Mid‐infrared Spectroscopy of Protonated Benzonitrile, 2‐Cyanonaphthalene, and 9‐Cyanoanthracene for Astrochemical Consideration

Abstract

The mid‐infrared (700–2300 cm⁻¹) absorption spectra of three protonated cyanosubstituted polycyclic aromatic hydrocarbons (CN‐PAHs), benzonitrile, 2‐cyanonaphthalene, and 9‐cyanoanthracene, measured by infrared multiple photon dissociation using the free‐electron laser Free‐Electron Laser for Infrared eXperiments are reported. The frequency of the CN stretch is found to be 2191 ± 10 cm⁻¹ for protonated benzonitrile and 2175 ± 10, and 2140 ± 10 cm⁻¹ for 2‐cyanonaphthalene, and 9‐cyanoanthracene respectively, showing a clear redshifting of the CN stretch frequency as the size of the aromatic system increases, contrary to neutral cyano‐PAHs that show nearly no shift as function of size. Quantum chemical calculations are performed to complement the experimental results at B3LYP/aug‐cc‐pVTZ level of theory. Density functional theory calculations reproduce the fingerprint region for all three CN‐PAHs, although they overestimate the CN stretching vibration frequency. These results likely rule out small protonated cyano‐PAHs as major contributors to the unidentified infrared bands observed in space. However, the largest species investigated in this study shows a promising match with the 4.75 μm band.

Experimental infrared absorption spectra of three protonated cyano‐substituted PAHs measured by infrared multiple photon dissociation using the free‐electron laser Free‐Electron Laser for Infrared eXperiments are studied. These spectra are compared to an emission spectrum measured by JWST, and the largest measured protonated cyano‐PAH, cyanoanthracene, shows promising match with the 4.75 μm band.

1. Introduction

Polycyclic aromatic hydrocarbons (PAHs) are a class of highly stable organic molecules consisting of two or more fused aromatic rings containing only carbon and hydrogen. They are of notable interest in a variety of fields ranging from combustion and environmental pollution^{[ 1} , ^{2 ]} on Earth to astrochemistry^{[ 3 ]} in the interstellar medium (ISM).

PAHs have long been considered ubiquitous in the ISM, accounting for ≈10–25% of interstellar carbon,^{[ 3} , ⁴ , ⁵ , ^{6 ]} but it is only recently that some individual PAHs have been positively identified in space. Of these detected PAHs only one is a pure PAH, namely indene,^{[ 7} , ^{8 ]} while the rest are cyano‐substituted PAHs (CN‐PAHs), where a cyanogroup replaces one of the peripheral hydrogens. The CN‐PAHs found so far include 1‐ and 2‐cyanonaphthalene,^{[ 9 ]} 2‐cyano‐indene,^{[ 10 ]} 1‐ and 5‐cyanoacenaphthylene,^{[ 11 ]} 1‐, 2‐ and 4‐cyanopyrene,^{[ 12} , ^{13 ]} and most recently cyanocoronene.^{[ 14 ]} Single‐ring aromatic molecules, including benzonitrile, have similarly been discovered in the interstellar medium.^{[ 15 ]} These CN‐PAHs have been detected in the cold Taurus Molecular Cloud (TMC‐1). Their detection was possible due to their notable dipole moment produced by the CN group,^{[ 16 ]} which enables detection of their rotational lines via radio astronomy. Pure PAHs generally lack a permanent dipole and therefore cannot be detected using radio telescopes.

Since these CN‐PAHs have been detected in the radio frequency range, it stands to reason that they could also be detected across other wavelengths. Such other wavelengths include the infrared (IR), where new high‐resolution data are available thanks to the James Webb Space Telescope (JWST). PAHs are believed to be the carriers of the IR emission features of the ISM, ranging from 3 to 20 μm, because of their characteristic vibrational frequencies. These bands are referred to as the unidentified infrared emission bands or the aromatic infrared bands (AIBs).^{[ 17} , ^{18 ]} Identification of specific carriers is difficult because the most prominent AIBs at 3.3, 6.2, 7.7, 8.6, and 11.2 μm are broad unresolved profiles associated with vibrational modes shared among many PAHs. Substituted PAHs, such as nitrogen‐containing PAHs (PANHs), oxygen‐functionalized PAHs (OPAHs), or CN‐PAHs, have been suggested as possible carriers of the AIBs.^{[ 18} , ¹⁹ , ^{20 ]} Protonated PAHs have also been suggested as possible carriers of the AIBs and could play an important role in the chemistry of the ISM.^{[ 21 ]} Of the about 330 molecules currently detected in the ISM, about 10% are cations, and a majority of them are protonated species.^{[ 22 ]} The proton is believed to originate from ${\mathrm{H}}_3^{+}$,^{[ 23 ]} which is a strong proton donor and has been detected in high abundance in both diffuse interstellar gas^{[ 24 ]} and dense molecular clouds.^{[ 25 ]}

Laboratory measurements of the spectroscopy of PAHs and PAH‐derivatives like CN‐PAHs provide important tools for understanding the presence and role of PAHs in the ISM. The spectroscopy of PAHs has been explored for the past few decades, and with the detection of CN‐PAHs in space, there has been more focus on obtaining spectroscopic reference data on these cyanospecies. This has been done both using experimental methods, such as messenger‐tag spectroscopy in cryogenic traps^{[ 26} , ²⁷ , ^{28 ]} and computational methods focusing on neutral CN‐PAHs.^{[ 28} , ²⁹ , ³⁰ , ^{31 ]}

Here, we report the IR spectra of three protonated cyanospecies, namely protonated benzonitrile (BZN, C₆H₅CNH⁺), 2‐cyanonaphthalene (2‐CNN, C₁₀H₇CNH⁺), and 9‐cyanoanthracene (9‐CNA, C₁₄H₉CNH⁺) obtained by infrared multiple photon dissociation (IRMPD) using the free‐electron laser Free‐Electron Laser for Infrared eXperiments (FELIX). Density functional theory (DFT) calculations were performed at B3LYP/aug‐cc‐pVTZ level of theory.

2. Results and Discussion

IR spectra of protonated BZN, 2‐CNN, and 9‐CNA are shown in black in Figure  1 . The vibrational frequencies are calculated using the Gaussian16^{[ 32 ]} program package at the harmonic B3LYP/aug‐cc‐pVTZ^{[ 33 ]} level of theory and scaled by a factor of 0.965^{[ 34 ]} and are shown as red sticks in Figure 1. Optimized structures and their vibrational frequencies are provided in the Supplementary Information. The predicted spectra shown in red are obtained by convoluting the stick spectra with a Gaussian line shape with a full‐width at half‐maximum (FWHM) of 35 cm⁻¹.

Figure 1.

IR action spectra of protonated a) BZN, b) 2‐CNN, c) 9‐CNA recorded at FELIX (black line). The red sticks are the vibrational frequencies calculated at the harmonic B3LYP/aug‐cc‐pVTZ level of theory and the red colored area shows calculated spectra convoluted with a Gaussian lineshape (FWHM: 35 cm⁻¹).

In the fingerprint region, we see absorption around 750 cm⁻¹ and between 1100–1600 cm⁻¹ for all three CN‐PAHs. The 745 cm⁻¹ mode for protonated BZN is a ring breathing mode, and the central ring breathing mode of protonated 9‐CNA is the strong peak at 1250 cm⁻¹. The 1100–1600 cm⁻¹ region is generally crowded by CC stretching and CH in‐plane bending modes, which become more numerous as the molecule grows bigger. The positions of the calculated peaks in the fingerprint region agree well with the measured spectra for all three cyanospecies, but the relative band intensities do not agree as well as the positions.

All three CN‐PAHs show a strong CN stretch between 2100 and 2200 cm⁻¹. The CN‐stretch lies at 2195 ± 10 cm⁻¹ (4.56 μm) for BZN and redshifts as the CN‐PAHs get larger to 2175 ± 10 cm⁻¹ (4.60 μm) for 2‐CNN and 2140 ± 10 cm⁻¹ (4.75 μm) for 9‐CNA. This stands in contrast to the neutral CN‐PAHs, where anharmonic quantum chemical computations on a range of different small neutral CN‐PAHs showed that the position of the CN stretch did not depend on size.^{[ 35 ]} Figure 1 shows that the computed frequency of the CN stretch deviates more from the experimental position than other bands in the fingerprint range. This is not surprising, as it has been seen before that DFT overestimates the CN stretch for CN‐PAHs.^{[ 28} , ^{36 ]} In the current study, the CN stretch is overestimated by about 45 cm⁻¹ for protonated BZN and about 35 cm⁻¹ for 2‐CNN. However, the position of the calculated CN stretch of the protonated 9‐CNA fits surprisingly well with the experiment. Table  1 shows the position of the CN stretch for BZN, 1‐CNN, 2‐CNN, and 9‐CNA in the protonated, radical cation, and neutral charge state, for which experimental values are available. The CN stretch of the protonated BZN is blueshifted compared to the radical cation, and the CN stretch for all three protonated CN‐PAHs is redshifted compared to the neutral species, as the extra proton increases the reduced mass of the oscillator.

Table 1.

Position of the CN stretch [cm⁻¹].

	Protonated	Radical cation	Neutral
BZN	2195a)	2130b)	2229d)
1‐CNN		2215c)	2220e)
2‐CNN	2175a)		2226e)
9‐CNA	2104a)		2207f)

^a)This work;

^b)Ref. [27];

^c)Ref. [26];

^d)Ref. [47];

^e)Ref. [36];

^f)Ref. [28].

The 4.3–4.8 μm region is perhaps one of the least well‐understood parts of the interstellar IR emission spectrum. This region is believed to arise from C–D stretching modes in deuterated PAHs^{[ 37} , ^{38 ]} and/or CN stretching modes in nitrile‐containing species.^{[ 39} , ^{40 ]} With the recent detections of CN‐substituted PAHs in TMC‐1, the nitrile hypothesis has regained interest, particularly because the D/H ratio required to attribute the observed IR features to CD stretching modes appears higher than what is expected. However, neutral CN‐PAHs^{[ 41} , ^{42 ]} and small radical cations^{[ 26} , ^{27 ]} do not seem to account for a significant portion of the IR emission in this spectral region. One particularly intriguing feature of this region is the emission band at 4.75 μm, recently reobserved with the JWST in the Orion Bar.^{[ 43 ]} In Figure  2 , we compare the spectrum from Dissociation Front 1 (DF1) of the Orion region where the 4.5 μm band is expected to be strongest, with our experimental IR spectra of protonated CN‐PAHs. It is evident that this family of species cannot account for the absorption feature at 4.5 μm or the strong emission at 4.64 μm. However, there is fairly good agreement between the 4.75 μm band and the spectrum of protonated 9‐CNA. Although it is unlikely that this single species is solely responsible for the emission, it would be interesting to investigate whether the redshift of the vibrational stretching mode in protonated CN‐PAHs converges as the size of the aromatic system increases. If so, a cumulative effect involving a range of large protonated CN‐PAHs could potentially account for the 4.75 μm band.

Figure 2.

IR action spectra of protonated BZN, 2‐CNN, and 9‐CNA in the region around the CN stretch compared to the emission spectrum measured by JWST in the DF1 in the Orion Bar (right y‐axis).^{[ 43 ]}

3. Conclusion

Mid‐IR spectra in the range of 700–2300 cm⁻¹ are presented for the protonated BZN, 2‐CNN, and 9‐CNA obtained by IRMPD spectroscopy using the free‐electron laser FELIX. The frequency of the CN stretch was found to redshift for larger CN‐PAHs, thereby showing a clear size dependency. The CN stretch was found to be at 2195 ± 10 cm⁻¹ for the protonated BZN, 2175 ± 10 cm⁻¹ for 2‐CNN, and 2104 ± 10 cm⁻¹ for 9‐CNA. The experimental spectra are compared to vibrational frequencies calculated by DFT at the B3LYP/aug‐cc‐pVTZ level of theory. The position of the calculated peaks agrees well with the experiment in the fingerprint region for all three cyanospecies. However, DFT overestimates the frequency of the CN stretch for both BZN and 2‐CNN, but predicts the frequency of the 9‐CNA quite well. Small protonated CN‐PAHs are unlikely to contribute significantly to the aromatic infrared bands. However, the largest species investigated in this study shows a promising match with the 4.75 μm band. This finding calls for further investigation of even larger species, to determine whether the CN stretching mode becomes size independent beyond a certain size of the aromatic system. This behavior could lead to an accumulation of intensity, which could explain the observed emission at 4.75 μm.

4. Experimental Section

The gas‐phase IRMPD spectra of three protonated cyanosubstituted PAHs (BZN, 2‐CNN, and 9‐CNA) were recorded using a modified 3D quadrupole ion trap (QIT, Bruker amaZon Speed ETD) coupled to FELIX.^{[ 44 ]} Each neutral CN‐PAH precursor was dissolved at a concentration of 100 μm in methanol with 1% of formic acid and introduced into an electrospray ionization source to generate the corresponding protonated ions. To record IR spectra, we isolate the ions at the nominal masses of interest, m/z 104, 154, and 204, corresponding to protonated BZN, 2‐CNN, and 9‐CNA, respectively. The trapped ion cloud is then irradiated with tunable IR laser light from FELIX. IR‐induced dissociation of the parent ions primarily results in the loss of HCN, yielding arylium ions that are highly reactive toward water, as previously observed in similar experiments on vacuum ultraviolet photodissociation.^{[ 45} , ^{46 ]} Minor additional dissociation channels were also observed. IR spectra of the parent ions are obtained by plotting the IRMPD yield as a function of IR frequency. The IRMPD yield is defined as the ratio of the sum of all fragment ion intensities over the sum of all ion intensities (fragments + parent). The yield is linearly corrected for frequency‐dependent variations in laser pulse energy. The light from FELIX was optimized in two ranges: 700–1900 cm⁻¹ for the fingerprint region and 1900–2300 cm⁻¹ for the CN stretch, and the spectra for each range were matched together. The relative intensities between these ranges are therefore not completely reliable, but the position of the peaks is.

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supplementary Material

Rasmussen Anne P., Rossi Corentin, Gans Bérenger, Finazzi Laura, Oomens Jos, Berden Giel, Jacovella Ugo, ChemPhysChem, 2025, 26, e202500642, 10.1002/cphc.202500642

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.