Theoretical Characterization of the Trends in Halogen Bonding - Halogen Bonding: Fundamentals and Applications - page 3

Since the interactions in these complexes are occasionally quite weak and
the interaction potentials are quite soft, only high-level quantum mechanical
methods that take account of electron correlation are suitable for a reliable
description of these compounds. The theoretical results obtained are indeed
quite sensitive to the level of electron correlation chosen. Moreover, the in-
clusion of the counterpoise (CP) correction to the basis set superposition
error (BSSE) in the course of the geometry optimization is often necessary,
even when large basis sets are applied. Additionally, the conventional Møller-
Plesset second-order (MP2) approach, which has been successfully applied
to the calculation of intermolecular potential energy surfaces in the case of
hydrogen bonding, has the tendency to overestimate the intermolecular in-
teraction in these complexes. This becomes visible when comparing it to the
methodically superior coupled cluster singles and doubles approach includ-
ing a triples contribution via perturbational theory, CCSD(T). Quite similarly,
several of the currently frequently applied density functional theory (DFT)
methods may fail badly in the description of these complexes while others
produce quite promising results.

2
XY ··· BComplexes

The interaction strength of complexes of dihalogens XY with different ac-
ceptors B spans a very wide range. The weakest complexes are those where
theacceptorisararegasatom.Experimentalandtheoreticalresultsonthe
structure, energetics, and dynamics of these rare gas-dihalogen complexes
in the electronic ground and excited states have been summarized in recent
review articles [1-3]. Their binding energies are all well below 1 kcal

mol,
quite often even substantially below 100 cm -1 . In the electronic ground state,
there are mostly two minima present, one for a linear arrangement and one
for a T-shaped structure. Quite apart from the interesting dynamical ques-
tions connected with these weakly bound complexes, the rare gas-dihalogen
interactions are of importance for all spectroscopic investigations of XY

/

B
complexes and halogenated organic molecules in inert rare gas matrices. Nev-
ertheless, the intermolecular interaction in these complexes is too weak to
justify the notion of a halogen bond. Therefore, they are excluded from this
review.
The interaction of dihalogen molecules XY with different acceptors B quite
often leads to vicious chemical reactions. In most cases, the 1 : 1 complexes
are extremely short-lived. To investigate these prereactive complexes exper-
imentally in a collision-free environment, pulsed-nozzle, Fourier-transform
microwave spectroscopy has turned out to be the ideal technique. Legon and
coworkers prepared a large number of these complexes and performed de-
tailed rotational spectroscopic analyses. Several series of simple molecules

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