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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2018-04, Vol.122 (15), p.3798-3804
• Main Authors: Nieuwjaer, N, Desfrançois, C, Lecomte, F, Manil, B, Soorkia, S, Broquier, M, Grégoire, G
• Format: Article
• Language: English
• Subjects: Chemical Sciences; or physical chemistry; Theoretical and
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/acs.jpca.8b01422

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 μm region, i.e., the phenol OH, Cβ–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 so-called IRMPD transparency. More interestingly, while the Cβ–OH is a proton acceptor group with moderate frequency shift for the different conformations, this mode is still inactive through IRPD.