Chemistry Reference
In-Depth Information
that basis set superposition errors (BSSEs) [153, 154] could be large in these
calculations, particularly at the 6-31G
∗
level.
More recently, Zhao and Truhlar created a benchmark database of non-
bonding interactions, including halogen bonded complexes, and compared
a large number of computational methods [155, 156]. Neither iodine nor
bromine electron acceptors were included in the surveys, but the results do
reveal some of the challenges that exist in this area. For example, it was
shown that all local spin density approximation (LSDA), generalized gradi-
ent approach (GGA), and hybrid GGA (GGA with HF exchange) function-
als systematically overestimate binding energies, though the latter approach
was typically more successful. The best functionals were found to be the
MPWB1K and MPW1B95, which are hybrid meta GGA functions (where the
functional also depends on kinetic energy density). These are recently de-
veloped functionals [157] that outperform the MP2 approach with these test
complexes. Other functionals, including BHandHLYP [158] and the widely
used B3LYP (both hybrid GGA) [159], also performed well.
3.2
Oxygen Electron Donors
While the first observations of halogen bonding were probably complexes
formed in aqueous solutions, there are relatively few investigations of the oxy-
gen halogen bond in the recent literature.
Davey, Legon, and Thumwood have studied several classes of donor-
acceptor interactions through combinations of rotational spectroscopy and
ab initio calculations. In a recent paper, they investigate the H
2
O
Cl
2
com-
plex, noting that it probably plays a role in both the ozone depletion cycle
(through the reaction HOCl + HCl
···
Cl
2
+H
2
O) and in water treatment
through the reverse reaction to form hypochlorous acid [160]. They exam-
ined seven isotopomers resulting from various combinations of deuterium
and
35
Cl/
37
Cl labeled complexes by pulsed-jet microwave spectroscopy. An-
alysis of the spectra provided rotational and nuclear quadrupole coupling
constants that were used in conjunction with models to give the geometry,
binding energy, and estimations of electron transfer in the complex. The
authors noted that the geometry of the complex is virtually identical to that
of the hydrogen bonded analogue H
2
O
→
HCl, though the binding is not as
strong. They also suggest that the extent of charge transfer from H
2
Oto
chlorine is very small, but the charge transfer of the bound chlorine to the
unbound chlorine is significantly larger. Thus the interaction is primarily
electrostatic in nature. Similar studies with ICl or IBr and a series of elec-
tron donors have shown that the extent of electron transfer decreases with the
order PH
3
>
NH
3
>
C
2
H
4
∼
···
HCN [142, 161].
Grabowski and Bilewicz have found an interesting effect in the study
of halogen bonding between formaldehyde and ClF (
1
) [162]. Using MP2/
H
2
S
>
C
2
H
2
>
H
2
O
>
CO
∼
Search WWH ::
Custom Search