3.2.1 | Geometries
Six 6 dimers of diiodoalkene were provided in the experimental supplementary data of ref. [29], including dimers 2, 3, 5, 6, 7b and 8. Each of these dimer structures was partially geometry optimized at the B3LYP-D3/6-311++G(d,p) and B3LYP-D3/def2-TZVP levels of theory. Because the dimers were too large to be fully geometry optimized, partial atoms or groups were fixed to ensure optimization is successful, with the fixed atoms or groups chosen to be far away from the locationsof the noncovalent bonds (e.g. I…I, I…O, I…C(π), I…H and O…O) formed. The details of fixed atoms or groups for each dimer are listed in the Supporting Information . The geometries optimized at the B3LYP-D3/6-311++G(d,p) level of theory are show in Figure 4. The noncovalent bond lengths of each dimer, including the values in crystal structures and the values calculated at the B3LYP-D3/6-311++G(d,p) and B3LYP-D3/def2-TZVP levels of theory, are listed in Table 3. The deviations between the crystal structure values and the values calculated at the two levels of theory are quite small. The average deviation between the noncovalent bond lengths optimized at the B3LYP-D3/6-311++G(d,p) level of theory and the values in crystal structures are 0.123 Å, and the corresponding deviation is 0.101 Å between the values optimized at the B3LYP-D3/def2-TZVP levels of theory and the crystal structure values. Again, the noncovalent bond lengths optimized at these two levels of theory are similar. Because the electron charge distribution of the halogen atom is anisotropic, the halogen can act both as the Lewis acid and as the Lewis base 44-47. This is why dihalogen bonds are possible in the 6 dimers.