Abstract
The interpretation of the distortions of the electron distribution in [2,2]cyclophanes (22-CPs) is controversial. Some studies indicate that there is an accumulation of electron density (ρ) outside the cavity of 22-CPs. The nature of through-space (ts) interaction is still under debate. The relative importance of ts and through-bond (tb) is an open question. In an attempt to clarify these points, we have investigated five 22-CPs and their corresponding toluene dimers by molecular orbitals analysis, electron density difference analysis, some topological analysis of ρ (quantum theory of atoms in molecules (QTAIM), electron localization function (ELF) and noncovalent interactions (NCI)), and energy decomposition analysis with natural orbitals for chemical valence (EDA-NOCV). ρ is concentrated inside the inter-ring region. All the analyses indicated that ts is predominant. The ts is composed by attractive dispersion and Pauli repulsion, with a small covalent contribution. Except for 1 and 6, all the compounds present inter-ring bond paths.
Highlights
Cyclophanes are compounds that bear one or more aromatic rings, decks, connected by aliphatic chains, bridges
The compounds had their geometry optimized, and the vibrational frequencies were calculated by the PW6B9536D3(BJ)37/def2-TZVP38 computational model; the Orca 4.04
The interacting quantum atoms (IQA) analyses were performed with AIMAll 19.02.13.40 The electron localization function (ELF) and non-covalent interaction (NCI) analyses and Hirshfeld partition of electron density were conducted with Multiwfn 3.6.41 For the energy decomposition analysis (EDA)-NOCV calculations, the geometries of all the studied compounds were optimized without restraints, and the vibrational frequencies were calculated from the BLYP42-D3(BJ)37 method and TZ2P43 basis set
Summary
Cyclophanes are compounds that bear one or more aromatic rings, decks, connected by aliphatic chains, bridges. In some [2,2]cyclophanes, as [2,2]paracyclophane, (1), the inter-ring distance is shorter than twice the carbon van der Waals radius. Despite several studies on the electronic structure of [2,2]cyclophanes, mainly 1, certain points still are not totally understood, some related to the transannular interactions, or the interaction between aromatic rings, π-π interaction, or stacking. The electron density (ρ) distribution inside or outside the inter-ring region, or cavity. Some authors observed a concentration of ρ outside the cavity, the “toothpaste-tube effect”. The use of photoelectron spectroscopy for [2,2]paracyclophanes with an increasing number of bridges (2PCPs) indicated that the electron density is not concentrated outside the inter-ring region.. Changes in the inter-ring distances reinforced the conclusions of both studies.
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