Iterative multiple linear regression analysis of substituent-induced chemical shifts in 3- and 4-substituted styrene derivatives is used to redefine σF and [Formula: see text] substituent constants (with respective standard errors of ca. 0.01 and 0.005). Correlations with the data for various model systems indicate that these substituent constants are applicable to data sets involving both chemical shift and acidity measurements in aprotic media. The approach we have used involving a homogeneous data set with a large number of variables is particularly suited for providing well-defined dual substituent parameter constants for the more common neutral substituents.Correlations for individual nuclei are entirely consistent with a previously proposed model of substituent effects involving field, resonance, and polarization effects. From these results and the results for other model systems, it is concluded that σF and [Formula: see text] are respectively pure field and resonance substituent constants. It is pointed out that a combination of detailed statistical testing and chemical modelling is essential for a thorough understanding of aromatic substituent effects.Parameters for predicting non-electronic contributions to ortho and meta chemical shifts are derived, thus allowing the patterns of electronic substituent effects for these carbons to be determined. However, only an approximate ipso factor can be deduced.

The techniques of factor analysis are used to investigate a data matrix constructed from magnetic resonance parameters for seventy-five 4-substituted styrenes, α-methylstyrenes, and α-tert-butylstyrenes and 3-substituted styrenes and α-methylstyrenes (with fifteen common substituents for each series). Two substituent factors (parameters) are clearly both necessary and sufficient to predict long-range chemical shifts within experimental uncertainty. Target testing indicates that Taft's σ1 and [Formula: see text] constants are appropriate for this purpose. One additional major factor is required for each of ipso and ortho carbons and a minor factor is apparently required for meta carbons.Conclusions do not appear to be sensitive to the weighting of individual variables and the specific variables included in the data matrix. However, the use of an extensive and well-chosen substituent set is essential.

σ+ Substituent constants for the ortho- and para-vinyl group have been determined by the application of the linear free-energy relationship to the nitration of the β-substituted styrene derivatives.Energy changes (relative to benzene system) for the proton and hydride ion transfer to individual positions in the styrene molecule have been calculated. Both approaches indicate that the vinyl group is capable of stabilizing both positively and negatively charged transition states. The interactions of the vinyl group with other substituents in the phenyl ring are also determined. Again, stabilizing effects with respect to both π-donor and π-acceptor substituents have been demonstrated for the vinyl group.

No abstract