3.2.3 | Natural Bond Orbital Analysis
To better understand the intermolecular interactions, natural bond orbital (NBO) analysis was carried out to characterize the weak interactions. Formation of complexes involing noncovalent bonds is associated with an orbital interaction between the bonding orbital in the electron donor and the antibonding orbital in the electron acceptor. Table 5 lists the second-order perturbation energy (E( 2)) and the charge transfer (∆q ) obtained by NBO analysis. BothE( 2) and ∆q represent the transfer from one molecule (donor) to the other molecule (acceptor) in the six dimers. Owing to the time-consuming nature of the B3LYP-D3/def2-TZVP level of theory, all NBO calculations were carried out at the B3LYP-D3/6-311++G(d,p) level of theory. The second-order perturbation energy represents the degree of charge transfer from the bonding orbital to the antibonding orbital, which is the degree of electron delocalization. Ultimately, the second-order perturbation energy allows us to quantitatively evaluate the charge transfer due to the formation of the halogen bond.
The results listed in Table 5 show that there is a positive relationship between the second-order perturbation energyE(2) and the charge transfer ∆q in the studied systems. Due to the centrosymmetry of dimer 3, the charge transfer from one monomer to another in both the crystal and optimized dimers is zero. Figure 5 presents the strong linear relationship between ∆q andE(2) with the exception of 7b. Dimer 7b forms more I…H halogen bonds compared to other dimers. The linear relationship between ∆q and E(2) indicates that charge transfer is an important factor in the noncovalent bonds seen in crystal systems.