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have been chosen as Lewis bases B, among them CO, C
2
H
2
,C
2
H
4
,H
2
O, H
2
S,
HCN, NH
3
,N(CH
3
)
3
,CH
3
CN, H
2
CO, PH
3
, aromatic rings like benzene, fu-
ran, thiophene, and saturated rings like oxirane and thiirane. These data have
been discussed in two extensive reviews [4, 5] and are also treated in the con-
tribution by Legon to this volume [6]. Of particular importance is the series
of complexes between dihalogens and the ammonia molecule, since in these
cases rich experimental data exist. Equally, a substantial number of theor-
etical calculations are already available. Therefore, the series of XY
···
NH
3
complexes with X,Y
is chosen as a representative example of anal-
ogous series with other Lewis bases. The most important structural and
vibrational spectroscopic properties of these complexes are thus discussed in
some detail in the following sections.
∈
{
F, C l , Br
}
2.1
XY
···
NH
3
Complexes
Five out of the six conceivable XY
NH
3
complexes have previously been
investigated with the aid of rotational spectroscopy by the group of Legon [7-
11]. Only the complex with BrF is missing because of the known instability
of this molecule toward a disproportionation reaction. A large number of
theoretical calculations have already been devoted to these complexes, rang-
ing in quality from conventional restricted Hartree-Fock (RHF) to MP2, and
various DFT calculations [12-33]. The trends in the calculated equilibrium
structures, stabilization energies, vibrational frequencies, dipole moments,
and nuclear quadrupole coupling constants (NQCCs) within the series have
been discussed by various groups [23, 26, 29-34]. Calculations beyond the
MP2 approximation are, however, quite rare. CCSD(T) calculations including
geometry optimizations are so far only available for the complexes with F
2
,
ClF, and Cl
2
[26].
In the following, a new set of data for the dihalogens BrF, BrCl, and Br
2
,as
obtained with MP2 and CCSD(T) calculations applying the aug-cc-pVTZ ba-
sis set, is presented. Comparison is made with the earlier MP2 and CCSD(T)
data for F
2
,ClF,andCl
2
[26]. Two of the currently often used DFT vari-
ants, B3LYP and BH&HLYP, are also included. Because of the well-known
BSSE error, optimized complex structures, as evaluated with and without
CP correction, are reported. The BSSE error is negligible for the RHF and
DFT calculations when using extended basis sets like aug-cc-pVTZ. How-
ever, for the MP2 and CCSD(T) approaches the BSSE error for structures
and stabilization energies is definitely nonnegligible. The set of data neces-
sary for the characterization of the halogen bond, namely intramolecular X-Y
and intermolecular Y
···
···
N distances, harmonic vibrational frequencies, fre-
quency shifts, and infrared intensity changes as obtained with MP2 and DFT
methods, is also discussed.
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