Chemistry Reference
In-Depth Information
CP
Counterpoise
CCSD(T)
Coupled cluster approach with single and double substitutions and inclusion
of noniterative triple excitations
DFT
Density functional theory
MP2
Second-order Møller-Plesset perturbation method
NQCC
Nuclear quadrupole coupling constant
RHF
Restricted Hartree-Fock method
1
Introduction
Intermolecular interactions in gas-phase complexes, liquids, and organic and
inorganic molecular crystals are, in general, characterized by intermolecular
distances in the range of van der Waals distances. The most important excep-
tion is the case of hydrogen bonding. There, considerably shorter intermole-
cular distances are observed. The formation of a hydrogen bond A - H
B
is usually encountered by a significant elongation of the A - Hbondandan
increase in the polarity of the A - H bond. Consequently, a spectroscopically
observable, substantial red shift of the A - H stretching frequency occurs, ac-
companied by a large increase of the infrared intensity.
Among the noncovalent interactions, there is a second interesting ex-
ception with shorter intermolecular contacts: the case of halogen bonding.
Originally, the term
halogen bonding
was introduced for complexes of di-
halogens XY with different Lewis bases as interaction partners. Previously,
complexes of this type were actually often characterized as charge-transfer
complexes or as electron donor-acceptor complexes. Later, a more general
definition of halogen bonding was suggested, following closely the analogy to
the hydrogen bonding case. Thus, any noncovalent intermolecular arrange-
ment A - X
···
···
B, where X is a halogen atom, is included in the definition. The
characteristic structural and spectroscopic properties of the halogen bond
have indeed much in common with those of the hydrogen bond. There too,
the X - YorA- X bond is usually elongated upon complex formation, the
A - X stretching frequency is red-shifted, and the corresponding infrared in-
tensity is enhanced.
In sharp contrast to the case of hydrogen bonding, most of the experi-
mental gas-phase investigations on the complexes between dihalogens and
Lewis bases stem exclusively from rotational spectroscopic investigations. For
these complexes, there are hardly any vibrational spectroscopic data avail-
able. Therefore, theoretical investigations are currently the only way to obtain
answers on all questions concerning the details of the intermolecular inter-
action, in particular, the structure changes induced by the formation of the
halogen bond and the frequency shifts taking place upon complex formation
in the interacting monomers.
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