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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

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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.