# Testing the photostability of 1-Cyanonaphthalene-(H2O)1,2 complexes to predict their resistance to the interstellar radiation field

Ana Niedojadlo, Alejandro Gutiérrez-Quintanilla, Jordan Dezalay, Isabelle

Couturier-Tamburelli, Juan Cruz Latorre, Meloottayil Vinitha, Anaïs

Massaloux, Gustavo Pino, Jennifer Noble

► To cite this version:

Ana Niedojadlo, Alejandro Gutiérrez-Quintanilla, Jordan Dezalay, Isabelle Couturier-Tamburelli, Juan Cruz Latorre, et al. Testing the photostability of 1-Cyanonaphthalene-(H2O)1,2 complexes to predict their resistance to the interstellar radiation field. Journal of Molecular Spectroscopy, 2026, 416-417, pp.112078. 10.1016/j.jms.2026.112078. hal-05524458

HAL Id: hal-05524458

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Submitted on 23 Feb 2026

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# Testing the photostability of 1-Cyanonaphthalene-(H₂O)₁,₂ complexes to predict their resistance to the interstellar radiation field

Ana Niedojadlo<sup>a,b,c</sup>, Alejandro Gutierréz Quintanilla<sup>d,e</sup>, Jordan Dezalay<sup>d</sup>, Isabelle Couturier-Tamburelli<sup>d</sup>, Juan Cruz Latorre<sup>a,b,c</sup>, Meloottayil V. Vinitha<sup>d,1</sup>, Anaïs Massaloux<sup>d,2</sup>, Gustavo A. Pino<sup>a,b,c,**</sup>, Jennifer A. Noble<sup>d,*</sup>

## ARTICLE INFO

### Keywords:

- Electronic spectroscopy
- 1-Cyanonaphthalene-water clusters
- Ionization potentials
- DFT calculations
- Evaporation

## ABSTRACT

The studies of spectroscopy, reactivity, and photochemistry of CN-substituted aromatic molecules have acquired particular relevance since their detection in TMC-1 in 2018. In this context, the REMPI spectroscopy of 1-cyanonaphthalene (1CNN) and its complexes with one (1CNN-W) and two (1CNN-W₂) water molecules (W = H₂O) was recorded and compared with theoretical calculations at the CAM-B3LYP/aug-cc-pVDZ level, validating the theoretical method and justifying its use in predicting some properties of the cationic species in their ground and excited electronic states. The adiabatic ionization potential (IP<sub>ad</sub>) for 1CNN monomer was experimentally determined for the first time (8.432 eV) and agrees to well within 1% with the theoretical value (8.417 eV). Ionization thresholds for 1CNN-W and 1CNN-W₂ complexes, as well as the water evaporation threshold in the cationic state of the latter complex, were also determined experimentally and were found to agree with the theoretical results to within 1%. Based on the available data, it is tentatively predicted that once 1CNN forms aggregates with ≥1 H₂O molecules, these complexes will be ionized by Ly-α irradiation, but they will be stable in their cationic state.

## 1. Introduction

Polycyclic aromatic hydrocarbons (PAHs) and their nitrile derivatives are of central interest in both molecular spectroscopy and astrochemistry due to their high stability, characteristic spectroscopy, and potential role as carriers of interstellar absorption features. Among these, the 1-cyanonaphthalene (1CNN) molecule has long been studied in terms of its molecular physics. Its vibronic spectroscopy was first characterized in the early 1980s by means of Laser Induced Fluorescence (LIF) in a supersonic jet expansion, which revealed the main electronic

transitions and associated vibrational structure [1–3]. The LIF excitation spectrum of 1CNN reveals a strong transition at 318.16 nm corresponding to the band origin (0₀) of the S₁—S₀ transition, followed by a progression of vibronic bands extending up to ~1000 cm⁻<fnref n="1" /> above the origin [1]. Later high-resolution fluorescence studies reported consistent spectral features, and several transitions were assigned by comparison with naphthalene [2].

1CNN provides an attractive model system for studying solvent interactions and clustering effects. While the nitrile group has little effect on the intrinsic spectroscopy of the aromatic chromophore, it

<footnote><sup>a</sup> Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Instituto de Investigaciones en Fisicoquímica de Córdoba (INFIQC), Haya de la Torre s/n, Ciudad Universitaria, X5000HUA Córdoba, Argentina</footnote>

<footnote><sup>b</sup> Universidad Nacional de Córdoba, Centro Láser de Ciencias Moleculares, Haya de la Torre s/n, Ciudad Universitaria, X5000HUA Córdoba, Argentina</footnote>

<footnote><sup>c</sup> Universidad Nacional de Córdoba, Facultad de Ciencias Químicas, Departamento de Fisicoquímica, Haya de la Torre s/n, Ciudad Universitaria, X5000HUA Córdoba, Argentina</footnote>

<footnote><sup>d</sup> Physique des Interactions Ioniques et Moléculaires (PIIM), CNRS, Aix-Marseille Université, Marseille, France</footnote>

<footnote><sup>e</sup> Université de Pau et des Pays de l'Adour, E2S UPPA, CNRS, IPREM, Pau 64053, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>** Corresponding author at: Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Instituto de Investigaciones en Fisicoquímica de Córdoba (INFIQC), Haya de la Torre s/n, Ciudad Universitaria, X5000HUA Córdoba, Argentina.
E-mail addresses: gpino@unc.edu.ar (G.A. Pino), jennifer.noble@univ-amu.fr (J.A. Noble).</footnote>

<footnote n="1">¹ Current address: CNR-ISM, Area della Ricerca Roma 1, Monterotondo Scalo, Roma 00015, Italy.</footnote>

<footnote n="2">² Current address: Autorité de Sûreté Nucléaire et de Radioprotection (ASNR), PSN-RES/SEREX/L2EC, Cadarache, France.</footnote>
significantly influences intermolecular interactions with polar and non-polar solvent molecules, often giving rise to multiple isomers [1,3–7].

Gas phase complexation with water molecules has been explored using various spectroscopic techniques. LIF spectroscopy demonstrated that complexation with a single water molecule exerts little influence on the electronic spectrum of 1CNN [7], an observation later confirmed by resonantly enhanced multiphoton ionization (REMPI) experiments combined with MP2 calculations [3]. Mass-selective UV-UV hole-burning studies identified two low-energy isomers of the 1CNN-W (1,1) complex corresponding to water binding at opposite sides of the CN group, with an energy difference of 2.71 kJ mol⁻¹ [3]. For the 1CNN-W₂ (1,2) complex, two of the three calculated isomers were detected experimentally with the same technique, showing an energy difference of 2.21 kJ mol⁻¹ [3]. Evaporation of the 1:2 complex into the 1:1 was observed [3].

Following the detection of interstellar benzonitrile in the cold dark cloud TMC-1 in 2018 [8], a number of PAHs and cyano-PAHs have been observed in the same object by the Gotham [9–14] and Quijote [15–18] survey teams, including 1CNN in 2021 [9]. These observations have prompted a resurgence in the study of the physical chemistry of cyano-PAHs, and notably, on their photon-induced reactivity. While it has been shown that the protonated cyanobenzene (benzonitrile) cation is highly UV unstable [19], and produces the same HCN photofragment as under IRMPD excitation conditions [20], this photoinstability is not common to all similar aromatic cations.

In the specific case of 1CNN⁺, the fast, efficient relaxation of energy via recurrent fluorescence permits this molecule to survive UV irradiation [21]. Its cold ion photofragmentation electronic spectroscopy [22] revealed a strong band origin for the D₂↔D₀ transition at 734.3 nm, which is independent of temperature in the 10–50 K range and allows an upper limit of 10¹³ cm⁻² to be placed on its interstellar column density, and a lower intensity D₃⁻D₀ transition at ~500 nm.

Given that 1CNN has been observed in TMC-1, a cold, dense cloud within the Taurus molecular cloud complex, where water ice has also been observed [23], it is interesting to extend upon the spectroscopy studies performed on 1CNN-Wₙ complexes [3,7] to look at the photostability of these species under UV irradiation. Recent joint experimental and DFT studies on microhydrated PAHs [24] and a DFT-based investigation of the series naphthalene, 1CNN and 2CNN [25] have revealed details of the PAH-water bonding geometries via IR spectroscopy and/or theoretical calculations, and have discussed UV photoreactivity induced within these systems.

In the present work, we provide experimental spectroscopic data: the adiabatic ionization potential (IP<sub>ad</sub>) of 1CNN, determined for the first time and in good agreement with theoretical calculations, as well as the ionization thresholds for 1CNN-W and 1CNN-W₂ complexes (W = H₂O), and the water evaporation threshold in the cationic state of the 1:2 complex. The main aim of this study is to experimentally explore the UV photostability of gas-phase 1CNN-W₁,₂ complexes in a broad excitation energy range. Since Lyman-α radiation (10.2 eV), generated as secondary photons following the interaction of cosmic rays with hydrogen, dominates the UV field within dense molecular clouds, we perform further quantum chemical calculations to predict the VUV photostability of these complexes in interstellar environments. It is tentatively predicted that once 1CNN aggregates with ≥1 H₂O molecules, these complexes will be ionized by Ly-α irradiation, but will be stable in their cationic state.

# 2. Methodology

## 2.1. *Experimental methods*

The experiments were carried out in a molecular beam setup, previously described in detail elsewhere [26,27]. Briefly, it is composed of two differentially pumped vacuum chambers (10⁻⁴ and 10⁻⁶ mbar) separated by a 6 mm diameter skimmer. In the first chamber, the cold

molecular beam is generated by a supersonic expansion, while the interaction with the lasers and subsequent extraction to the time-of-flight mass spectrometer (TOF-MS) occurs in the second chamber.

The solid 1CNN reagent was placed in a stainless steel reservoir inside the first vacuum chamber where it was heated to 150 °C to promote vaporization. Helium was used as a carrier gas (2–6 bar) to pick up 1CNN from the reservoir and then expanded through a 200 µm diameter nozzle placed at the exit of a pulsed valve (10 Hz, General Valve). A few drops of distilled water were added to the helium line to favor the formation of the hydrated aggregates in the gas phase.

In the second chamber, a TOF-MS (Jordan TOF Products, Inc.) is used for the detection of charged species formed using two UV-visible OPO lasers (EKSPLA model-NT342B) tunable in the 225–709 nm region (FWHM = 8 cm⁻¹), with 10 ns pulse width, a minimum scanning step of 0.02 nm and 10 Hz repetition rate.

The molecular clusters were analyzed by two-color-two-photon REMPI (1 + 1') for which the two laser beams were temporally synchronized and spatially overlapped in the middle of the repulsion and extraction plates of the TOF-MS, the pump laser excites the clusters at a tunable wavelength, while the probe laser ionizes them at a fixed wavelength (258 nm).

## 2.2. *Theoretical methods*

Electronic structure calculations and spectral simulations were performed using the Gaussian 16 software suite [28]. Optimized geometries, vibrational frequencies, and energies were calculated using density functional theory (DFT) and its time-dependent version (TD-DFT) for the ground and excited states, respectively, with the CAM-B3LYP functional and the aug-cc-pVDZ basis set, including the D3 Grimme's dispersion correction. This level of theory has previously been used to successfully describe aromatic molecule-water clusters [29] and it has been shown to be an appropriate method to simulate vibronic spectra, and predict the energy and geometry of different radical cations, as previously reported [30,31].

Vibronic spectra were simulated from the optimized geometries using the PGOPHER software package [32] to calculate the Franck-Condon factors (FCF). Computed frequencies for each state are harmonic, and scaling factors were not applied. The vibrational and rotational temperature in the spectral simulations was set to 20 K.

# 3. Results and discussion

As previously mentioned, the aim of this work is to test the UV photostability of 1CNN-W₁,₂ complexes and predict their ISM survivability (i.e. extrapolate to Ly-α). Therefore, although the theoretical method chosen to perform the calculations presented here (CAM-B3LYP/aug-cc-pVDZ) is known to be a good compromise between precision and computational cost for similar systems [29–31,33], it needs to be validated for this particular one. Thus, experimental information about the S₁ and D₀ states of 1CNN and the 1:1 and 1:2 complexes was obtained in order to compare with the theoretical results, as a validation probe.

## 3.1. *Experimental and theoretical spectra of 1CNN and 1CNN-W₁,₂*

The REMPI (1 + 1') spectra of 1CNN, 1CNN-W, and 1CNN-W₂, recorded on *m/z* = 153, 171, and 189, respectively, are presented in Fig. 1. The frequency in the x-axis corresponds to the excitation photon (S₁↔S₀ transition) while the ionization photon wavelength was kept constant at 258 nm (4.806 eV / 38,764 cm⁻¹). This ionization wavelength was chosen to ensure that the total energy (excitation + ionization) is above the ionization threshold of the bare molecule and the clusters.

The spectra reported in Fig. 1 are in very good agreement with those previously reported by other authors [3,7]. 1CNN has only one isomer as
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** (1 + 1') REMPI spectra obtained for 1CNN monomer (black) and its complexes with one (1CNN-W) and two (1CNN-W₂) water molecules (red and blue, respectively). The ionization photon was fixed at 258 nm. The spectra were vertically shifted for better visualization. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

shown in Table 1, while the 1CNN-W and 1CNN-W₂ complexes have been reported to have two isomers each, since the water molecules can interact on one side or the other of the –CN group in the molecular plane (Z-matrices are reported in the Supplementary information (S.I.)). All isomers were calculated and are presented in Table 1 together with their stabilization energy relative to the bare 1CNN and one or two free water molecules, for the 1CNN-W and 1CNN-W₂ complexes, respectively (ΔE).

The energy difference between both isomers of the 1CNN-W and 1CNN-W₂ complexes is very low, in agreement with previous works [7], and they can be considered quasi-isoenergetic.

The assignment of the band origin (0₀₀₀) for the different isomers has been previously performed by Brenner et al. [7] and Fujiwara et al. [3]. and their results are presented in Table 1 together with corresponding values determined in this work and the calculated adiabatic energy for the S₁–S₀ transition (E_ad), including the zero-point-energy difference between both states (ΔZPE). The differences between the experimental values obtained are within the expected uncertainties of the methods used for determinations (±30 cm⁻¹). A good agreement between experimental and theoretical values is observed, with a difference between both sets of values that is within only ±1.5% of the experimental value.

Finally, the Franck-Condon factors (FCF) for the two isomers of the 1:1 complex and the two isomers of the 1:2 complex were computed and they are shown in Fig. 2 in comparison with the experimental results. A satisfactory agreement between experimental and calculated FCF and excited state vibrational frequencies is observed in Fig. 2.

Most experimental transitions are observed to be narrow, and they are well reproduced by the FCF calculations using a FWHM of 8 cm⁻¹ to account for the laser bandwidth. An exception is seen in the 1CNN-W₂

**Table 1**

Optimized structures for the S₀ and S₁ states of 1CNN and the 1CNN-W and 1CNN-W₂ complexes with water, calculated energies relative to the S₀ state of 1CNN and, where appropriate, one or two free water molecules (ΔE) and vertical (E_v) and adiabatic (E_ad) S₁–S₀ transition energies, together with the experimental values for the band origin.

<table><thead><tr><th rowspan="2">Cluster</th><th>S<sub>0</sub></th><th colspan="4">S<sub>1</sub></th><th rowspan="2">Structure</th></tr><tr><th>Structure</th><th>ΔE (eV)</th><th>E<sub>v</sub> (eV)</th><th>E<sub>ad</sub> + ΔZPE (eV)</th><th>S<sub>1</sub> ↔ S<sub>0</sub> exp. (eV)</th></tr></thead><tbody><tr><td>1CNN</td><td></td><td>0.000</td><td>4.384 (35,358 cm<sup>-1</sup>)</td><td>3.959 (31,931 cm<sup>-1</sup>)</td><td>3.896 (31,421 cm<sup>-1</sup>)<br/>a) 31,400 cm<sup>-1</sup><br/>b) 31,412 cm<sup>-1</sup></td><td></td></tr><tr><td>1CNN-W<br/>I</td><td></td><td>-0.217</td><td>4.392 (35,422 cm<sup>-1</sup>)</td><td>3.963 (31,964 cm<sup>-1</sup>)</td><td>3.900 (31,453 cm<sup>-1</sup>)<br/>a) Not reported<br/>b) 31,439 cm<sup>-1</sup></td><td></td></tr><tr><td>1CNN-W<br/>II</td><td></td><td>-0.230</td><td>4.312 (34,779 cm<sup>-1</sup>)</td><td>3.897 (31,429 cm<sup>-1</sup>)</td><td>3.882 (31,308 cm<sup>-1</sup>)<br/>a) 31,284 cm<sup>-1</sup><br/>b) 31,356 cm<sup>-1</sup></td><td></td></tr><tr><td>1CNN-W<sub>2</sub><br/>I</td><td></td><td>-0.582</td><td>4.382 (35,342 cm<sup>-1</sup>)</td><td>3.960 (31,939 cm<sup>-1</sup>)</td><td>3.901 (31,462 cm<sup>-1</sup>)<br/>a) 31,435 cm<sup>-1</sup><br/>b) 31,449 cm<sup>-1</sup></td><td></td></tr><tr><td>1CNN-W<sub>2</sub><br/>II</td><td></td><td>-0.570</td><td>4.261 (34,366 cm<sup>-1</sup>)</td><td>3.842 (30,986 cm<sup>-1</sup>)</td><td>3.867 (31,187 cm<sup>-1</sup>)<br/>a) 31,153 cm<sup>-1</sup><br/>b) 31,188 cm<sup>-1</sup></td><td></td></tr></tbody></table>

a) Brenner et al. [7].

b) Fujiwara et al. [3].
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** (1 + 1') REMPI spectra of the 1CNN monomer (a. left), 1CNN-W complex (b. middle) and 1CNN-W₂ complex (c. right) and the corresponding simulated spectra for the different isomers, whose structures are also shown as insets. The 0₀ transition of each calculated spectrum was shifted to match the experimental one, and the intensities were scaled to match those of the origin peaks of each species.</figcaption></figure>

complex spectrum, where a mixture of wide and narrow bands is observed. According to these calculations, the broadening observed in the first band is attributed to spectral congestion due to low frequency vibrations of the 1CNN-W₂ II isomer, associated with the 1₀ vibrational mode, mainly related to the in-plane motion of the water molecules with respect to the 1CNN, and the 6₁₁⁰ vibrational mode, associated with the out-of-plane displacement of the O—H bond in one of the water molecules, as shown in Fig. S1 (S.I.).

The good agreement between theoretical and experimental 0₀ transition energies and spectral shape (FCF and frequencies) for all five species validates the accuracy of the method for predicting ground and excited state energies, structures, and frequencies of neutral species. It should be noted that the calculated E<sub>ad</sub> + ΔZPE values differ by only 0.4–1.6% from the experimental transition energy.

The E<sub>v</sub> and E<sub>ad</sub> + ΔZPE for the S<sub>n</sub>—S<sub>0</sub> (n = 2, 3, and 4) for the three species have also been calculated and are reported in Table S1 (S.I.) together with the corresponding oscillator strength (O.S.).

## 3.2. Experimental and theoretical ionization energy and water evaporation in the D₀ state of 1CNN-W<sub>1,2</sub>

The ionization thresholds of 1CNN, 1CNN-W and 1CNN-W₂ complexes were carefully determined according to the following procedure. The 1CNN-W II and 1CNN-W₂ II isomers were chosen to explore because they both present a relatively high signal.

In order to determine the ionization thresholds of 1CNN, 1CNN-W II and 1CNN-W₂ II complexes and the water evaporation threshold from the 1CNN-W₂ complex, resonant two-color two-photon ionization experiments were conducted. The excitation laser was tuned resonantly to the 0₀ transition of the selected species (i.e. 3.896 eV for 1CNN, 3.882 eV for 1CNN-W II complex and 3.867 eV for 1CNN-W₂ II complex), while the ionization laser was scanned in the range 4.4–5.1 eV, and the ions intensities were integrated on the different mass channels.

Typically, in similar systems, spectral congestion increases with the cluster size, leading to a structureless and continuous absorption spectrum for large clusters, which lies underneath the vibronic transitions of small clusters [3,34,35]. Thus, their contribution is the same on-resonance and out-of-resonance of the 0₀ transition of a size-selected small cluster. Therefore, to subtract the contribution of larger clusters, the experiment was repeated but with the excitation laser slightly moved (317.3 nm / 3.907 eV) to be out-of-resonance of the 0₀ band of each species (Fig. S2, S.I.).

The out-of-resonance signal was subtracted from the on-resonance signal, leading to the corrected signal for the spectroscopically-selected molecule or complex. The corrected spectra are shown in Fig. 3.

The corrected signal recorded on *m/z* = 153 (1-CNN⁺) upon excitation of the bare molecule (1CNN) depicted in Fig. 3a clearly shows a first onset at 8.432 eV followed by several steps until 8.604 eV where a sudden enhancement of the signal is observed. These features will be discussed in the following paragraphs.

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Photoion yield curves recorded on different mass channels, as indicated in the figure, upon excitation of 1CNN (a. left), 1CNN-W (b. middle), and 1CNN-W₂ (c. right). The curves are the result of the difference of the signals recorded with the excitation laser tuned on-resonance of the 0₀ transition (3.896 eV for 1CNN, 3.882 eV for 1CNN-W II and 3.867 eV for 1CNN-W₂ II) minus the signals recorded with the excitation laser tuned out-of-resonance (3.907 eV) of the 0₀ band of each species.</figcaption></figure>
<caption>**Table 2**</caption>

Experimental and theoretical ionization energies and evaporation thresholds for 1CNN, 1CNN-W II and 1CNN-W₂ II, and optimized structures in the neutral ground state (S₀) and cationic ground state (D₀). All energies are reported in eV.

<table><thead><tr><th rowspan="2">Optimized structures of the neutrals</th><th colspan="2">Experimental</th><th colspan="3">Theoretical</th><th rowspan="2">Optimized structure of the ions</th></tr><tr><th>Ionization threshold (eV)</th><th>Evaporation threshold (eV)</th><th>IP<sub>v</sub> (eV)</th><th>IP<sub>ad</sub> + ΔZPE (eV)</th><th>Evaporation threshold (eV)</th></tr></thead><tbody><tr><td><img src="image_0_0.png" alt="structure of 1CNN-W II with 8.432<sup>a</sup> (adiabatic) and 8.604<sup>b</sup> (vertical) ionization threshold"></td><td><img src="image_0_1.png" alt="structure of 1CNN-W II with 8.432<sup>a</sup> (adiabatic) and 8.604<sup>b</sup> (vertical) ionization threshold"></td><td>-</td><td>8.558</td><td>8.417</td><td>-</td><td><img src="image_0_2.png" alt="structure of 1CNN-W II with 8.432<sup>a</sup> (adiabatic) and 8.604<sup>b</sup> (vertical) ionization threshold"></td></tr><tr><td><img src="image_0_3.png" alt="structure of 1CNN-W II with 8.436<sup>a</sup> (vertical) ionization threshold"></td><td><img src="image_0_4.png" alt="structure of 1CNN-W II with 8.436<sup>a</sup> (vertical) ionization threshold"></td><td>-</td><td>8.476</td><td>8.251</td><td>8.649</td><td><img src="image_0_5.png" alt="structure of 1CNN-W II with 8.436<sup>a</sup> (vertical) ionization threshold"></td></tr><tr><td><img src="image_0_6.png" alt="structure of 1CNN-W II with 8.393<sup>a</sup> (vertical) ionization threshold"></td><td><img src="image_0_7.png" alt="structure of 1CNN-W II with 8.393<sup>a</sup> (vertical) ionization threshold"></td><td>8.591<sup>a</sup></td><td>8.440</td><td>8.155</td><td>8.595</td><td><img src="image_0_8.png" alt="structure of 1CNN-W II with 8.393<sup>a</sup> (vertical) ionization threshold"></td></tr></tbody></table>

In the case of the selective excitation of the 1CNN-W II complex (Fig. 3b), a single onset at 8.436 eV is observed. We were unable to observe any signature of water evaporation on the mass channel *m/z* = 153 when this complex is excited up to 9.00 eV, i.e. 0.60 eV above its ionization threshold.

Finally, upon excitation-ionization of the 1CNN-W₂ II complex (Fig. 3c), ions were observed on the mass channels *m/z* = 189 and 171, corresponding to the ionic species 1CNN-W₂ II and 1CNN-W⁺, respectively. On the one hand, the onset in the *m/z* = 189 channel (8.393 eV) corresponds to the ionization energy of the 1CNN-W₂ complex. On the other hand, the onset in the *m/z* = 171 channel (8.592 eV) is related to the threshold for the evaporation of one water molecule in the ionic state of the 1CNN-W₂ II complex and the consequent appearance of the signal at the 1CNN-W⁺ complex's mass. No evidence of two water molecule evaporation was found in channel *m/z* = 153 up to 9.00 eV, i.e. 0.61 eV above 1CNN-W₂ ionization threshold.

The vertical and adiabatic Ionization Potentials (IP<sub>ad</sub> and IP<sub>v</sub>, respectively) as well as the water evaporation energies (E<sub>evap</sub>) in the ionic state of the complexes were calculated, and are shown in Table 2 together with the experimental results from this work and others from

the literature, when available.

By comparison with the theoretical values reported in Table 2, we can assign the experimental onsets to different processes. In the case of 1CNN the first onset at 8.432 eV is assigned to the IP<sub>ad</sub>, calculated at 8.417 eV, while the onset at 8.604 eV is assigned to the IP<sub>v</sub> calculated at 8.558 eV and in good agreement with previous reported values [36,37]. This is the first experimental report of the IP<sub>ad</sub> of 1CNN, since previous works [36,37] have only reported the IP<sub>v</sub>. The steps observed in Fig. 3a between IP<sub>ad</sub> and IP<sub>v</sub> are assigned to vibronic structure of the 1CNN⁺ radical cation.

The onsets observed for the 1CNN-W II and 1CNN-W₂ II complexes at 8.436 eV and 8.393 eV, respectively, are closer to the corresponding calculated IP<sub>v</sub> (8.476 eV and 8.440 eV, respectively) than to their calculated IP<sub>ad</sub> (8.251 eV and 8.155 eV, respectively). For these complexes, the position/s of the water molecule/s undergo/es a large change from the S₀ state to the D₀ state, as observed in Table 2. This large geometry change leads to negligible FCF in the adiabatic region and, as a consequence, an undetectable signal below the sensitivity of the experimental set-up. The ion signal becomes detectable close to the IP<sub>v</sub> in the FC region.

<caption>**Table 3**</caption>

Calculated evaporation energies of the neutral and ionic 1CNN-W II and 1CNN-W₂ II complexes in different electronic states. The three possible evaporation channels for 1CNN-W₂ are included: evaporation of one water molecule, two single water molecules or the water dimer. All values are reported in eV.

<table><thead><tr><th rowspan="2">E<sub>evap</sub> (eV)</th><th>1CNN-W II →</th><th colspan="3">1CNN-W<sub>2</sub> II →</th></tr><tr><th>→ 1CNN + H<sub>2</sub>O</th><th>→ 1CNN-W II + H<sub>2</sub>O</th><th>→ 1CNN + 2 H<sub>2</sub>O</th><th>→ 1CNN + (H<sub>2</sub>O)<sub>2</sub></th></tr></thead><tbody><tr><td>S<sub>0</sub></td><td>0.232</td><td>0.343</td><td>0.575</td><td>0.411</td></tr><tr><td>S<sub>1</sub></td><td>0.294</td><td>0.398</td><td>0.692</td><td>0.459<sup>a</sup></td></tr><tr><td>E<sub>evap</sub> (eV)</td><td>1CNN<sup>+</sup>-W II →</td><td>1CNN<sup>+</sup>-W<sub>2</sub> II →</td><td>→ 1CNN<sup>+</sup> + 2 H<sub>2</sub>O</td><td>→ 1CNN<sup>+</sup> + (H<sub>2</sub>O)<sub>2</sub></td></tr><tr><td>D<sub>0</sub></td><td>0.399</td><td>0.440</td><td>0.838</td><td>0.674</td></tr><tr><td>D<sub>1</sub></td><td>0.388</td><td>0.3<sup>b</sup></td><td>0.3<sup>b</sup></td><td>0.3<sup>b</sup></td></tr><tr><td>D<sub>2</sub></td><td>0.369</td><td>0.3<sup>c</sup></td><td>0.3<sup>c</sup></td><td>0.3<sup>c</sup></td></tr><tr><td>D<sub>3</sub></td><td></td><td>0.567</td><td>0.937</td><td>0.773</td></tr></tbody></table>

<footnote>a This work.</footnote>

<footnote>b Klasinc et al. [37].</footnote>

<footnote>c Utsunomiya et al. [36].</footnote>

<footnote>a Reported by Queiroz et al. [25] at the DFT-D B97D3/6-311++G(d,p) level.</footnote>

<footnote>b The optimization does not converge. See Table 4.</footnote>

<footnote>c D₂ state of 1CNN⁺-W₂ II complex converges to D₃ state. See Table 4.</footnote>
The observed threshold for the evaporation of one water molecule from the ionic state of the 1CNN-W₂ II complex (8.592 eV) is in very good agreement with the calculated value (8.595 eV). Considering the good agreement between most of the calculated and experimental values reported in Table 2, the theoretical IP<sub>ad</sub> of the 1CNN-W₂ II complex might be used to determine the binding energy of the water molecule in the D₀ state, which is estimated to be 0.440 eV.

The same theoretical method was used to predict the evaporation energies that couldn't be determined experimentally. The calculated evaporation energies in the D₀ state are shown in Table 3, together with those in the S₀ state for comparison. Also, evaporation energies in different excited states of the neutral and ionic species are reported in Table 3 and will be discussed below.

As a general trend, more energy is needed to remove one water molecule from the ionic ground state (D₀) than from the neutral ground state (S₀) due to the stronger ion-dipole interaction in the D₀ state than the dipole-dipole interaction in the S₀ state, as would be expected.

In all cases involving two water molecules, the evaporation of the water dimer (H₂O)₂ is more favorable than the sequential evaporation of the two molecules, because the energy needed for the evaporation is compensated by the energy of the H-bond between both water molecules.

The evaporation energy of (H₂O)₂ from the S₀ state of the 1CNN-W₂ II complex has been previously calculated by other authors at a similar level to that in this work and is in good agreement with the present report, as shown in Table 3.

### 3.3. Effect of Lyman-α (Ly-α) on 1CNN and 1CNN-W₁,₂ complexes

The IP<sub>ad</sub> of 1CNN has been determined in this work to be 8.432 eV. In addition, the experimental spectra for the D₂← D₀ and D₃← D₀ transitions of 1CNN⁺ have been reported recently [22], and the corresponding band origins were found at 1.69 eV and 2.62 eV, respectively. Therefore, the energy of Ly-α photons (10.2 eV) is enough to ionize 1CNN and produce 1CNN⁺ in the D₀, D₁ and D₂ states.

Considering that the IP<sub>ad</sub> of the 1CNN-W (8.251 eV) and 1CNN-W₂ (8.155 eV) complexes are lower than the IP<sub>ad</sub> of 1CNN, in the interstellar medium (ISM), it would be expected that these complexes could also be produced in the excited electronic state(s) of their radical cations upon absorption of Ly-α radiation (10.2 eV). To test this scenario, the first three excited states (Dₙ) of the ions were optimized. The calculated vertical (E<sub>v</sub>) and adiabatic (E<sub>ad</sub> + ΔZPE) energies for the Dₙ← D₀ transitions of 1CNN⁺, 1CNN-W⁺ II and 1CNN-W₂⁺ II, and their corresponding oscillator strength are reported in Table 4, together with the D₀← Sₙ adiabatic transition energies.

For 1CNN⁺, the calculated values for the D₂← D₀ and D₃← D₀ transitions are 1.707 eV and 2.785 eV, respectively, in very good agreement with the recently reported experimental values of 1.69 eV

and 2.62 eV [22].

According to the results reported in Table 4, the energy of one photon from Ly-α radiation exciting the 1CNN-W II complex is enough to produce the 1CNN-W⁺ II complex in the D₀, D₁ and D₂ states, as was the case for the 1CNN monomer.

In the case of the 1CNN-W₂⁺ II complex, the optimization of the D₁ state was not feasible. In addition, the optimization of the D₂ and D₃ states always led to the optimized D₃ state, suggesting that the D₂ and D₃ states cross and that, in the adiabatic region, the D₃ state lies below the D₂ state. This crossing could favor internal conversion from D₂ to D₃, which is accessible upon one Ly-α photon absorption from the S₀ state.

Additional FCF calculations were performed for those states that could be optimized, and the results are presented in Fig. 4.

The complexity of the spectra increases with the number of water molecules because their presence introduces some flexibility in the complex that leads to geometry changes between the S₀ and Dₙ states and, as a consequence, to a very high FCF activity, which is observed as spectral congestion.

These geometry changes also affect the intensity of the transitions, resulting in an almost negligible intensity for the D₃← S₀ transition of the 1CNN-W₂⁺ II complex (see Fig. 4c).

Upon absorption of one Ly-α photon by 1CNN-W in the S₀, the 1CNN-W⁺ II complex could be produced in the D₁ or D₂ excited states with an excess energy of 1.031 eV or 0.214 eV, respectively. In the case of the 1CNN-W₂⁺ II complex, the absorption of Ly-α radiation by 1CNN-W₂ in the S₀ could produce the ion in the D₃ state with an excess energy of 0.440 eV and if the D₂ state is excited, calculations predict that it will internally convert to the D₃ state. The latter is supported by the convergence of our D₂ optimization to the D₃ state (see Table 4).

In any case, the electronically excited ionic complexes can undergo three competing processes: 1) Internal conversion to a lower electronic state of the ion, 2) fluorescence and 3) water evaporation.

The evaporation of one or more water molecules from the excited electronic states affects the survival lifetime of the complexes in the ISM. Considering the evaporation energies reported in Table 3, this process is energetically allowed only for the D₁ state of the 1CNN-W⁺ II complex. However, the absorption cross-section for the D₁← S₀ transition of the 1CNN-W⁺ II complex at 10.2 eV is negligible (Fig. 4b) and more likely, the complex will be excited to the D₂ state with only 0.214 eV of internal energy. This excess energy is not enough to induce water evaporation, as observed in Table 3.

The internal conversion to lower electronic states of the ion, can also result in the evaporation of water in the final state. For instance, if the 1CNN-W⁺ II complex in the D₂ state undergoes internal conversion to the D₀ or D₁ states, the excess energy in these states (1.949 eV or 1.031 eV, respectively) is enough to overcome the evaporation energy (0.399 eV and 0.388 eV, respectively).

However, the internal conversion competes with radiative processes

<caption>**Table 4**</caption>

Calculated vertical (E<sub>v</sub>), adiabatic (E<sub>ad</sub> + ΔZPE) energies and O.S. (oscillator strength) for the Dₙ← S₀ and Dₙ← D₀ transition of 1CNN⁺, 1CNN-W⁺ and 1CNN-W₂⁺. All energies are given in eV.

<table><thead><tr><th rowspan="2">Transition</th><th colspan="3">1CNN<sup>+</sup></th><th colspan="3">1CNN-W<sup>+</sup> II</th><th colspan="3">1CNN-W<sub>2</sub><sup>+</sup> II</th></tr><tr><th>E<sub>v</sub> (eV)</th><th>E<sub>ad</sub> + ΔZPE (eV)</th><th>O.S.</th><th>E<sub>v</sub> (eV)</th><th>E<sub>ad</sub> + ΔZPE (eV)</th><th>O.S.</th><th>E<sub>v</sub> (eV)</th><th>E<sub>ad</sub> + ΔZPE (eV)</th><th>O.S.</th></tr></thead><tbody><tr><td>D<sub>1</sub>← D<sub>0</sub></td><td>1.235</td><td>0.908</td><td>0.000</td><td>1.249</td><td>0.918</td><td>0.0001</td><td>1.268</td><td>Does not converge</td><td>0.0002</td></tr><tr><td>D<sub>1</sub>← S<sub>0</sub> E<sub>ad</sub> + ΔZPE</td><td>9.326 eV</td><td></td><td></td><td>9.169 eV</td><td></td><td></td><td>–</td><td></td><td></td></tr><tr><td>D<sub>2</sub>← D<sub>0</sub></td><td>1.872</td><td>1.707<br>(1.69 eV)<sup>a</sup></td><td>0.067</td><td>1.916</td><td>1.735</td><td>0.062</td><td>1.871</td><td>Converges to D<sub>3</sub></td><td>0.053</td></tr><tr><td>D<sub>2</sub>← S<sub>0</sub> E<sub>ad</sub> + ΔZPE</td><td>10.124 eV</td><td>(10.12 eV)<sup>b</sup></td><td></td><td>9.986 eV</td><td></td><td></td><td>–</td><td></td><td></td></tr><tr><td>D<sub>3</sub>← D<sub>0</sub></td><td>3.062</td><td>2.785<br>(2.62 eV)<sup>a</sup></td><td>0.003</td><td>2.888</td><td>2.121</td><td>0.008</td><td>2.493</td><td>1.607</td><td>0.017</td></tr><tr><td>D<sub>3</sub>← S<sub>0</sub> E<sub>ad</sub> + ΔZPE</td><td>11.202 eV</td><td>(11.05 eV)<sup>b</sup></td><td></td><td>10.372 eV</td><td></td><td></td><td>9.762 eV</td><td></td><td></td></tr><tr><td>D<sub>4</sub>← D<sub>0</sub></td><td>–</td><td></td><td></td><td>–</td><td></td><td></td><td>3.091</td><td>2.723</td><td>0.020</td></tr><tr><td>D<sub>4</sub>← S<sub>0</sub> E<sub>ad</sub> + ΔZPE</td><td>–</td><td></td><td></td><td>–</td><td></td><td></td><td>10.877 eV</td><td></td><td></td></tr></tbody></table>

<footnote>a Experimental value from Daly et al. [22].</footnote>

<footnote>b Experimental value combining experimental IP<sub>ad</sub> from this work and electronic transitions from ref. [22].</footnote>
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Simulated spectra for different $D_n$—$S_0$ transitions of 1CNN (a. left), 1CNN-W II (b. middle) and 1CNN-W$_2$ II (c. right). The vertical dashed lines in each panel indicate the position of the Ly-α line.</figcaption></figure>

(fluorescence or recurrent fluorescence) and under the extreme conditions of the ISM, radiative cooling can play an important role in stabilizing these complexes [21,38,39].

It was recently reported that vibrationally excited 1CNN⁺ is efficiently stabilized by recurrent fluorescence (RF) [21]. This process involves the inverse internal conversion from highly vibrationally excited states of the ground state to an electronically excited state, followed by fluorescence. This RF takes place on the µs-ms timescale, which is longer than the timescale for a normal fluorescence mechanism. However, it is relevant for isolated molecules with long lifetimes, such as those in the ISM, and thus, RF becomes an effective radiative cooling mechanism in such environments.

Therefore, we expect that RF is also an operative mechanism in the water complexes of 1CNN⁺ studied in this work, as it is for the bare chromophore, and it could allow them to survive Ly-α radiation, if they were present in the ISM.

From these data, we tentatively extrapolate that once a 1CNN molecule attaches one or more water molecules, it is likely to be stable (in its radical cation form) on timescales relevant to the ISM. It would be interesting to perform dynamics studies to further explore this hypothesis.

## 4. Conclusions

In this work, we carried out an experimental and theoretical study of the REMPI spectroscopy and of the ionization and water evaporation thresholds of 1CNN and its complexes with one and two water molecules.

Although several authors had previously reported the REMPI spectroscopy, the redetermination in this work allows comparison of the experimental results with theoretical calculations, with the aim of validating the theoretical method used to predict other properties that have not been experimentally determined yet.

The $O_0^0$ transition of each species was chosen to selectively excite them to determine the ionization and water evaporation thresholds. In the case of the bare 1CNN molecule, the adiabatic ionization potential was determined for the first time (8.432 eV) in good agreement with the calculated value (8.417 eV), while the experimental vertical ionization potential (8.603 eV) is in agreement with previous experimental determinations (8.61 and 8.59 eV [36,37]). In the case of the 1CNN-W and 1CNN-W$_2$ complexes, the experimental ionization threshold is closer to the calculated vertical ionization potential than to the adiabatic one.

The evaporation of one water molecule was observed in the ionic

state of the 1CNN-W$_2$ complex and the evaporation threshold (8.591 eV) satisfactorily agrees with the theoretical value (8.595 eV). The evaporation of two water molecules or one water molecule upon ionization of the 1CNN-W$_2$ and 1CNN-W complex, respectively, was not observed, highlighting the stability of the 1CNN-W⁺ cationic complex.

The photoionization spectra of 1CNN, 1CNN-W and 1CNN-W$_2$ to different electronic states of the cations ($D_0$, $D_1$, $D_2$, $D_3$) were calculated theoretically. Based on the available data we determine that these complexes may be ionized and electronically excited by Ly-α irradiation, but that they will be stable in their cationic states. This remains to be studied in the context of competing radiative and non-radiative processes, including recurrent fluorescence (RF).

We suggest that for a more definitive evaluation of the interstellar relevance of these systems, at least a theoretical study on the excited state dynamics of the cations, including the competing relaxation pathways (water evaporation, fluorescence, RF, etc.) should be performed and we encourage theoretical groups to carry it out.

### CRediT authorship contribution statement

**Ana Niedojadlo:** Writing – original draft, Visualization, Validation, Investigation, Formal analysis. **Alejandro Gutierrez Quintanilla:** Writing – review & editing, Supervision, Methodology, Investigation, Formal analysis. **Jordan Dezalay:** Writing – review & editing, Investigation, Formal analysis. **Isabelle Couturier-Tamburelli:** Writing – review & editing, Supervision, Investigation, Conceptualization. **Juan Cruz Latorre:** Investigation. **Melottayil V. Vinitha:** Investigation. **Anaïs Massaloux:** Investigation, Formal analysis. **Gustavo A. Pino:** Writing – original draft, Validation, Supervision, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. **Jennifer A. Noble:** Writing – original draft, Validation, Supervision, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.

### Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Jennifer A. Noble - Ana Niedojadlo report financial support was provided by Institut Origines - Aix Marseille University, A-MIDEX. Jennifer A. Noble - Gustavo A. Pino report financial support was provided by ECOS-Sud - MinCyT. Jennifer A. Noble reports financial support was provided by French National Research Agency. Gustavo A. Pino reports
financial support was provided by LIA - LEMIR. Gustavo A. Pino reports financial support was provided by FONCyT - SeCyT. Other authors declare no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

The project was carried out in the context of the ECOS Sud - MinCyT A22U01 program and the LIA-LEMIR (CNRS-CONICET) and was supported by CONICET, FONCyT (PICT2019 N° 241 and PICT2021 Aplicación N° 006), SeCyT-UNC, MinCyt-Córdoba, and ANR Research Grants ANR2010BLANC040501-ESPEM and ANR17CE05000502-Wsplit.

JAN and MVV acknowledge funding from the Agence Nationale de la Recherche (ANR, HYDRAE project ANR-21-CE30-0004-01). JAN and AN acknowledge the Origins Institute; the project leading to this publication has received funding from Excellence Initiative of Aix-Marseille University - A*MIDEX, a French "Investissements d'Avenir" programme (AMX-21-IET-018, Origines). This work used computational resources from UNC Supercómputo (CCAD), which is part of SNCAD, Argentina. The authors acknowledge access to the computing centre MésoLUM managed by ISMO (UMR8214) and LPGP (UMR8578), University Paris-Saclay (France).

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jms.2026.112078.

## Data availability

Data will be made available on request.

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