3.3 Binding energies
The uncorrected binding energy, ΔE , was calculated using the supermolecular procedure proposed by Pople.83 That is, ΔE is the difference between the sum of the total electronic energies of two monomer species (OX2 and A) and the total electronic energy of the complex X2O···A (A = F, Cl, Br, CN, Br3, SCN, NCO, NO3). We found that the most stable complexBr2O···SCN has a ΔE of –90.05 kcal mol-1, while the least stable complex Cl2O···OCN has a ΔE of –2.30 kcal mol-1Table 1).
The ΔE of F2O···CN (–25.52 kcal mol-1) is significantly larger than that of F2O···NC (–4.32 kcal mol-1), indicating that the C-end of the CN anion is significantly more nucleophilic that the N-end. Similarly, the ΔE of the complex X2O···NCO (X = F, Cl, Br) is significantly larger than that of the corresponding X2O···OCN complex, showing that the O in OCN is a poorer nucleophile than N. This feature is consistent with SCN as well, where the X2O···SCN (X = F, Cl) complexes are markedly stronger than the corresponding X2O···NCS complexes. In the case of Br2O···SCN and Br2O···NCS, the energy minimization procedure changed the geometry of the latter complex into that of the former, with a ΔE of –94.61 kcal mol-1. This suggests that the Columbic repulsion between the N and O atoms of the interacting monomers is predominant, which prevents the N-end of NCS atttracting the O-end of Br2O, revealing that S is a better nucleophile (hence, more reactive) than N. The complex Br(NCS)O···Br (31) is equivalent to Br2O···SCN (29-30). However, a ΔE of –93.11 kcal mol-1 for the O···Br bond in this system, given that Br(NCS)O and Br are the two monomers of the system, reveals a potentially covalently bound interaction. Although the complex systems 29 and 30 represent the same Br2O···SCN,the system is numbered differently to show that no matter which end of the SCN ion is bonded with Br2O the ΔE between Br2O and SCN is the same (Table 1) given that the total electronic energies of these two isolated systems were used for the calculation of ΔE . The effect of basis set superposition error (BSSE) on ΔE is not negligible for all systems examined, with BSSE values ranging from –4.54 kcal mol-1 for the complex Br2O···SCN (29-30) to –0.50 kcal mol-1 for the complex F2O···NC (5).
The ΔE (BSSE) values for the complexes of X2O···A and NCFO···A (A = F, Cl, Br) are comparable with those widely recognized to arise from charge-assisted hydrogen-bonded complexes. For example, the energy of intermolecular interaction for HF⋅⋅⋅F was reported to be –38.6 kcal mol-1.43,44 Wolters and Bickelhaupt50 have examined a representative selection of hydrogen‐, fluorine‐ and iodine‐bonded model complexes, DX⋅⋅⋅A, using ZORA‐BP86/TZ2P (where D = F and I; X = H, F and I; and A = F, Cl, Br and I) and found the interaction energy to be between −80.6 and −14.5 kcal mol-1 (for IH⋅⋅⋅F and IF⋅⋅⋅Cl, respectively). Intermolecular interactions of similar strength have been reported for halogen bonded systems, such as −N–X+O–N+, wherein an oxygen atom served as an unusual halogen bond acceptor.49 The data in Table 1 show a wide range of strengths of the ion-molecule interactions, which can be classified as ultra-strong in some cases (29-31), very strong (8-9,15-17), strong (1-4, 6-7,18-21,23, 26-28, 32), moderate (11, 13, 24), or weak (5, 14, 22, 25, 33-34), based on the degree of interaction between the anion and the molecule, and the recommendation provided in an earlier study.48 There is no obvious relationship notable between the ΔE (BSSE) and the intermolecular distances within the entire series of complexes examined.
Our results indicate that the O atoms in molecules such as X2O (X = F, Cl, Br) and NCFO have tremendous potential to form unusually strongly bound complexes, driven by chalcogen bonding. This is especially when the electrophile on the O atom in molecules is exposed to anions.