The nature of the C–Cl⋯Cl–C intermolecular interactions found in molecular crystals: a general theoretical-database study covering the 2.75–4.0 Å range

Abstract

The nature of C–Cl⋯Cl–C interactions in molecular crystals has been evaluated at the MP2/aug-cc-pVDZ computational level after test computations on simple model systems showed that such a computational level predicts for model dimers the same angular dependence and impact of the C(ipso) atom hybridization as those at the MP2/CBS computational levels. Thus MP2/aug-cc-pVDZ calculations predict a C(spⁿ)–Cl⋯Cl–C(spⁿ) strength of −0.73, −0.87, and −0.96 kcal mol⁻¹ for n = 3, n = 2 (non-aromatic), and n = 1 (all BSSE-corrected values) at their most stable orientation, while for the same orientations and hybridization MP2/CBS calculations predict values of −1.14, −1.29, and −1.40 kcal mol⁻¹. The first group of computations on the model dimers allows conclusion that the strength of the C–Cl⋯Cl–C interactions depends on (a) the number of short-distance Cl⋯Cl contacts involved, (b) the hybridization of the C(ipso) atom [E(Csp²) > E(Csp) > E(Csp³)], (c) the degree of chlorination of the C(ipso) atom, and (d) the relative orientation of the two C–Cl groups. Two types of minima were found in the E(θ₁, θ₂) potential energy surface [θ₁ = <C(1)–Cl(2)⋯Cl(3) and θ₂ = <Cl(2)⋯Cl(3)–C(4)]: type I minima, where θ₁ = θ₂ = 90°, and type II minima, which is energetically more stable, where θ₁ = 180° and θ₂ = 90° or θ₁ = 90° and θ₂ = 180° (the orientation where θ₁ = θ₂ = 155° (a type I geometry) is a saddle point in E(θ₁, θ₂) that connects the two type II minima). The interaction energy was also computed for 45 C–Cl⋯Cl–C containing dimers extracted from the Cambridge Crystallographic Database (CCDB) and having a Cl⋯Cl distance smoothly distributed within the 2.75–4.0 Å range. The nature of these interactions was further characterized by looking at (a) the dominant component of the dimer interaction energy and (b) the characteristic properties of their only Cl⋯Cl bond critical point (evaluated from an atoms-in-molecules (AIM) analysis of the dimer electron density). Their interaction energy is dominated by the dispersion component, although a much weaker electrostatic component is also present in some cases. These interactions fail to fulfill the strength–length distribution that should correlate E_int and the Cl⋯Cl distance. A previously proposed correlation between the electron density at the Cl⋯Cl bond critical point and the strength of the C–Cl⋯Cl–C interaction is shown to fail for short Cl⋯Cl distances.