For further identification, we recorded IR spectra of the protonated ions of the compounds and compared those to theoretical IR spectra of candidate structures. Figure shows the IR spectrum of compound A compared with the theoretical IR spectrum of protonated MDZ ([MDZ+H] + , m/z 326). Overall, the observed vibrational bands are well reproduced in the theoretical spectrum, both in terms of peak positions as well as relative intensities. Some of the very weak bands in the 850 -1050 cm -1 range in the computational spectrum appear to escape observation. Their low absorption cross sections do not allow the ion to absorb enough photons within the duration of the laser pulse to reach the dissociation threshold. Moreover, some deviations between observed and computed relative band intensities are noticed. A slight redshift of the C=N and C=C stretching vibrations near 1600 cm -1 is observed, probably due to the anharmonic shift deviating slightly from average. However, in general, computed and theoretical IR spectra show a convincing match, confirming the structural assignment of compound A as protonated MDZ.