Substituent effect in aromatic proton NMR spectra—IV Proton NMR spectra of monosubstituted (poly) methylbenzenes

Proton NMR spectra of 1‐substituted 2,4‐dimethylbenzenes ( 2 ), 1‐substituted 2,6‐dimethylbenzenes ( 3 ) and 1‐substituted 2,4,6‐trimethylbenzenes ( 4 ) were determined and the SCS values compared with those of monosubstituted benzenes ( 1 ). SCS of 1 are assumed to be primarily due to the effects of π‐electron charge density, substituent electric field and substituent diamagnetic anisotropy, and the van der Waals interaction: thus\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm SCS = SCS}_{\pi {\rm}} + {\rm SCS}_{\rm E} + {\rm SCS}_{\rm A} + {\rm SCS}_{\rm V} $$\end{document}When, however, the substituent is sterically hindered, then:\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm SCS}^{\rm '} {\rm = SCS}_{{\rm \pi}^{\rm '}} {\rm + SCS}_{{\rm E}^{\rm '}} {\rm + SCS}_{{\rm A}^{\rm '}} {\rm + SCS}_{\rm V} $$\end{document}\documentclass{article}\pagestyle{empty}\begin{document}$$ = {\rm c}_{\rm \pi} {\rm SCS}_{\rm \pi} {\rm + c}_{\rm E} {\rm SCS}_{\rm E} {\rm + c}_{\rm A} {\rm SCS}_{\rm A} {\rm + c}_{\rm V} {\rm SCS}_{\rm V} $$\end{document}where c π∼V may be constants predictable from the theories associated with each component. By estimating SCS π∼V or c π∼V , a quantitative separation of SCS into their components was attempted. It is shown, however, that the data available, as well as the nature of this approach, cannot necessarily be sufficient for this purpose. Various effects which might also contribute to the proton NMR chemical shifts of sterically hindered molecules are also discussed.