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.