FIGURE 1 is near here
2
| COMPUTER METHOD
Data for halogen bonds observed in crystal structures of
1,2-diiodoolefins reported by Hettstedt et al.29 have
been used as references for quantum chemical calculations to analyze
noncovalent interactions. The structures of monomers and dimers were
obtained from the Cambridge Crystal Structure Database (CCSD). Geometry
optimization, molecular electrostatic potential, and interaction energy
calculations were carried out using the Gaussian09 program
package30-33. The DFT-D3 method, which is recommended
in studying noncovalent interactions34-37, was applied
to the monomer and dimer optimizations. Both Kolar et al. and Banza et
al.38, 39 verified that the B3LYP-D3 method combined
with the “def2” basis set series can be used to successfully examine
halogen bonds and the properties of σ-holes. Therefore, the
B3LYP-D3/6-311++G(d,p) and B3LYP-D3/def2-TZVP levels of theory were used
to optimize the structures of monomers. Frequency calculations were run
to be sure that the geometry was a potential energy minimum (no negative
frequencies were obtained). The keyword “counterpoise” was used for
the calculations of corrected interaction energies (∆E (AB))
including the inherent basis set superpositon error
(BSSE)40 according to Eq. (1).
∆E (AB) = E (A,B) – { E (A) + E (B) } + BSSE
(1)
Here, E (AB) is the total energy of dimer AB and E (A) andE (B) are the energies of monomers A and B, respectively.
The electrostatic potential on the molecular surfaces of all monomers
was analyzed in order to gain insight into the nature and directionality
of the halogen bond interactions being considered herein. The
electrostatic potentials were considered to be an outer contour of the
electron density, and were cut off at the 0.001 au
(electrons/bohr-3) surface, as proposed by Bader et
al. 41. The most positive value of the potentials (the
local maximum) is referred to as VS,max. Natural bond
orbital (NBO) calculations were performed using the NBO 3.1
program42 as implemented in Gaussian09. The QTAIM
theory was applied to find critical points and these were analyzed in
terms of electron density and the Laplacian. The topological properties
at the bond critical points (BCPs) of halogen bonds were computed with
the program-AIMALL 200043.