Chemistry Reference
In-Depth Information
The LANL2DZ basis set was augmented by a set of six d polarization func-
tions as successfully used by Bopp [169] and others. Complex geometries
were optimized and the effects of the acetonitrile solvent were modeled using
the polarized continuum model of Tomasi and coworkers [176, 177]. The na-
ture of the halogen bond was described using NBO analysis. The authors
found a rough correlation between the electron donor HOMO and the
V
OC
.
The higher the HOMO, and thus the smaller the energy difference between
that orbital and the I
2
LUMO, the higher the
V
OC
. Likewise, decreasing
G
of
complex formation (increasing halogen bond strength) also correlated with
V
OC
.
Valdes and Sordo have studied the interaction of PH
3
and NH
3
with BrCl
using either all-electron or effective core potential (ECP) MP2 methods, and
several different basis sets [178]. They found that ECPs could give results
nearly equivalent to their all-electron counterparts. PH
3
was found to form
weaker halogen bonds than NH
3
.
∆
3.4
Sulfur and Selenium Electron Donors
The interaction of dihalogens, particularly diiodine, with sulfur and selen-
ium electron donors has been an area of increasing interest over the past
decade because of potential biological, pharmaceutical, and electronic mate-
rials applications [35, 179]. Devillanova and coworkers have recently reviewed
the solution behavior of a large number of chalcogenides and I
2
,partic-
ularly thiones, selones, sulfides, and selenides [180]. Correlations between
computational methods, thermodynamic parameters, and spectroscopic data
(UV/Vis,
13
C NMR, Raman, UPS) were discussed.
The interaction of chalcogenides and dihalogens can lead to a large variety
of complexes and salts. The initial interaction in both the gas phase [15] and
in solution [180] seems to be the formation of a halogen bonded structure.
The available literature shows that Se electron donors bind I
2
more strongly
than do S electron donors, and that thiones and selones bind more strongly
than the corresponding sulfides and selenides. The electron donor ability of S
or Se depends greatly on the environment, and the formation constants vary
widely. For compounds containing the framework R - C(
=
X)-R (X=S, Se),
the
K
values decrease according to the following sequence of R: NR
≥
alkyl
≥
aromatic
O.
Bouab, Yanez, and coworkers have noted that thiocarbonyl electron
donors can form two distinctly different charge-transfer complexes in so-
lution [181]. Calculations at the HF/LANL2DZ
∗
and MP2(full)/LANL2DZ
∗
//
LANL2DZ
∗
levels indicate that a charge-transfer band in the 300-nm region
results when the halogen bond lies in the plane of the C=Sgroup,while
abandat350 nm results when the bond is perpendicular to that plane (
4
and
5
, respectively). The halogen bond in both types of complexes is primarily
≥
S
≥
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