# Infrared and ultraviolet spectra of monohydrated N-methyl-dopamine -methylation effect on structure and excited state dynamics

Keisuke Hirata, Ken-Ichi Kasai, Masaaki Fujii, Gilles Grégoire, Shun-Ichi Ishiuchi

► To cite this version:

Keisuke Hirata, Ken-Ichi Kasai, Masaaki Fujii, Gilles Grégoire, Shun-Ichi Ishiuchi. Infrared and ultraviolet spectra of monohydrated N-methyl-dopamine -methylation effect on structure and excited state dynamics. Journal of Physical Chemistry A, 2025, 130 (1), pp.80-87. 10.1021/acs.jpca.5c06307 . hal-05486219

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Infrared and ultraviolet spectra of monohydrated N-methyl-dopamine – methylation effect on structure and excited state dynamics

Keisuke Hirata,¹ Ken-ichi Kasai,²,³, Masaaki Fujii ³,⁴,⁵* Gilles Grégoire,⁴,⁶* and Shun-ichi Ishiuchi¹*

¹ Department of Chemistry, School of Science, Institute of Science Tokyo, 2-12-1 4259 Ookayama, Meguro-ku, Tokyo, 152-8550, Japan

² School of Life Science and Technology, Institute of Science Tokyo, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa, 226-8503, Japan

³ Laboratory for Chemistry and Life Science, Institute of Integrated Research, Institute of Science Tokyo, 4259 Nagatsu-ta-cho, Midori-ku, Yokohama, 226-8503, Japan

⁴ Tokyo Tech World Research Hub Initiative (WRHI), Institute of Integrated Research, Institute of Science Tokyo, 4259, Nagatsuta-cho, Midori-ku, Yokohama, 226-8503, Japan

⁵ Research and Development Initiative, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan

⁶ Université Paris-Saclay, CNRS, Institut des Sciences Moléculaires d'Orsay, F-91405 Orsay, France

E-mail: gilles.gregoire@universite-paris-saclay.fr, ishiuchi.s.aa@m.titech.ac.jp, mfujii@res.titech.ac.jp

ORCID: Keisuke Hirata: 0000-0003-4472-7992, Gilles Gregoire: 0000-0002-8577-3621, Masaaki Fujii: 0000-0003-4858-4618, Shun-ichi Ishiuchi: 0000-0002-4079-818X,

**Abstract**

Ultraviolet and infrared photodissociation spectra (UV PD and IRPD) of monohydrated N-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, the one 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 a 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.

**Introduction**

Methyl group is a common substituent in molecules and plays important roles in
chemistry and biochemistry. In physical chemistry, one of its characteristic properties is
hyperconjugation, which allows methyl group to interact with π electrons even though it is
bound to the aromatic ring by a σ bond. Another interesting feature is the large-amplitude
rotational motion that cannot be described by a harmonic, normal mode approximation or a
point group. Usually, its symmetry is described by the G3 (or G6) group in the permutation
inversion group, and the energy levels are classified as a₁, a₂ or e symmetry. This unique
feature also disturbs the dynamics of the excited states of molecules,¹⁻³ and tends to accelerate
intramolecular vibrational redistribution (IVR).⁴⁻⁶ Symmetry selectivity in intersystem crossing⁷
has also been reported. The effect of methylation on the dynamics stems from the perturbation
of the electronic state by rotation as well as the significant increase of the density of states, but
has not been studied in detail in chemical reactions such as proton transfer.

We already studied the solvation effect on the intramolecular excited state proton
transfer (ESPT) in protonated molecules. Tryptophan, one of the first biomolecules studied in
gas phase,⁸⁻¹¹ showed a broad ultraviolet spectrum after protonation (TrpH⁺) even under ultra-
cold conditions as those achieved in a cryogenic ion trap.⁸,⁹ This contrasts with its neutral form,
for which the UV spectrum shows well-resolved vibronic transitions under isolated cold
conditions obtained in a molecular beam.¹² Later, it is discovered that the optical spectrum of
TrpH⁺ became resolved again by attaching two water molecules.¹³ The similar phenomenon
have been observed in dopamine: the broad UV spectrum of protonated dopamine (DAH⁺)
turns to be resolved after attaching three water molecules.¹⁴,¹⁵ The broad excitation spectrum
of TrpH⁺ and DAH⁺ can be explained by an excited state proton transfer from the protonated
amino group of the alkyl chain to the aromatic ring. The drastic spectral change due to the
hydration effect (n=2 for TrpH⁺, n=3 for DAH⁺) is rationalized by the suppression of ESPT in
clusters where the water is found in the reaction coordinate, i.e. in conformer where the water
is inserted between the protonated moiety and the aromatic ring, so called ring conformer.
Further studies on TrpH⁺ and DAH⁺ have revealed that even a single water molecule can
prevent ESPT ¹⁶. In all cases, we have evidenced the relationship between the structure and
photo reactivity of hydrated complexes. The ground state energy of the different conformers of
these hydrated complexes determines their relative population in the cryogenic ion trap, and
therefore the amplitude of the broad background signal relative to the sharp vibronic transitions.
We also found essentially identical solvent effect on ESPT when replacing water by methanol.¹⁷
It appears that this model provide a general framework for explaining the photo physics of
these protonated aromatic biomolecules containing small solvent clusters.

In this work, we studied the effect of methylation of the amino group on the ESPT
reaction in protonated N-methyl-dopamine complexes with a single water molecule
(NMeDAH⁺(H₂O)₁). By using UV and IR cryogenic ion photodissociation spectroscopy, UVPD
and IRPD spectra have been recorded. In addition, conformer-selected IR spectra are
measured by double resonance IR-UV hole burning spectroscopy. Based on spectral features
and theoretical calculations, we discuss the relationship between the conformation of the
complex, the excited state dynamics and the effect of methylation.

## Methods

The details of the experimental setup have already been reported elsewhere.¹⁸, ¹⁹ It
consists of an electrospray ionization source, two quadrupole mass spectrometers (the first
and the second QMSs), a temperature-controlled linear octupole ion trap,¹⁹, ²⁰ a cryogenic
quadrupole ion trap¹⁸, ²¹⁻²³ (QIT), and a time-of-flight mass spectrometer. A methanol solution
containing N-methyl-dopamine hydrochloride (Synchem, 2×10⁻⁵ M) with 0.5% formic acid was
electrosprayed and transported through a heat capillary at 70 °C to evaporate the solvents
from the fine droplets. The ions of interest were mass-selected by the first QMS and guided to
the linear ion trap maintained at 180 K, in which water vapor was introduced by a pulsed valve
(Parker Hannifin, General Valve Series 9), to produce hydrated clusters. The cluster ions were
mass-selected by the second QMS and only mono-hydrated clusters are introduced into the
cryogenic QIT, maintained at 4 K by a closed-cycle two-stage He refrigerator (Sumitomo, RDK-
408D2). The buffer He gas was introduced into the QIT by a pulsed valve (Parker Hannifin,
General Valve Series 9). The collisions with the cold buffer gas allow the clusters to be cooled
down to ~10 K (vibrational temperature). In this setup, formation of dehydrated cluster in QIT
by the remaining water vapor is negligible because of ultra-cold surface of QIT (4K) and
sufficient differential pumping speed.

The cold ions were irradiated with a frequency-doubled output of a tunable dye UV
laser (Lumonics). The subsequent photofragments were mass-analyzed using the time-of-
flight mass spectrometer. The UV photodissociation (UVPD) mass spectrum of
NMeDAH⁺(H₂O)₁ in Figure S1 shows the photodissociation loss channels of water (m/z 150)
and methylamine (m/z 137). UVPD spectra were measured by scanning the yield of the
photodissociation ions (m/z 137 and 150) as a function of laser frequency.

IR-UV dip spectroscopy was used to measure a conformer-selected IR spectrum. A
tunable OPO/OPA IR laser (LaserVision) illuminated the clusters ~50 ns prior to the UV
irradiation, whose frequency was set to a specific transition of the UVPD spectrum. When cold
ions are vibrationally excited by the IR laser, the photofragment yield is decreased due to the
depopulation of the ground state. IR-UV dip spectra were measured by monitoring the
decrease of the UVPD signal.

Two-color IR photodissociation (IRPD) spectroscopy was performed to measure the IR-active vibrations of all the conformers present in the cold QIT.²⁴ The first laser (IR1) illuminated the ions in the QIT ~100 ns before the irradiation with the second laser (IR2). When IR1 excites the clusters to a vibrationally excited state, the photon energy is redistributed to the vibrational degrees of freedom. The vibrationally excited hot clusters can absorb another photon of IR2, in which frequency is fixed at a hot band of a vibrational transition (~3577 cm⁻¹) with approximately five times stronger laser fluence (~5 mJ/pulse) than IR1, allowing the water molecule to be detached from the clusters. Two-color IRPD spectra were measured by monitoring the water loss channel.

Density functional theory (DFT) calculations were performed for structural optimization and harmonic frequency analysis. Initial structures were constructed by replacing one of the H atoms with a methyl group based on the reports on the DAH⁺(H₂O)₁.¹⁵ The solvent water molecule can be located above the catechol ring (“ring” conformer), to the side (“side” conformer), or above the ammonium group (“up” conformer).¹⁷ All the calculations at the electronically ground state (S₀) were performed at the B3LYP-D3BJ/cc-pVTZ level¹⁷ using the Gaussian 16 package. The harmonic frequencies were corrected for the anharmonicity with a scaling factor of 0.965 (NH and catechol OH stretches), 0.962 (H-bonded OH stretches), and 0.975 (CH stretches).¹⁷

Excited state calculations were performed with the TURBOMOLE program package²⁵
making use of the resolution-of-the-identity (RI) approximation for the evaluation of the
electron-repulsion integrals.²⁶,²⁷ Equilibrium geometries in the electronic ground state (S₀) and
first excited state (S₁) were evaluated at the CC2 level with the correlation-consistent polarized
valence double-ζ aug-cc-pVDZ basis set augmented with diffuse functions.²⁸ In particular, the
spin-component scaled approach (SCS) is applied since it improves the accuracy of CC2
calculations for the charge transfer states.²⁹ The vibrational modes of the ground state and the
first excited state have been calculated at the same level in order to obtain the adiabatic
excitation energy corrected for the difference in zero-point energy between the two states.
Finally, the reaction path connecting the locally excited state with the ESPT structure is
estimated using the reaction path program,³⁰ as implemented in TURBOMOLE, which uses
equal spacing via constrained optimization assuming a quadratic potential. 14 initial structures
were first generated by a linear synchronous transit method and further optimized in order to
estimate the energy of the transition state which defines the height of the ESPT barrier.

Results and Discussion

Figure 1 shows the UVPD spectra of a) monohydrated NMeDAH⁺ and b)
monohydrated DAH⁺.¹⁴ The latter spectrum consists of a background starting at 34500 cm⁻¹ and a relatively intense broad band centered at 34960 cm⁻¹. These spectral features were assigned to two conformers in which the water molecule is located above (noted up) or on the side (noted side) of the amine chain. A conformer in which a water molecule is inserted between the amine chain and aromatic ring (noted ring) is expected to give a sharp UV spectrum, however the presence of such conformer is not clear in the UV spectrum. Excited state proton transfer (ESPT) involves significant geometry changes in the excited state. When the ESPT is barrierless as for protonated tryptophan or goes through a low energy barrier as for protonated dopamine, this leads to a broad UV spectrum, which can be explained by bad Franck-Condon factors and fast proton transfer dynamics. The UVPD spectrum of NMeDAH⁺(H₂O)₁ also shows a background signal and an intense broad band, but a significant number of sharp vibronic transitions also appear. The presence of sharp peaks suggests that methylation of dopamine promotes new conformers that exhibit sharp vibronic transitions, reflecting relatively good Franck-Condon activity and slow excited state dynamics.

To understand the relationship between the dynamics and structures of NMeDAH⁺(H₂O)₁, conformer-selected IR spectra shown in Figure 2 are measured using IR-UV dip spectroscopy by setting the UV laser frequencies to 34765 cm⁻¹ and 34896 cm⁻¹, which correspond to two sharp vibronic transitions indicated by letter “A” and “B” in Figure 1, respectively. Thus, the IR-UV dip spectrum obtained by setting the UV laser on band A (B) shows the IR transition of the corresponding single conformer (herein after denoted conformer A and conformer B). The two IR-UV dip spectra show four vibrational transitions in the OH stretch region, from 3550 cm⁻¹ to 3700 cm⁻¹. Two vibrational bands are commonly observed in both spectra (3595 cm⁻¹ and 3660 cm⁻¹) which are close to the OH stretching vibrations of the catechol ring.¹⁵ These catechol OH stretches are not sensitive to the dopamine rotamer (G1 and G2)³¹ nor to the conformation of the water complex. The other two bands at ~ 3700 cm⁻¹ and ~ 3570 cm⁻¹ should be assigned to the two water OH stretches. In particular, the transition at 3700 cm⁻¹ is close to the expected free OH stretch commonly observed in hydrated species, while the redshifted one below 3600 cm⁻¹ should be related to a bonded OH stretch.

The two-color IRPD spectrum is also shown in Figure 2. This spectrum is not selective in terms of conformation, but allows the detection of IR transitions of all conformers present in the ion trap at low temperatures by enhancing detachment of the H₂O molecule by the second IR laser with fixed frequency. By comparing the two IR-UV spectra with the IRPD spectrum, it is clear that additional transitions appear in the latter. This reveals that other conformers contribute to the IRPD spectrum. In particular, in the OH stretching spectral region mentioned above, two transitions at 3640 cm⁻¹ and 3715 cm⁻¹ (highlighted in blue) appear clearly. These two transitions are close to the symmetric and antisymmetric stretching modes of free OH groups in water, which would indicate that at least one conformer for which the water molecule does not form a hydrogen bond is present. Additionally, a set of transitions in the CH-NH stretching region from 3050 cm⁻¹ to 3300 cm⁻¹ are observed.
To assign conformations, dispersion-corrected DFT calculations (B3LYP-D3BJ/cc-pVTZ) were performed for NMeDAH⁺(H₂O)₁. Nine conformers were selected after a systematic conformational search, their structures are shown in Figure S2. G1 and G2 are the two gauche rotamers of DAH⁺ which differ in the orientation of the alkyl chain above the catechol ring. Side, up and ring conformers indicated the position of the water molecule relative to DAH⁺. The six lowest-energy conformers have Gibbs energies (180 K) within 2.5 kJ/mol. Such a small difference in energy does not allow them to be sorted with certainty based solely on their relative energy. Theoretical spectra of calculated conformers are shown in Figure S3 and are compared with the IRPD spectrum. As already explained, the IRPD spectrum includes the contribution of at least three different conformers. In Figure 2, the structures and theoretical spectra of G1-side (ΔG = 0 kJ/mol at 180 K), G1-ring (+1.8 kJ/mol) and G2-ring (+2.1 kJ/mol) are compared with the conformer-selected IR spectra. Among all low-energy structures, only conformers G1-ring and G2-ring, in which the water molecule forms a hydrogen bond with the catechol ring, have a predicted OH stretch frequency shifted to the red that is consistent with the two transitions observed at 3560 cm⁻¹ and 3572 cm⁻¹ in the IR-UV dip spectra in Figure 2. In addition, both also have an NH stretch frequency calculated at 3312 cm⁻¹, very close to the low intensity band observed in the IRPD spectrum. These two conformers can therefore be confidently assigned to conformers A and B. The theoretical IR spectrum of the most stable conformer G1-side shows intense bands at 3200 cm⁻¹ and ~3050 cm⁻¹, however, such strong vibrational bands are not found in the two IR-UV dip spectra, which confirms that these intense and broad bands are not assigned to the ring conformers. Besides, the calculated symmetric and antisymmetric water OH stretches of G1-side are close to the experimental transitions observed in the IRPD spectrum (highlighted in blue in Figure 2). It should be mentioned that the energetically close side/up conformers, G2-side (+0.8 kJ/mol), G2-up (+1.4 kJ/mol), and G1-up (+2.4 kJ/mol) would also contribute to the broad bands highlighted in blue in Figure 2 because they show similar IR spectra to that of G1-side.

The conformer-selected UV dip spectra of conformer A (G1-ring), conformer B (G2-ring) were recorded by fixing the IR laser to the corresponding vibrational transitions (3560 cm⁻¹ and 3572 cm⁻¹, respectively) and are shown in Figure 3. Both spectra contain only sharp transitions. Comparison with the UVPD spectrum shows that the sharp transitions from 34500 cm⁻¹ to 34800 cm⁻¹ are assigned to conformer A, while those around 34920 cm⁻¹ which corresponds to the more intense band are assigned to conformer B. Unfortunately, we could not obtain the conformer-selected UV spectrum of the side/up conformers which exhibit a broad, background signal. Nevertheless, by subtracting the spectra of conformers A and B from the UVPD spectrum, the broad background signal is then attributed to the UV transition of G1-side and the other side/up conformers (see the difference spectrum shown in Figure 3d). This strongly suggests that the side/up conformers have fast excited state dynamics.

CC2 calculations were performed for the conformers assigned by IR spectroscopy. This includes the G1-ring, G2-ring in which water interacts with the catechol ring, and G1-side
where the protonated amino group forms a hydrogen bond with the catechol. Geometry optimization and frequency calculations of the ground state and first excited state ($\pi\pi^*$) were calculated to estimate the adiabatic excitation energy of the S₁ state. The G1-side has the lowest excitation energy at 33540 cm⁻¹, followed by G1-ring and G2-ring whose band origins are calculated at 33850 cm⁻¹ and 34263 cm⁻¹, respectively. The order of the calculated band origins of the three conformers corresponds to the ones measured in the IR-UV spectra, in which the vibronic transitions of G1-ring (A) start at 34530 cm⁻¹ while those of G2-ring start at 34860 cm⁻¹. CC2 calculations predict the band origin of these two conformers within 600 cm⁻¹ of the experimental value, i.e., an absolute error of less than 2%, which is remarkable for such a molecular size. It is interesting to note that the G1-side has the lowest excitation energy among these conformers, which is consistent with the onset of the background signal in the UVPD below 34500 cm⁻¹.

Excited proton transfer (ESPT) between the protonated amino group and the aromatic ring has been extensively studied in related systems, such as amino acids³² and catechol compounds.³³ Here again, ESPT is expected to occur in complexes where the water molecule does not block the interaction between the protonated group and the ring, for instance G1-side. ESPT is highly exothermic with an energy of 0.8 eV lower than the locally excited S₁ state. In the ESPT structure, the hybridization of the nitrogen of the amine group has become sp² with a planar NHCH₃ group. The electronic configuration of the S₁ state has therefore changed from a $\pi-\pi_{ring}^*$ transition localized on the catechol to a $\pi_{NH2}-\pi_{ring}^*$ transition (MOs plotted in Figure S4). This had already been noted in the case of hydrated dopamine.¹⁵ The optimized reaction path connecting the minimum of the S₁ state ($\pi\pi_{ring}^*$) and the ESPT structure for G1-side is shown in Figure 4. This reaction path is characterized by a very low barrier of 80 meV, which corresponds to the position of the proton between the amino and catechol groups. This barrier is significantly lower than the zero-point energy of the NH stretch mode. It is also interesting to note that the orientation movements of the protonated group around the catechol to optimize the proton transfer reaction require almost no additional energy. In conclusion, ESPT is thought to occur in such a complex even with low excess energy in the S₁ ($\pi\pi^*$) state.

To make the methylation effect clear, the two-color IRPD spectra of NMeDAH⁺(H₂O)₁ and DAH⁺(H₂O)₁ (drawn by red and gray, respectively) are compared in Figure 5. The most significant change is the relative population of ring conformers. Vibrational transitions of ring conformers (G1- and G2-ring) are indicated by red circles while those of the side/up conformers are marked by blue circles. The OH stretches of ring- and side (and up) conformers in NMeDAH⁺(H₂O)₁ appear almost at the same positions as those of DAH⁺(H₂O)₁ so that enhancement of ring-conformer's bands and suppression of side-conformer's ones can be seen in the figure. We estimate the relative population of ring- and side (up) conformers of NMeDAH⁺(H₂O)₁ and DAH⁺(H₂O)₁ by normalizing the fitted area of these bands (Figure S5) using calculated IR strengths, which are shown by bar chart in the figure. It clearly shows that the population of the ring-conformers is doubled by methylation. Though the side and up
conformers cannot be distinguished by OH stretches, the NH stretches are different between side and up conformers. Notably, the H-bonded NH stretch of the up conformer at ~3160 cm⁻¹ in NMeDAH⁺(H₂O)₁ is significantly reduced, whereas the free NH stretch of the up conformer in DAH⁺(H₂O)₁ is comparable to that of the side conformer. The up conformers are slightly less stable by methylation as shown in Figure S2.

These observations can be rationalized by the steric effect of methylation (see Figure 6). Methylation of the DAH⁺ can impose a steric repulsion between the methyl group and the catechol moiety in a certain conformation. The *anti* configuration is preferable to avoid the steric effect. Although side and ring conformers can take the *anti*-type conformation by methylating the H atom on the opposite side of the catechol ring, the up conformer cannot take such a conformation because this site is H-bonded to the water molecule. In the calculated up conformer, the H atoms are preferentially methylated in the side position and the methylation at the ring position could not be obtained as a local structure. The methylation at the ring position seems sterically unfavorable, given that the G1-side-2 conformer, in which the methyl group is in the ring position, is less stable by 17 kJ/mol. Although the methylation at the side position is less sterically hindered than the ring position, it still suffers from more steric repulsion due to the gauche configuration (Figure 6). The different populations can be ascribed to the less contribution of the up conformers.

In summary, we have measured UVPD and IRPD spectra of monohydrated N-methyl-dopamineH⁺ NMeDAH⁺(H₂O)₁ by cryogenic ion spectroscopy. NMeDAH⁺(H₂O)₁ shows sharp vibronic transitions on a broad background signal, suggesting coexistence of two or more conformers in which the excited state dynamics are very different. From conformer-selected IR spectra and theoretical calculations, sharp bands are assigned to G1-ring and G2-ring conformers in which the water molecule is inserted between the amine chain and the aromatic ring and prevents NH → π ESPT. The broad background is attributed to the most stable G1-side conformer and the other side/up conformers in which the N-H bond of the amino group directly interacts with the catechol ring. Calculated reaction path of NH → π ESPT in the most stable conformer exhibits a barrier lower than 100 meV, thus the broad UV spectrum is explained by the fast ESPT. The main effect induced by methylation of the amino group is an increase in the stability and relative population of cyclic conformers compared to DAH⁺(H₂O)₁. This increase in population can be explained by the steric hindrance imposed by methylation. The results also confirm the model that explains the absence of PT in the excited state by inserting water into the PT reaction pathway.

**Associated contents**

**Supporting information**
UV photodissociation mass spectrum (S-1), Optimized conformers (S-2), Calculated IR spectra (S-3), Frontier Molecular Orbitals (S-4), and Gauss fitting IR spectra (S-5).

## Acknowledgements

This work is supported in part by KAKENHI (JP21H04674, JP21K14585) and the Core-to-core program of JSPS (JPJSCCA2024002), World Research Hub Initiative in Institute of Science Tokyo and Smoking Research Foundation. The computations are performed at the Research Center for Computational Science, Okazaki, Japan, and the HPC resources from the "Mésocentre" computing center of Centrale Supélec and École Normale Supérieure Paris-Saclay, supported by CNRS and Région Île-de-France (http://mesocentre.centralesupelec.fr/) and the HPC resources MAGI from Université Sorbonne Paris Nord, France.

## Notes

The authors declare no competing financial interest.

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## TOC Graphics

<figure><img src="image_2.png" /></figure>
<figure><img src="image_3.png" /><figcaption>Figure 1 UVPD spectrum of NMeDAH⁺(H₂O)₁ in comparison with that of DAH⁺(H₂O)₁.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 2 IR-UV dip spectra of NMeDAH⁺(H₂O)₁ probed at 34765 (conf. A) and 34896 cm⁻¹. The blue-colored bands correspond to the unassigned ones except for conf. A and B.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Figure 3 UV spectra of NMeDAH⁺(H₂O)₁ measured in cryogenic ion trap: a) UVPD, b) IR-UV hole-burning spectra probed at 3560 (conf. A), c) that probed at 3572 (conf. B) and d) difference spectrum of a-b-c.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 4 Optimized reaction path (SCS-CC2/aug-cc/pVDZ) connecting the locally excited state $\pi\pi^*$ with the ESPT state for G1-side</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Figure 5 a) Two-color IRPD spectrum of NMeDAH⁺(H₂O)₁ (red) in comparison with that of DAH⁺(H₂O)₁ (black). Transitions of ring and side & up conformers are indicated by red and blue circles, respectively. b) Estimated population of ring and side & up conformers.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Figure 6 Schematic illustration of the origin of the population difference between NMeDAH⁺(H₂O)₁ and DAH⁺(H₂O)₁.</figcaption></figure>