6.2 Hydrogen- and Halogen-Bonded Complexes
Systematic experimental studies of the hydrogen- and halogen-bond complexes in
the gas phase are carried out by the A.C. Legon’s and J.W. Bevan’s scientific
groups. An analysis of rotational spectra of isolated weakly-bound complexes
formed in a B/XY/Rg supersonic molecular beam in conjunction with ab initio
quantum-chemical calculations gives a clue to the symmetry and geometry of the
complexes. Besides, it allows determining other spectroscopic characteristics of the
complexes (parameters of the one-dimensional potential energy function to the
dissociation coordinate, the intermolecular dissociation energy, D r , the dipole
moment, and some others, see [7–15] and references).
The quadratic intermolecular stretching force constant k r provides a measure of
the force required for an infinitesimal unit increase in the separation of the B and
XY subunits along the intermolecular B
… HX or B
… XY bonds. It has been shown
(see [7, 8] and references) that
k r ¼ cNE;
ð6:2:1Þ
where c is a constant, N and E are numbers representing the B nucleophilicities and
HX (XY) electrophilicities [16, 17]. The intermolecular dissociation energy D r is
directly proportional to k r
D r ¼ C r k r ;
ð6:2:2Þ
C r = 1.50(3)Á10
3 m
2 mol
−1 for both, hydrogen- and halogen- bonds. The electrophilicity of the H and halogen atoms depends on their electronegativities [4]. It
increases with electronegativity, which is equal to 2.20 (H), 3.96 (F), 3.16 (Cl), 2.96
(Br), 2.66 (I) in the Pauling scale [16]. Therefore, the D r increases in the HI–HBr–
HCl–HF raw for the hydrogen-bonded complexes, HI (1007 cm
−1 )–HF
(2958 cm
−1 ) for B = H 2 O, for example, and in the F 2 –Cl 2 –Br 2 –ClF–BrCl–ICl raw
for halogen-bonded complexes, F 2 (387 cm
−1 )– ICl (2059 cm
−1 ) for B = H 2 O, for
example. Nucleophilities and D r increase in the N 2 , CO, HCN, H 2 O, C 2 H 2 , H 2 S,
NH 3 raw [7], N 2 (591 cm
−1 )– NH 3 (3905 cm
−1 ) for XY = ICl, for example (see
[8]).
6.3 Van der Waals Complexes
6.3.1 General
In this book, the author uses the following definition of the vdW complexes: ‘Van
der Waals complexes are weakly-bound molecular species in which the dominant
attractive contribution are weak intermolecular attractions’. The weak
6.2 Hydrogen- and Halogen-Bonded Complexes
201
Systematic experimental studies of the hydrogen- and halogen-bond complexes in
the gas phase are carried out by the A.C. Legon’s and J.W. Bevan’s scientific
groups. An analysis of rotational spectra of isolated weakly-bound complexes
formed in a B/XY/Rg supersonic molecular beam in conjunction with ab initio
quantum-chemical calculations gives a clue to the symmetry and geometry of the
complexes. Besides, it allows determining other spectroscopic characteristics of the
complexes (parameters of the one-dimensional potential energy function to the
dissociation coordinate, the intermolecular dissociation energy, D r , the dipole
moment, and some others, see [7–15] and references).
The quadratic intermolecular stretching force constant k r provides a measure of
the force required for an infinitesimal unit increase in the separation of the B and
XY subunits along the intermolecular B
… HX or B
… XY bonds. It has been shown
(see [7, 8] and references) that
k r ¼ cNE;
ð6:2:1Þ
where c is a constant, N and E are numbers representing the B nucleophilicities and
HX (XY) electrophilicities [16, 17]. The intermolecular dissociation energy D r is
directly proportional to k r
D r ¼ C r k r ;
ð6:2:2Þ
C r = 1.50(3)Á10
3 m
2 mol
−1 for both, hydrogen- and halogen- bonds. The electrophilicity of the H and halogen atoms depends on their electronegativities [4]. It
increases with electronegativity, which is equal to 2.20 (H), 3.96 (F), 3.16 (Cl), 2.96
(Br), 2.66 (I) in the Pauling scale [16]. Therefore, the D r increases in the HI–HBr–
HCl–HF raw for the hydrogen-bonded complexes, HI (1007 cm
−1 )–HF
(2958 cm
−1 ) for B = H 2 O, for example, and in the F 2 –Cl 2 –Br 2 –ClF–BrCl–ICl raw
for halogen-bonded complexes, F 2 (387 cm
−1 )– ICl (2059 cm
−1 ) for B = H 2 O, for
example. Nucleophilities and D r increase in the N 2 , CO, HCN, H 2 O, C 2 H 2 , H 2 S,
NH 3 raw [7], N 2 (591 cm
−1 )– NH 3 (3905 cm
−1 ) for XY = ICl, for example (see
[8]).
6.3 Van der Waals Complexes
6.3.1 General
In this book, the author uses the following definition of the vdW complexes: ‘Van
der Waals complexes are weakly-bound molecular species in which the dominant
attractive contribution are weak intermolecular attractions’. The weak
6.2 Hydrogen- and Halogen-Bonded Complexes
201