining
calculations. NBO 6.0 calculations were performed on fully optimised
geometries, as well as QTAIM calculations. Natural Steric Analysis
calculations were first performed at B3LYP/6-31G** level of theory and
subsequently calculated at HF/6-31G** to search for eventual
artifacts.[27] All optimisations, frequency calculations and NBO
calculations were performed in Gaussian09 program, Revision D.01.
DLPNO-CCSD(T)/cc-pVTZ single point calculations were performed in ORCA
4.2.1.[28], [29] QTAIM calculations were performed in AIMALL
package, version 19.10.12.[30]
Results and Discussion
Helical geometries with C-C-C-C or Si-Si-Si-Si dihedral angles of
~160o were the overall preference for
all molecules - perfluoroalkanes, perchloroalkanes, perfluorosilanes and
perchlorosilanes. The calculated molecular central dihedral angle values
are showed in Figure 1.