The strongest peak in the three IR spectra in Fig. can be assigned to the carbonyl stretching mode ν(C=O) (around 1800 cm -1 ). We limit the following discussion of the IR spectra to this vibrational mode and compare the solvent shifts of the maximum of the carbonyl stretching mode to the experimental results by Nyquist et al. (see Table ). Al- used cyclohexane, which has very similar solvent properties. Therefore, we expect the shifts to be comparable. We also assume that the density of the acetone vapor in the experiment was low enough to make it comparable to acetone in vacuum.