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.