¹H and ¹³C Nuclear Magnetic Resonance Spectra of Dimethylnaphthalenium Ions. Calculation on the Site of Protonation and Correlation between ¹³C Chemical Shifts and Electron Density by the Huckel MO Method

All ten dimethylnaphthalenes have been protonated with FSO₃H-SbF₅ (1:1) in SO₂C1F. The resulting dimethylnaphthalenium ions were studied by ¹H NMR and ^l3C NMR spectroscopy. The 1,8-dimethvlnaphthalenium ion 8 isomerizes upon raising the temperature with formation of the protonated 1,7-dimethylnaphthalene 6. The ¹H NMR spectrum of protonated 1,4-dimethylnaphthalene is temperature dependent and indicates the presence of two pairs of ions which rapidly equilibrate via simple 1,2-hydrogen shifts. One of the ions involved is the ipso protonated species 17. The site of protonation of the dimethylnaphthalenes is as predicted from the localization energies obtained by simple Hückel π-electron molecular orbital calculations. The ¹H and ^l3C chemical shifts of the individual ring positions of the various ions correlate with the calculated charge densities (q). A satisfactory correlation between Δδ_c and Δq is observed if the stabilizing features of the methyl group are accounted for. On this basis new inductive parameters for the methyl substituent are offered.