Infrared and Ultraviolet Spectra of the Monohydrated <i>N</i> -Methyl-Dopamine–Methylation Effect on Structure and Excited State Dynamics

Ultraviolet and infrared photodissociation spectra (UVPD and IRPD) of monohydrated <i>N</i>-methyl-dopamineH⁺ NMeDAH⁺(H₂O)₁ are measured by cryogenic ion spectroscopy and compared with those of monohydrated dopamineH⁺ DAH⁺(H₂O)₁ to reveal the methylation effect on intramolecular excited state proton transfer (ESPT). Whereas the UVPD spectrum of DAH⁺(H₂O)₁ is broad due to the proton transfer from the amine chain to the aromatic ring, that of NMeDAH⁺(H₂O)₁ shows sharp vibronic transitions in addition to a broad background signal. This suggests the coexistence of two or more conformers in which the excited state dynamics are very different. Based on conformer-selected IR spectra and theoretical calculations, sharp bands are assigned to two conformers in which a water molecule is inserted between the amine chain and the aromatic ring and prevents NH → π ESPT. This will lead to slow excited state dynamics. This water-inserted conformer is very minor in DAH⁺(H₂O)_1, and its enhancement by methylation is discussed. The broad UVPD background is attributed to the most stable conformer, in which the N-H bond of the amino group directly interacts with the catechol ring. The reaction path for NH → π ESPT in the most stable conformer shows a low barrier of less than 100 meV, which explains the broad UVPD spectrum by fast ESPT. The results confirm the model based on the relationship between the structure of these hydrated complexes and their photo reactivity associated with the excited state proton transfer.