Experimental Details

Solution preparation was done in a home‐made glob‐box under dry Ar atmosphere to minimize the water content in the acetonitrile solution. Anhydrous CD₃CN 99.8 % (Sigma‐Aldrich), and CF₃SO₃H (triflic acid) 99 % extra pure (Strem Chemicals) were used as received. HClO₄ (Sigma‐Aldrich), was 70.7 % by weight (x=H₂O/acid=2.3) and was further concentrated down to about x=1.3 by drying the concentrated acid in acetonitrile solutions using P₂O₅ as the drying agent. HI (Merck) was 67 % by weight (H₂O/acid=3.5) and the acid was further dried by P₂O₅ in acetonitrile solutions down to about x=1.8.

Steady state IR spectra of 0.6 M or 0.5 M of the acids in CD₃CN, were carried out with CaF₂ windows using optical path lengths of 15 and 25 μm.

The measurements were done with Nicolet IS10 FTIR instrument with 0.4–1 cm⁻¹ resolution while applying constant dry N₂ flow to keep moisture out of the surrounding atmosphere. The data as shown represent the average of 20 FTIR absorption scans after subtraction of the absorption of the pure CD₃CN solvent.

¹H NMR experiments were carried out using BrukerBiospin GmbH 400 MHz spectrometer of 0.6 M solutions of triflic acid in deuterated acetonitrile (CD₃CN). The measurements were done in a sealed NMR tube, temperature stabilized and cooled by liquid nitrogen. The residual ¹H impurity signal of CHD₂CN in CD₃CN solvent (δ ¹H=1.94 ppm on the TMS scale) was used as the NMR standard chemical‐shift.