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.