6.9 Structure of Weakly Bound Complexes
151
Table 6.11 Equilibrium structure of the heavy-atom skeleton of trans-hexatriene (distances in pm,
angles in degrees) a
Method
r b
e
CCSD(T)
Extrapol. c
r se
e
r se
e
Basis set
wCVQZ_ae
MP2/VTZ
MP2/VTZ d
B3LYP/VTZ d
r(C1C2)
134.01
133.94
133.89
133.90(8)
133.89(9)
r(C2C3)
145.03
145.01
144.95
144.94(7)
144.95(9)
r(C3C4)
134.65
134.60
134.55
134.61(14)
134.62(16)
∠(C1C2C3)
123.71
123.72
123.70
123.6(4)
123.68(5)
∠(C2C3C4)
123.72
123.71
123.70
123.67(8)
123.69(9)
Reprinted with permission from The Journal of Physical Chemistry A (2015) 119: 195–204.
Craig NC, Demaison J, Groner P, Rudolph HD, Vogt N Electron delocalization in polyenes: a
semiexperimental equilibrium structure for (3E)-1,3,5-hexatriene and theoretical structures for
(3Z,5Z)-, (3E,5E)-, and (3E,5Z)-1,3,5,7-octatetraene. Copyright 2014 American Chemical Society
a Craig et al. (2015)
b r BO
e = CCSD(T)_fc/VTZ + MP2_fc/VQZ – MP2_fc/VTZ + MP2_ae/wCVQZ – MP2_fc/wCVQZ
c Mean value of the extrapolation of the B and C rotational constants with the rovibrational correction
calculated at the MP2/VTZ level
d Level of theory used to calculate the cubic force field
6.9 Structure of Weakly Bound Complexes
6.9.1 Introduction
A chemical bond may involve the electrostatic interaction between oppositely
charged atoms (ionic bonding) or the sharing of electron pairs between atoms. There
are also weaker interactions due to van der Waals’s forces (see Appendix 2.19.5),
which explain the formation of cluster molecules.
The study of weakly bound cluster molecules is a very active field of research
because the forces that hold the cluster together are responsible for the interactions
of molecules in the liquid and solid states. They also explain the stability of some
conformers.
Spectroscopy with the conventional Stark spectrometer was dominant up to the
1980s. However, this method is not well-suited to the study of complexes because the
spectra are recorded using an equilibrium gas mixture at low pressure. Nevertheless,
the spectra of a few strong complexes were recorded and analyzed, one of the first
being the hydrogen-bonded dimer CF 3 COOH· · · HCOOH (Costain and Srivastava
1961). This example was followed by the detection of other dimers of carboxylic
acids (Costain and Srivastava 1964) and, later, of the CH 3 CN· · · HF hydrogen-bonded
dimer (Bevan et al. 1975).
A much better method of observation is to cool the gas mixture (to improve
the equilibrium composition) and, at the same time, avoid condensation. It can be
done by adiabatic expansion through a nozzle of a gas mixture seeded in a rare
gas (Ar for instance). Several techniques are available, the most used one being a
151
Table 6.11 Equilibrium structure of the heavy-atom skeleton of trans-hexatriene (distances in pm,
angles in degrees) a
Method
r b
e
CCSD(T)
Extrapol. c
r se
e
r se
e
Basis set
wCVQZ_ae
MP2/VTZ
MP2/VTZ d
B3LYP/VTZ d
r(C1C2)
134.01
133.94
133.89
133.90(8)
133.89(9)
r(C2C3)
145.03
145.01
144.95
144.94(7)
144.95(9)
r(C3C4)
134.65
134.60
134.55
134.61(14)
134.62(16)
∠(C1C2C3)
123.71
123.72
123.70
123.6(4)
123.68(5)
∠(C2C3C4)
123.72
123.71
123.70
123.67(8)
123.69(9)
Reprinted with permission from The Journal of Physical Chemistry A (2015) 119: 195–204.
Craig NC, Demaison J, Groner P, Rudolph HD, Vogt N Electron delocalization in polyenes: a
semiexperimental equilibrium structure for (3E)-1,3,5-hexatriene and theoretical structures for
(3Z,5Z)-, (3E,5E)-, and (3E,5Z)-1,3,5,7-octatetraene. Copyright 2014 American Chemical Society
a Craig et al. (2015)
b r BO
e = CCSD(T)_fc/VTZ + MP2_fc/VQZ – MP2_fc/VTZ + MP2_ae/wCVQZ – MP2_fc/wCVQZ
c Mean value of the extrapolation of the B and C rotational constants with the rovibrational correction
calculated at the MP2/VTZ level
d Level of theory used to calculate the cubic force field
6.9 Structure of Weakly Bound Complexes
6.9.1 Introduction
A chemical bond may involve the electrostatic interaction between oppositely
charged atoms (ionic bonding) or the sharing of electron pairs between atoms. There
are also weaker interactions due to van der Waals’s forces (see Appendix 2.19.5),
which explain the formation of cluster molecules.
The study of weakly bound cluster molecules is a very active field of research
because the forces that hold the cluster together are responsible for the interactions
of molecules in the liquid and solid states. They also explain the stability of some
conformers.
Spectroscopy with the conventional Stark spectrometer was dominant up to the
1980s. However, this method is not well-suited to the study of complexes because the
spectra are recorded using an equilibrium gas mixture at low pressure. Nevertheless,
the spectra of a few strong complexes were recorded and analyzed, one of the first
being the hydrogen-bonded dimer CF 3 COOH· · · HCOOH (Costain and Srivastava
1961). This example was followed by the detection of other dimers of carboxylic
acids (Costain and Srivastava 1964) and, later, of the CH 3 CN· · · HF hydrogen-bonded
dimer (Bevan et al. 1975).
A much better method of observation is to cool the gas mixture (to improve
the equilibrium composition) and, at the same time, avoid condensation. It can be
done by adiabatic expansion through a nozzle of a gas mixture seeded in a rare
gas (Ar for instance). Several techniques are available, the most used one being a