Messenger Tagging Vibrational Spectroscopy of Protonated Nicotinamide

We present the vibrational photodissociation spectrum of cryogenically prepared complexes of protonated nicotinamide with N₂ in the NH stretching region (3250 to 3600 cm^-1). We analyze the spectrum using quantum chemical methods and vibrational perturbation theory to capture anharmonic effects. The spectrum shows that only the protonation site on the pyridine ring is significantly populated. The calculations indicate that the N₂ tag is attached to the charged site and that it leads to a significant red-shift of the NH⁺ stretching frequency compared to protonated nicotinamide without the presence of the tag.