Photodissociation Spectroscopy of Cold Protonated Synephrine: Surprising Differences Between IR-UV Hole-Burning and IR Photodissociation Spectroscopy of the O-H and N-H Modes.

We report the UV and IR photofragmentation spectroscopies of protonated synephrine in a cryogenically cooled Paul trap. Single (UV or IR) and double (UV UV and IR UV) resonance spectroscopies have been performed and compared to quantum chemistry calculations, allowing the assignment of the lowest energy conformer with two rotamers depending on the orientation of the phenol hydroxyl (OH) group. The IR UV hole burning spectrum exhibits the four expected vibrational modes in the 3 pm region, i.e., the phenol OH, C-beta-OH, and two NH2+ stretches. The striking difference is that, among these modes, only the free phenol OH mode is active through IRPD. The protonated amino group acts as a proton donor in the internal hydrogen bond and displays large frequency shifts upon isomerization expected during the multiphoton absorption process, leading to the socalled IRMPD transparency. More interestingly, while the C-beta-OH is a proton acceptor group with moderate frequency shift for the different conformations, this mode is still inactive through IRPD.