Chemistry Reference
In-Depth Information
region carried out by Benesi and Hildebrand in 1949 [2] was the initial focus
of the important work of Mulliken [3] on the theory of electron donor-
acceptor complexes in the 1950s and 1960s. During that period, Hassel and
co-workers [4, 5] carried out X-ray diffraction studies of crystals of add-
ition complexes formed by dihalogen molecules with Lewis bases. They
concluded that the hydrogen bridge and halogen bridge were closely re-
lated. Of particular interest in the context of the work to be described here
is Hassel's statement that, in complexes formed between halogen molecules
and electron-donor molecules possessing lone pairs of electrons, it is to
be assumed “that halogen atoms are directly linked to donor atoms with
bond directions roughly coinciding with the axes of the orbitals of the lone
pairs in the non-complexed donor molecule”. Hassel's investigations involved
crystals of the adducts, so that the complexes were therefore mutually in-
teracting, albeit quite weakly. Complexes in effective isolation in cryogenic
matrices were studied by infrared spectroscopy in the 1980s, particulary by
Pimentel [6], Ault [7-10] and Andrews [11-14]. The so-called fast-mixing
nozzle [15] incorporated into a pulsed-jet, Fourier-transform microwave
spectrometer [16, 17] allowed complexes formed from simple Lewis bases
(such as NH
3
,H
2
CCH
2
, etc.) and dihalogen molecules to be isolated and
probed by microwave radiation before they could undergo the (sometimes vi-
olent) reaction that attends normal mixing. This technique allowed the power
and precision of rotational spectroscopy to be brought to bear on many sim-
ple complexes. Moreover, the Lewis base and the dihalogen molecule could
be systematically varied to reveal conclusions of general interest about the
binding that holds the two components together.
1.2
Definitions and Nomenclature
The aim of this chapter is to show that there is a strong parallelism be-
tween the measured properties of halogen-bonded and hydrogen-bonded
complexes and, consequently, that the terms
halogen bond
and
hydrogen bond
carry similar connotations. After extensive consultations and discussions,
the IUPAC Working Party on the hydrogen bond, and other molecular in-
teractions, put forward the following definition of the hydrogen bond for
consideration by the Chemistry community [18]:
“
The hydrogen bond is an attractive interaction between a group X-H and
an atom or a group of atoms, in the same or different molecule(s), when there
is evidence of bond formation
.”
Of several properties simultaneously recommended as providing criteria
of such evidence, the most important in the present context are:
1.
The physical forces involved in the hydrogen bond must include electro-
static and inductive forces in addition to London dispersion forces
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