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.