Figure and 1c compare the computed IR spectra of protonated 1'-OH-MDZ and the two tautomers of protonated 4-OH-MDZ to the experimental IR spectra. Computed geometries are inlayed in each panel. It is seen that the 1'-OH-MDZ spectrum is reasonably well explained by the computational spectrum, except for the intense band calculated just below 1200 cm -1 , which is redshifted compared to the experiment. This band corresponds to an OH-bending vibration of the 1'-OH-group, which may be redshifted due to the hydrogen-bond with the protonated nitrogen (see optimized geometry in the figure) that is not well modelled within the harmonic approximation. Moreover, a few predicted low-frequency bands are absent in the experimental IR spectrum, probably because of their low intensity, not allowing the ion to absorb enough photons to reach the dissociation threshold. The IR spectrum of 4-OH-MDZ is well explained by the computed spectrum of the enol-imine tautomer except for the obvious absence of the carbonyl stretching vibration, which is very intense in the predicted spectrum for the amide-tautomer. This suggests that the majority of the ion population corresponds to the enol-imine tautomer, despite its substantially higher predicted energy relative to the amide-tautomer (+49.0 kJ/mol). Perhaps the hydroxyl group of the enol-imine structure imparts greater solution-phase stability and no tautomerization occurs upon ESI transfer to the gas phase. Previous studies have provided ample spectroscopic evidence for such scenarios. Note that an alternative enol-imine tautomer, having an N=C(-OH) double bond, of 4-OH-MDZ can be imagined as well. This tautomer is 11.2 kJ/mol lower in energy than the enol-imine tautomer in Figure , but its predicted IR spectrum does not match well with experiment (see Figure in the . SI) and it is therefore not likely to contribute to the ion population. Figure compares the experimental IR spectrum of 4-OH-MDZ to a weighted average of the computed IR spectra of the two assigned tautomers employing a 98:2 enol-imine/amide ratio. Note that the doublet around 1150 cm -1 in the 1'-OH-MDZ spectrum versus the singlet in the 4-OH-MDZ spectrum, is well predicted by the computations. This difference is due to the C-OH stretch vibration of the hydroxyl group, which shifts on top of a C-H in-plane bending mode depending on its attachment to an aromatic ring versus a methylene group, making this band of diagnostic value.