The nature of C-Cl center dot center dot center dot 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(sp(n))-Cl center dot center dot center dot Cl-C(sp(n)) 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 center dot center dot center dot Cl-C interactions depends on (a) the number of short-distance Cl center dot center dot center dot Cl contacts involved, (b) the hybridization of the C(ipso) atom [E(Csp(2)) > E(Csp) > E(Csp(3))], (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(theta(1), theta(2)) potential energy surface [theta(1) = <C(1)-Cl(2)center dot center dot center dot Cl(3) and theta(2) = <Cl(2)center dot center dot center dot Cl(3)-C(4)]: type I minima, where theta(1) = theta(2) = 90 degrees, and type II minima, which is energetically more stable, where theta(1) = 180 degrees and theta(2) = 90 degrees or theta(1) = 90 degrees and theta(2) = 180 degrees (the orientation where theta(1) = theta(2) = 155 degrees (a type I geometry) is a saddle point in E(theta(1), theta(2)) that connects the two type II minima). The interaction energy was also computed for 45 C-Cl center dot center dot center dot Cl-C containing dimers extracted from the Cambridge Crystallographic Database (CCDB) and having a Cl center dot center dot center dot Cl distance smoothly distributed within the 2.75-4.0 angstrom 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 center dot center dot center dot 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 center dot center dot center dot Cl distance. A previously proposed correlation between the electron density at the Cl center dot center dot center dot Cl bond critical point and the strength of the C-Cl center dot center dot center dot Cl-C interaction is shown to fail for short Cl center dot center dot center dot Cl distances.
