X-ray Structures and Electronic Spectra of the Halogen Complexes between Halogen Donors and Acceptors with-Receptors - Halogen Bonding: Fundamentals and Applications - page 138

consideration of the electronic spectra, the structures, and the energetics
of intermolecular complexes of halogen derivatives; and most importantly,
these facets provide significant insight into the electronic nature of halogen
donor/acceptor interactions and the reactivity of halogen derivatives.
Accordingly, this chapter begins with a description of the diagnostic fea-
tures of donor/acceptor associates, as shown by their distinctive charge-
transfer absorption bands with the transition energies that are closely cor-
related with the redox potentials, i.e., the ionization potential and electron
affinity of the donor and acceptor, respectively [29-31]. The spectral char-
acterization of the 1 : 1 complexes formed in solution is followed by the
structural elucidation of the corresponding associates in the solid-state. (For
a review of gas-phase studies, see [1] and chapter by Legon in this volume).
In order to avoid obfuscation of the relatively weak long-distance halogen
interactions by electrostatics, metal-ion induced effects, etc., principal atten-
tion in this work is paid to various halogen complexes with neutral organic
(donor/acceptor) -receptors. Notably, our consideration is focused on the

experimental (spectral, structural, thermodynamics) data as well as their
implications on the nature and properties of halogen interactions. The high-
level quantum mechanical computations (see e.g., [32-38]) of such bondings
are beyond the scope of this review.

π

2
Electronic Spectroscopy and Thermodynamics
of Donor/Acceptor Interactions with Halogen Derivatives

2.1
Dihalogen Complexes with Arenes and Alkenes

The (UV-Vis) spectroscopic detection of the interaction of dihalogen accep-
tors with aromatic -donors was one of the first studies of intermolecular

donor/acceptor complexes. Indeed, more than 50 years ago, Benesi and Hilde-
brand published their seminal work describing the distinct spectral (UV-Vis)
changes that accompany the spontaneous complexation of various aromatic
hydrocarbons (ArH) with diiodine in non-polar solvents such as CCl 4 ,C 6 H 14 ,
etc. [39]. Shortly thereafter, these studies were extended to include dibromine,
dichlorine and interhalogen acceptors [26-28, 40]. For example, when pure
benzene is added incrementally in small amounts to a 5 mM solution of
dibromine in carbon tetrachloride, the red-brown color changes almost im-
perceptibly. However, inspection of the UV-Vis spectrum readily reveals the
progressive growth of a new absorption band at

π

λ max = 285 nm (Fig. 1), while
the “local” band of the dibromine moiety is essentially unchanged relative to
theabsorptionoffreedibromine,asshownbytheseriesofinvariantspectra
at λ > 350 nm [41].