**1. Introduction**

Noncovalent interactions play key roles in crystal growth and design. In 1989, Desiraju defined the term "crystal engineering" as "the understanding of intermolecular interactions in the context of crystal packing and the utilization of such understanding in the design of new solids with desired physical and chemical properties" [1]. There are many kinds of noncovalent interactions, such as the hydrogen bond, π···π stacking interaction, dispersive interaction, σ-hole interaction, π-hole interaction, etc. [2–6]. In the preceding paper of this Special Issue, Alkorta, Elguero, and Frontera presented an excellent review of the noncovalent interactions formed by the electron-deficient elements of groups 1, 2, and 10–18 in the periodic table [7]. Here, we want to stress that the term noncovalent interaction is more general than the term noncovalent bond, and some of the noncovalent interactions such as the dispersive interactions are not noncovalent bonds [4]. What is a noncovalent bond? According to Bader's theory of atoms in molecules, in a noncovalent bond there usually is a bond path connecting the two interacting atoms and a (3, –1) bond critical point between the two interacting atoms [8]. Evidently, crystal packing is the result of the synergistic contributions of different types of strong or weak noncovalent interactions. Hence, in the field of crystal engineering, it is always significant and important to study the cooperativity and competition of these noncovalent interactions.

The π··· π stacking interaction is one of the most common noncovalent interactions in crystal engineering [9]. The benzene dimer is always considered as a model for the study of the π··· π stacking interaction [10]. Comparing with the π··· π stacking interaction in the benzene dimer, the π··· π stacking interaction in the complex between hexamethylbenzene (HMB) and hexafluorobenzene (HFB) is much stronger due to the strong attractive quadrupole–quadrupole electrostatic interaction between the two monomers. The quadrupole–quadrupole interaction is repulsive in the face-to-face structure of the benzene dimer with an interaction energy of +13.68 kcal/mol, whereas the quadrupole–quadrupole interaction is attractive in the face-to-face structure of the complex between HMB and HFB with an interaction energy of −8.53 kcal/mol [11]. The crystal structure of the complex between HMB and HFB has been reported in 1972, in which the partner molecules are stacked alternately to form infinite columns [12]. Naturally, HMB can also form π-stacked complexes with other electron-deficient perfluoro aromatic compounds. Perfluoroiodobenzenes are such compounds that we are very interested in because they are always employed as the halogen atom donors for the halogen bonds [13]. Figure 1 shows the molecular electrostatic potentials on the 0.001 a.u. electron density isodensity surfaces of HMB and 1,3-diiodotetrafluorobenzene (1,3-DITFB) along with some selected surface minima and surface maxima. The computational details of the molecular electrostatic potentials are given in the following section. As shown in Figure 1, the most negative electrostatic potentials of −22.47 kcal/mol on the surface of HMB are located 1.76 Å above or below the center of mass of HMB; the most positive electrostatic potentials of +30.76 kcal/mol on the surface of 1,3-DITFB are located on the extensions of the C–I bonds. For the 1,3-DITFB, besides the two electropositive σ-holes on the extensions of the C-I bonds, there are also electropositive regions ( π-holes) that are perpendicular to the molecular plane. As a result, 1,3-DITFB can form the C–I··· π halogen bond with HMB on the one hand, and on the other hand it can also form the strong π··· π stacking interaction with HMB [13–15]. Certainly, the π··· π stacking interactions can also be formed between two HMB molecules or between two 1,3-DITFB molecules. What is the order of strengths of all these noncovalent interactions? Will one of them, some of them, or all of them contribute to the formation of the cocrystal between HMB and 1,3-DITFB? Will there be other noncovalent interactions that we could not predict in the cocrystal structure? In this study, we solve these issues by employing a combined theoretical and crystallographic method.

**Figure 1.** The molecular electrostatic potentials on the 0.001 a.u. electron density isodensity surfaces of hexamethylbenzene (HMB) (**left**) and 1,3-diiodotetrafluorobenzene (1,3-DITFB) (**right**). The numbers are in kcal/mol. Some selected surface minima and surface maxima are also shown.

This paper is organized as follows: First, we present and discuss the calculated results for the abovementioned noncovalent interactions in the gas phase; then, after describing the structure of the cocrystal between HMB and 1,3-DITFB, we calculate and analyze the noncovalent interactions

in the crystal structure in detail. Finally, we give explanations for the formation of the special cocrystal structure.

## **2. Materials and Methods**

## *2.1. Quantum Chemical Calculation*

The geometries of the monomers and complexes in the gas phase were fully optimized at the PBE0-D3(BJ)/def2-TZVPP level of theory [16–19]. According to the corresponding frequency calculations, all the structures of these monomers and complexes are true minima on their respective potential energy surfaces. The interaction energies were calculated at the same theory level. For the complexes in the crystal structure, their geometries were directly extracted from the crystal structure, and only single-point interaction energies were calculated at the PBE0-D3(BJ)/def2-TZVPP theory level. All the interaction energies were calculated with the supermolecule method and corrected for the basis set superposition error using the conventional counterpoise method [20]. The molecular electrostatic potentials on the 0.001 a.u. electron density isodensity surfaces of HMB and 1,3-DITFB were also calculated at the PBE0-D3(BJ)/def2-TZVPP level of theory. The "ultrafine" integration grids were used for the PBE0-D3(BJ)/def2-TZVPP calculations to eliminate possible integration grid errors. All the calculations were performed with the GAUSSIAN 09 program package [21].

For the calculations of strong noncovalent interactions, many computational methods can give comparable results with experiments. The main challenges for the electronic structure calculations lie in the accurate descriptions of weak noncovalent interactions. In previous studies, we employed the PBE0-D3(BJ)/def2-TZVPP method to calculate the interaction energies of the di fferent configurations of the complex between benzene and hexahalobenzene, the complex between benzene and naphthalene, and the complex between fullerene C60 and benzene [22–24]. It was found that the results from the PBE0-D3(BJ)/def2-TZVPP calculations are in excellent agreemen<sup>t</sup> with the results from the "gold standard" coupled-cluster calculations. Considering that the noncovalent interactions studied in this work are very similar to those in previously studied complexes, the results from the PBE0-D3(BJ)/def2-TZVPP calculations should be reliable throughout this paper.

## *2.2. Crystal Preparation*

The chemical reagents HMB and 1,3-DITFB were purchased from J&K Scientific Ltd. in China and used as received. The solvent for the crystallization in this study was trichloromethane and also used without further purification. The HMB (0.0162 g, 0.10 mmol) and 1,3-DITFB (0.0402 g, 0.10 mmol) were dissolved in 10 mL trichloromethane, and the mixture was refluxed gently with stirring for half an hour. Then, the solution was filtered, and the filtrate was naturally volatilized at room temperature. After about three days, colorless block crystals that are suitable for the X-ray di ffraction analyses were obtained.
