Dependence of 1H and 13C NMR chemical shifts on the PAH∶FSO3H ratios for 4H-cyclopenta[def ]phenanthrenium and pyrenium cations in SO2ClF. Possible existence of cation–anion interactions and coexistence of PAHH+–PAH †

Abstract

For two representative delocalized PAH-carbocations, namely the C-1 protonated 4H-cyclopenta[def]phenanthrenium cation 1H+ and the C-1 protonated pyrenium cation 2H+, generated in FSO3H–SO2ClF, the FSO3H∶PAH molar ratio and the absolute concentration of the arenium ions were systematically varied over a wide range (FSO3H∶PAH molar ratios were varied from 71 to 1 for 1H+ and from 69 to 5 for 2H+ in 0.3 mL of SO2ClF and 0.05 mL of CD2Cl2 at –75 °C). In selected cases, the H/H COSY and C/H HETCOR spectra were used to fully assign the resonances. Shielding of the PAH-arenium ion protons and carbons was observed with decreasing FSO3H∶PAH ratios without noticeable line-broadening. This is attributed to cation–anion interactions in the low FSO3H∶PAH domain and possible formation of contact ion pairs. Lowering the ratio (below 35 in the case of 1 and below 10 for 2) gave separate sets of resonances for the unprotonated PAH, indicative of the presence of the equilibrium: PAH + FSO3H⇌PAHH+ + FSO3–. The observed shielding of the unprotonated PAH in the equilibrium as compared to the starting material in CDCl3, is attributed to solvation by FSO3H, although possible formation of PAHH+ FSO3––PAH ion pair–molecule clusters is not ruled out.