*2.1. Population of Hal*−···*CH3Y Tetrel Bonds in Crystals*

The search of short contacts between a halide anion and the methyl carbon, Hal−···CH3–Y, in Cambridge Structural Database (CSD) v.5.39 (The Cambridge Crystallographic Data Centre, Cambridge, UK) [39] was performed with the following restrictions: organic derivatives without disordered, polymeric, powder, organometallic and repeating structures have been considered. The main condition was to choose interactions in which the halide anion, Hal− = Cl, Br, I, was placed on the extension of the C–Y covalent bond, where Y = C, N, O. We set that condition using the angle θ (Hal−–C–Y), the value of which was in the range from 160◦ to 180◦. At the same time, we selected the structures with interatomic distances, d(Hal, C), falling into the range (rvdw(C) + rvdw(Hal) ± 0.2 Å), where rvdw is the Bondi atomic radius [40]. The total number of selected structures that satisfied those conditions was 164. The analysis of the obtained sample has shown that the Y atom covalently bound with the CH3 group is nitrogen in most cases. We have found 43 cases of Cl−···CH3–N interactions, 36 cases of Br−···CH3–N interactions and 53 cases of I−···CH3–N interactions. Oxygen and carbon are involved in such covalent bonds much less frequently and approximately equally. In these cases, the methyl group forms more interactions with the Cl− anion than with the Br− and I− anions taken together. It can be concluded that the polarity of the covalent bond CH3–Y affects the probability and strength of the tetrel bond formation. It should be noted that the distances d(Hal, C) for the cases of Hal−···CH3–C interactions everywhere exceed the sum of van der Waals radii (Figure 1). If the CH3-group is bound with oxygen (Hal−···CH3–O), the distances d(Hal, C) can be less than the sum of van der Waals radii, though all of these distances are more than this sum for I− cases.

Therefore, we conclude that the studied type of interactions, Hal−···CH3–Y, are not widely spread within crystals listed in CSD, but they are not exceptional.

#### *2.2. Evidence of Electrophilic Sites for the CH3-Groups Bound in Tetrel Bonds*

Let us now look at the same examples of halide crystal structures containing tetrel bonds (Figure 2). All the results considered in this chapter were obtained for crystal structures by the calculations with the periodic boundary conditions. In the crystalline *N*,*N*,*N* ,*N* -tetramethylchloroformamide chloride, LONGEB [41], the Cl− anion forms non-covalent interactions with the Cl, H, C atoms, which are characterized by interatomic distances smaller than the sums of van der Waals radii. The Cl−···Cl–C non-covalent interaction with a distance of 3.122 Å refers to a typical charge-assisted halogen bond; the next five hydrogen bonds, Cl−···H–C, are characterized by interatomic distances ranging from 2.924 to 2.664 Å. Finally, the Cl−···CH3 interaction of 3.425 Å can be called a tetrel bond. In the crystalline dimethylmethyleneammonium chloride VAPREJ [42], the chloride anion forms eight

Cl−···H–C interactions, which are shorter than the sum of van der Waals radii and two Cl−···CH3 tetrel bonds. In the dimethylmethylenimine bromide crystal, LILLOH [43], in addition to multiple Br−···H–C interactions, there are two tetrel bonds: Br−···CH3 (3.533 Å) and Br−···CH2 (3.503 Å). Quantum-topological analysis of the electron density in all considered crystals have confirmed the presence of the Hal−···C bond path and bcp of electron density (Table 1). Our series of tetrel bonds in the considered crystals does not vary much, and we have observed the small changes in electron density at the bond critical points, (rbcp), which are in the range 0.0042–0.0070 a.u. for Cl−···CH3–Y and 0.0060–0.0068 a.u. for Br−···CH3–Y.

**Figure 1.** The distribution of the Hal−···CH3–Y interactions in crystals for Cl<sup>−</sup> (green), Br<sup>−</sup> (brown), I − (violet); the blue lines mark the sum of van der Waals radii.

**Figure 2.** Fragments of structures with tetrel bonds and other non-covalent interactions in crystals LONGEB (**a**), VAPREJ (**b**), POSTUM02 (**c**), LILLOH (**d**). The interatomic distances are given in Angstroms.


**Table 1.** Experimental and calculated tetrel and C–Y bond lengths, D (Å), angles Hal−···C–Y, θ and calculated electron density, (rbcp) (a.u.) at bond critical points for considered crystals.

It is possible to demonstrate the electrophilic site on a carbon atom using the electrostatic potential (ESP) mapped on the isosurface of electron density or the distribution of Electron Localization Function (ELF) [44] for CH3-group, which participate in a tetrel bond. For example, relatively higher positive values of ESP on the isosurface of electron density (0.003 a.u.) we can see in the region of the σ-hole, which belongs to the C atomic basin in trimethylammonium cation (Figure 3a,b).

**Figure 3.** (**a**) ESP in the trimethylammonium cation on the isosurface of electron density of 0.003 a.u.; (**b**) contour map of ESP in the plane N-C-H, red point indicates the maximum of ESP on the van der Waals surface (red line) and belongs to C atomic basin.

Moving strictly along the line linking the Cl(1)− and C(4) atoms of CH3-group (Figure 4a), we reach the extension of the covalent bond formed by CH3-group. Along this line the ELF is less than 0.5 near the carbon atom, showing the region of the reduced probability of electron pairing. It can be considered as the manifestation of the carbon atom σ-hole. The values of ELF (rbcp) at the bond critical points do not exceed 0.05. This excludes the hypothesis about significant covalent character of the Hal−···CH3Y tetrel bonds. At about 0.8 Å from the chloride anion nucleus the maximum values of ELF are distributed around the circumference. Figure 4b, depicting the tetrel bonds formed by the bromide anion in LILLOH crystal, shows a similar ELF distribution. Near the carbon atom and along the Br(1)<sup>−</sup> ···C(2) line the ELF does not attain high values, but it increases sharply, affecting the basins of hydrogen atoms, if we slightly deviate from this line.

**Figure 4.** The ELF for (**a**) the halogen and tetrel bonds Cl(2)···Cl(1)−···C(4) in LONGEB crystal; (**b**) the tetrel bond in LILLOH crystal.

Now let us consider and evaluate how the electronic criterion works for the cases of non-covalent interactions formed by the carbon atoms of methyl groups in halide crystals. In Figure 5a it can be seen that in the LONGEB crystal, the Cl− anion forms two non-covalent interactions at least, as follows from the presence of corresponding bcp. In both cases, Cl(2)···Cl(1)−···C(4), the one-dimensional ESP minimum is closer to electron donating anion, Cl(1)− in a crystal. The electron density minima along the Cl(2)···Cl(1)<sup>−</sup> and Cl(1)−···C(4) lines are located on the side of the Cl(2) and C(4) atoms. They indicate the electrophilic site providers and dictate the name of the non-covalent bonding.

**Figure 5.** The disposition of electron density and electrostatic potential minima (a.u.) (**a**) along two interatomic lines Cl(2)···Cl(1)−···C(4), (Å), in LONGEB; (**b**) along the tetrel bond in LILLOH. The arrows point to the electrophilic site provider.

According to the proposed electronic criterion, the first interaction, Cl(2)···Cl(1)−, can be categorized as a charge-assisted halogen bond, and the second one, Cl(1)−···C (4), is a tetrel bond enhanced by charges. In Figure 5b, the minimum of electron density along the Br(1)−···C(2) line is located on the side of the C(4) atom, while the minimum of ESP is closer to Br(1)−. Such disposition of minima shows that the carbon atom accepts electrons along the Br(1)−···C(2) line and that interaction can be called a tetrel bond.
