Substituent-induced chemical shifts in 3- and 4-substituted styrenes: definition of substituent constants and determination of mechanisms of transmission of substituent effects by iterative multiple linear regression

Abstract

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.

Substituent-induced chemical shifts in 3- and 4-substituted styrenes: factor analysis of the styrene data matrix

“…The question of whether predicted a, (3, and Q are pure 0 factors or rather linear combinations of a,, a,, and the actual factors is probably best addressed by correlation analysis. This is reported in the subsequent paper (29).…”

“…The first two are totally independent of a, and (T:. This allows us to separate electronic ((T, and (T:) contributions from other short-range, non-electronic contributions (29 chemical shifts in a large number of haloaromatic and haloalkene derivatives. They concluded that two major and one minor factor were necessary to account for the chemical shifts, compared to four major and one minor factor according to this investigation.…”

“…Furthermore, while a third parameter is undoubtedly necessary for short-range chemical shifts, different parameters are needed for ipso and ortlzo positions. Finally, although Q may be an appropriate parameter for ortho chemical shifts, it is sufficiently strongly correlated with a : (or R ) that the weighting coefficients in a multiple linear regression analysis of ortho shifts are likely to be of dubious reliability (29).…”

Electronic Effects of Nitro, Nitroso, Amino and Related Groups

“…However, the value for the nitro group was confirmed as 0.15. 156 based a similar operation on 13 C substituent chemical shifts of meta-and para-substituted styrenes. Iterative multiple regression was used for the redefinition of the I and 0 R scales.…”

Molecular orbital calculations and 1H nuclear magnetic resonance spectra as indicators of internal rotational potentials in some methyl derivatives of styrene

“…The I3C chemical shifts of many styrene derivatives, as well as the 'H shifts of their olefinic protons, have been extensively studied and interpreted (29)(30)(31)(32)(33), notably by Reynolds and coworkers. Here we note that H-4 in styrene is shielded by 0.10 ppm relative to benzene at the same concentration (footnote in Table 1).…”