Transmission of substituent effects in para-disubstituted benzenes: 13C chemical shift and Mulliken charge modeling of para carbon atoms via the Reynolds DSP model.

• 13C SCS significantly correlates PM3 charge (q) of disubstituted benzene para carbon. • δ (13C SCS or q) = ρ F σ F + ρ R σ R o • The concept of π-polarization reflects the variation in ρ F. • Enhancement of ρ F parallels π-polarization stabilization of para carbon. • Generally, series' ρ R have the same trend of q,13C SCS and extended conjugation. The PM3 calculations of the Mulliken charge and the experimental 13C substituent chemical shifts (SCS) of para carbon for 17 para- disubstituted benzene series were investigated by correlation analysis. A significant correlation between the charge and the 13C SCS of the para carbon of the same series was observed. In mono-substituted benzene, the correlation of para 13C SCS with Mulliken charges calculated by PM3 and Mulliken charge calculated by DFT(B3LYP) with basis set 6 311(dp)G of the same carbon is similar. All the substituent effects on the charge and 13C SCS were modeled by the Reynolds' dual substituent parameter model (charge or 13C SCS = ρ F σ F + ρ R σ R o), where σ F and σ R o are the Reynolds' field and resonance constants, respectively, and ρ F and ρ R are the field and resonance effects, respectively. The coefficients of determination for all series were significant. The concept of π-polarization was used to analyze variation in ρ F. The interpretation of ρ F showed parallels between the charge and 13C SCS. An enhanced ρ F for a given series was attributed to the stabilization of the π-polarization charge at the para carbon. The ρ R was a function of extended conjugation within the side chain. Parallelism of charge and 13C SCS of the ρ R of the compared series were also observed, except in series with the reverse-substituent effect and the possession of a strong electronegative group, such as a nitro or azo group. [Display omitted] [ABSTRACT FROM AUTHOR]