222
8 Models of Chemical Bonding and “Empirical” Methods
There are other interesting correlations with the spin-spin coupling constants.
For instance, Muller and Pritchard (1959) noted a correlation between the J(
13 C–H)
spin-spin coupling constant and the r(CH) bond length.
8.5.3 Other Correlations
Most molecular properties have a common origin. Therefore, they are expected to
be related to one another. Indeed, there are many empirical relationships including
structural parameters. They are found to correlate with the frequency of vibration
(see 8.5.1), force constant, amplitude of vibration (see Chap. 7), spin-spin coupling
constant (see for instance 8.5.2), electronegativity, Taft constant, dissociation energy,
coordination number, etc. Some of them are listed in (Mastryukov and Simonsen
1996). Most of these relationships are either moderately accurate or their spectrum
of application is limited. Only the most useful will be briefly discussed.
8.5.3.1 Correlation with the Electronegativity
In Sect. 8.2.2, it has been shown that the length of a bond can be roughly estimated
from the covalent radii and the electronegativities of the two atoms forming the
bond. The electronegativity may be used in many other correlations. A few typical
examples are listed below.
• Hargittai (1985) could predict the S=O bond length in sulfones, XSO 2 Y from the
electronegativity of the ligands X and Y.
• Domenicano found that the ipso angle in monosubstituted benzenes, C 6 H 5 X,
increases linearly with the electronegativity of the substituent X (Domenicano
1992). Furthermore, the ipso angle and the non-bonded distance C1…C4 are
linearly correlated for symmetrically para-disubstituted benzenes.
• It was also observed that the C–H bond length in CH 3 X increases with the electronegativity of X, whereas it decreases in HC≡CX. The C≡N bond length in
XC≡N decreases with the electronegativity of X. The C≡C bond length in X
C≡CY decreases with the electronegativity of Y. etc. (Demaison et al. 2003)
• The C–C bond length in ethyl derivatives was found linearly dependent on the
electronegativity of the substituent (Hayashi and Adachi 1982)
As these correlations are nearly linear, they can be used for semi-quantitative
predictions.
8.5.3.2 Correlations Between Geometrical Parameters of One Molecule
The set of independent geometrical parameters defining the structure of a molecule
are actually independent only in first approximation. For instance, there is a linear
8 Models of Chemical Bonding and “Empirical” Methods
There are other interesting correlations with the spin-spin coupling constants.
For instance, Muller and Pritchard (1959) noted a correlation between the J(
13 C–H)
spin-spin coupling constant and the r(CH) bond length.
8.5.3 Other Correlations
Most molecular properties have a common origin. Therefore, they are expected to
be related to one another. Indeed, there are many empirical relationships including
structural parameters. They are found to correlate with the frequency of vibration
(see 8.5.1), force constant, amplitude of vibration (see Chap. 7), spin-spin coupling
constant (see for instance 8.5.2), electronegativity, Taft constant, dissociation energy,
coordination number, etc. Some of them are listed in (Mastryukov and Simonsen
1996). Most of these relationships are either moderately accurate or their spectrum
of application is limited. Only the most useful will be briefly discussed.
8.5.3.1 Correlation with the Electronegativity
In Sect. 8.2.2, it has been shown that the length of a bond can be roughly estimated
from the covalent radii and the electronegativities of the two atoms forming the
bond. The electronegativity may be used in many other correlations. A few typical
examples are listed below.
• Hargittai (1985) could predict the S=O bond length in sulfones, XSO 2 Y from the
electronegativity of the ligands X and Y.
• Domenicano found that the ipso angle in monosubstituted benzenes, C 6 H 5 X,
increases linearly with the electronegativity of the substituent X (Domenicano
1992). Furthermore, the ipso angle and the non-bonded distance C1…C4 are
linearly correlated for symmetrically para-disubstituted benzenes.
• It was also observed that the C–H bond length in CH 3 X increases with the electronegativity of X, whereas it decreases in HC≡CX. The C≡N bond length in
XC≡N decreases with the electronegativity of X. The C≡C bond length in X
C≡CY decreases with the electronegativity of Y. etc. (Demaison et al. 2003)
• The C–C bond length in ethyl derivatives was found linearly dependent on the
electronegativity of the substituent (Hayashi and Adachi 1982)
As these correlations are nearly linear, they can be used for semi-quantitative
predictions.
8.5.3.2 Correlations Between Geometrical Parameters of One Molecule
The set of independent geometrical parameters defining the structure of a molecule
are actually independent only in first approximation. For instance, there is a linear