152
6 Equilibrium Structures from Spectroscopy
pulsed-nozzle Fourier transform microwave spectroscopy (Balle and Flygare 1981).
A further advantage of this technique is its very high resolution and sensitivity.
6.9.2 Classification of Non-covalent Bonds E (Legon 2017)
An E bond occurs when there is evidence of a net attractive interaction between an
electrophilic region associated with an E atom in a molecular entity and a nucleophilic
region (e.g., a n-pair or π-pair of electrons) in another or the same molecular entity.
This definition also applies to the hydrogen bond whose IUPAC definition is: the
hydrogen bond is an attractive interaction between a hydrogen atom from a molecule
or a molecular fragment X–H, in which X is more electronegative than H, and an
atom or a group of atoms in the same or a different molecule, in which there is
evidence of bond formation (Arunan et al. 2011).
A typical non-covalent bond is denoted by the three dots in R–X· · · Y. Analysis
of the electron density topology of hydrogen-bonded systems usually shows a bond
path connecting H and Y and a bond critical point between H and Y (see Sect. 2.18).
In most cases, the distance between X and Y is found to be less than the sum of
their van der Waals radii (Sect. 8.2.3), but it is true only for rather strong bonds.
The infrared spectrum also reveals evidence of complexes. First, new vibrations due
to the intermolecular interaction appear. Then, the frequencies of the vibrations of
the bonds close to the location of the intermolecular interaction are affected. For
instance, for a hydrogen-bonded complex X–H· · · Y–Z, there is a red shift in the
infrared X–H stretching frequency.
6.9.3 Rotational Constants and Structure
If the interaction linking the monomers is strong enough, the semirigid rotor model
as described in Chap. 4 may be used. On the other hand, when the interaction is weak,
large-amplitude motions appear and the Taylor series expansion of the potential is no
longer valid. The determination of rotational constants may even be problematic. In
such a case, the determination of an experimental or semiexperimental equilibrium
structure is extremely difficult. But, the rotational constants may still be used to
obtain semiquantitative information on the structure such as the relative positions of
the monomers.
To derive intermolecular structural parameters from the experimental rotational
spectrum, four main methods are used
1. Kraitchman equations (Sect. 6.4.3).
2. Least-squares fit of the moments of inertia with bond lengths and bond angles of
the monomers constrained to their free-molecule values (Sect. 6.4.2).
6 Equilibrium Structures from Spectroscopy
pulsed-nozzle Fourier transform microwave spectroscopy (Balle and Flygare 1981).
A further advantage of this technique is its very high resolution and sensitivity.
6.9.2 Classification of Non-covalent Bonds E (Legon 2017)
An E bond occurs when there is evidence of a net attractive interaction between an
electrophilic region associated with an E atom in a molecular entity and a nucleophilic
region (e.g., a n-pair or π-pair of electrons) in another or the same molecular entity.
This definition also applies to the hydrogen bond whose IUPAC definition is: the
hydrogen bond is an attractive interaction between a hydrogen atom from a molecule
or a molecular fragment X–H, in which X is more electronegative than H, and an
atom or a group of atoms in the same or a different molecule, in which there is
evidence of bond formation (Arunan et al. 2011).
A typical non-covalent bond is denoted by the three dots in R–X· · · Y. Analysis
of the electron density topology of hydrogen-bonded systems usually shows a bond
path connecting H and Y and a bond critical point between H and Y (see Sect. 2.18).
In most cases, the distance between X and Y is found to be less than the sum of
their van der Waals radii (Sect. 8.2.3), but it is true only for rather strong bonds.
The infrared spectrum also reveals evidence of complexes. First, new vibrations due
to the intermolecular interaction appear. Then, the frequencies of the vibrations of
the bonds close to the location of the intermolecular interaction are affected. For
instance, for a hydrogen-bonded complex X–H· · · Y–Z, there is a red shift in the
infrared X–H stretching frequency.
6.9.3 Rotational Constants and Structure
If the interaction linking the monomers is strong enough, the semirigid rotor model
as described in Chap. 4 may be used. On the other hand, when the interaction is weak,
large-amplitude motions appear and the Taylor series expansion of the potential is no
longer valid. The determination of rotational constants may even be problematic. In
such a case, the determination of an experimental or semiexperimental equilibrium
structure is extremely difficult. But, the rotational constants may still be used to
obtain semiquantitative information on the structure such as the relative positions of
the monomers.
To derive intermolecular structural parameters from the experimental rotational
spectrum, four main methods are used
1. Kraitchman equations (Sect. 6.4.3).
2. Least-squares fit of the moments of inertia with bond lengths and bond angles of
the monomers constrained to their free-molecule values (Sect. 6.4.2).