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.