Assessing the attractive/repulsive force balance in axial cyclohexane C–Hax···Yax contacts: A combined computational analysis in monosubstituted cyclohexanes

Abstract

The interactions of axial substituents in monosubstituted cyclohexane rings are studied in this work using an array of different computational techniques. Additionally, the anomalous axial preference for some bulky substituents is related to stabilizing dispersion interactions. We find that the C–Hax···Yax contacts for various substituents with distances ranging from 2 to ∼5 Å may include attractive dispersion forces that can affect the conformational equilibrium; these forces co-exist with Pauli repulsive forces effected by Yax group due to van der Waals sphere penetration. At distances between 2 and 3 Å stabilizing electron transfer interactions were calculated and the combination of natural bond orbital and QTAIM analysis showed that, in certain cases, Yax = tBu, Cax–O or Cax = O or Sax = O or Cax = S this interaction can be characterized as an improper H-bond. DFT-D3 and non-covalent interactions calculations (NCIs) in cyclohexane derivatives with Yax = SiOR3 including HYax···Hcy surfaces at distances ranging between 4 and 6 Å suggest that dispersion has a clear effect on the experimentally observed stabilization of the axial conformer. NCIs computed from the reduced density gradient help to visually identify and analyze these interactions. © 2016 Wiley Periodicals, Inc.