Protonenresonanz-Spektroskopie Ungesättigter Ringsysteme—XIV: Zur Frage der Substituenten-Einflüsse auf die Resonanzfrequenzen und Vicinalen Kopplungskonstanten von protonen in substituierten Benzolen

The substituent effect on proton resonances and vicinal H, H-coupling constants in benzene derivatives is studied using linear regression analysis. The change in shielding observed on substitution, relative to the proton resonance in benzene, Δσ, can be expressed as . This equation enables calculations of substituent effects on the basis of the dipolar and quadratic field effect (E_z, E²) and the π-charge density changes on the neighbouring C-atom (Δρ). The vicinal coupling constants show no correlation with the HMO -π-bond order. A linear regression using the well known dependence of the coupling constant on substituent electronegativity yields where ΔE_α and ΔE_β are the changes in electronegativity caused by substitution in α- or β-position, respectively, to the HCCH-fragment considered.