We propose a new scale of group electronegativities, derived from benzene ring deformations in Ph−X
molecules. A recent analysis of such deformations (Campanelli, A. R.; Domenicano, A.; Ramondo, F. J.
Phys. Chem. A
2003, 107, 6429) has shown that two orthogonal linear combinations of the internal ring
angles, termed S
E and S
R, are directly related to the electronegativity and resonance effects of the substituent,
respectively. In the present paper, we show that S
E increases linearly with the electronegativity of X within
each of the first two rows of the periodic table, acting as a sensitive indicator of the polarity of the Ph−X
bond. By using S
E values from ab initio quantum chemical calculations, we have derived the electronegativities
of 100 organic and inorganic groups. Nonbonded interactions with the ortho hydrogens and carbons may
fictitiously alter the electronegativity of a group; in most cases, however, they are easily eluded by changing
the conformation of the substituent with respect to the benzene ring. Although the atom directly linked to the
ring tends to dominate the electronegativity of a group, the role of its adjacent atoms is also important. Their
effect depends markedly on the nature of chemical bonding and electron density distribution within the group.
The deformation of the carbon skeleton of the benzene ring under substituent impact has been analyzed from the structures of 74 monosubstituted derivatives, as determined by ab initio MO calculations. The geometry of the substituted ring is shown to contain valuable information on the electronegativity, resonance, and steric effects of the substituent, and also on other, more subtle effects, affecting primarily the length of the C ipso -C ortho bonds. The results obtained substantially augment previous knowledge from the analysis of experimental geometries (Domenicano, A.; Murray-Rust, P.; Vaciago, A. Acta Crystallogr., Sect. B 1983, 39, 457). Varying the electronegativity of the substituent causes a concerted change of the ring angles at the ipso, ortho, and para positions, coupled with a change in the C ipso -C ortho bond length. The values of the ipso angle span a remarkably wide range, 113-126°. Enhancing the resonance interaction between a substituent and the ring causes a complex pattern of angular distortions, arising from the superposition of two separate effects. The first originates from the decreased length of the C-X bond, and consists primarily in a concerted change of the ipso and ortho angles. It occurs irrespective of whether the substituent is a π donor or a π acceptor. The second effect is associated with π-charge alternation on the ring carbons. It involves all the internal ring angles, and depends on the substituent being a π donor or a π acceptor. These angular changes are generally accompanied by changes in all C-C bond lengths, as expected from an enhanced contribution of polar canonical forms to the electronic structure of the molecule. By using symmetry coordinates, we have derived two orthogonal linear combinations of the internal ring angles, S E and S R , measuring the electronegativity and resonance effects of a substituent, respectively, as seen from their impact on the ring geometry. S E and S R values are affected in a typical way by steric effects.
The variance of the geometry of monosubstituted benzene rings has been analysed systematically from a sample of 199 rings, representing a wide spectrum of substituents. The sample is composed of structures determined accurately by X-ray crystallography, neutron crystallography and microwave spectroscopy. Part of the analysis has been carried out using symmetry coordinates based on the irreducible representations of the D6h point group. The application of bivariate analysis to these coordinates shows that the angular variance of monosubstituted benzene rings is fully described by two orthogonal components of distortion, involving angular changes in different ratios. The distortion that accounts for most of the variance is a simultaneous change of the internal angles at the ipso, ortho and (to a minor extent)para positions of the ring, and is definitely related to the tr electronegativity of the substituent. The second distortion involves mainly the internal angles at the meta and para positions and is controlled to a large extent by the n-donor/acceptor power of the substituent. The separation of the two distortions is not based on chemical assumptions, but stems directly from the statistical analysis of the data. When bond-length variation is included in the model, factor analysis reveals the existence of three orthogonal components of distortion. These account for ca 90% of the variance, the remaining 10% being essentially due to random experimental error. The first distortion, accounting for ca 40% of the variance, now also involves a change of the Ctpso-Corth o bond length. The second distortion accounts for ca 20% of the variance
The angular deformations induced in the carbon skeleton of the benzene ring by substitution of a hydrogen atom with a second-row element have been investigated, using the best structural data available in the literature. It is shown that a linear correlation exists between the internal bond angle at the carbon atom at which substitution takes place and Pauling's electronegativity of the substituent. From this correlation, the actual electronegativities of phosphorus and sulphur in several classes of related compounds and individual molecules are evaluated. Similar correlations are shown to exist for the phenyl derivatives of first-and third-row elements.
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