Beyond Idealized PAHs: Infrared Signatures of Carbon-Chain Defects from Shock Synthesis

Abstract: Polycyclic aromatic hydrocarbons (PAHs) are widely recognized as carriers of the aromatic infrared bands (AIBs). However, most spectral models rely on idealized structures that fail to capture the energetic environments of interstellar PAH formation. This work investigates the infrared (IR) signatures of PAHs formed under shock conditions and explores whether produced defective structures can explain observational features unpredicted by standard, idealized models. We combine two-stage reactive molecular dynamics simulations of PAH formation via condensation and shock processing with density functional theory spectral calculations, and compare our theoretical results with James Webb Space Telescope (JWST) observations of NGC 7023 and MRK 1066. Shock processing produces PAHs featuring fullerene-like carbon skeletons and linear carbon-chain attachments. These structural defects yield distinct IR signatures, including prominent carbon-chain stretching features at 4.6-5.5 micron that is absent in ideal PAHs, and significantly enhanced out-of-plane skeletal modes in the 14.5-20 micron regime. Our findings attribute the observed 5.2 micron band in NGC 7023 and MRK 1066 to carbon-chain vibrations and the 15-18 micron emission to curved skeletal modes, providing observational support for the prevalence of defective, shock-formed PAHs in the interstellar medium.

• The paper induces that shock-driven synthesis forms carbon-chain defects in PAHs.
• The paper identifies new infrared signatures: 4.6–5.5µm carbon-chain band (from $sp$ chains) and a 14.5–20µm out-of-plane skeletal emission from curved, defect-rich PAHs.
• Reexamines standard diagnostic infrared signatures. Concludes the cursing structure from shock processing has stronger emission in the infrared range than planar PAHs.

Motivation and scope

Most models of the aromatic infrared bands (AIBs) rest on an implicit assumption: that interstellar polycyclic aromatic hydrocarbons (PAHs) are structurally close to ideal benzenoid molecules. Even when databases such as the NASA Ames PAH IR Spectroscopic Database (PAHdb) include modified species, the perturbations considered — 5–7 ring pairs, heteroatom substitution, functional groups, vacancies — leave the molecular scaffold essentially intact. Tan et al. argue this assumption is untenable for PAHs formed under realistic conditions, since formation occurs through energetic processing in shocks, photodissociation regions (PDRs), and outflows that intrinsically produce irregular architectures. The paper addresses this gap by combining two-stage reactive molecular dynamics (MD) simulations of PAH formation with density functional theory (DFT) spectral calculations, and confronts the resulting spectra with JWST/MIRI-MRS observations of NGC 7023 and Mrk 1066.

Two-stage simulation of shock-driven synthesis

The workflow follows a commonly invoked two-step mechanism. In Stage 1, C and H atoms condense onto a graphene surface at 50 K over 5000 ps, at H/C ratios from 0.0 to 0.8, using the AIREBO reactive force field. This produces kinetically trapped, irregular hydrocarbon clusters containing carbon chains, pendant aliphatic groups, and $sp^3$ carbons. Hydrogen-poor conditions favor extended ring closure; hydrogen-rich conditions terminate dangling bonds and favor open chains.

In Stage 2, precursors are heated to 3500 K — above the physical 1000–3000 K range of C-type shocks, adopted to compress reaction timescales into the simulation window — using the ReaxFF force field, which the authors prefer over AIREBO because it does not artificially bias toward planar $sp^2$ structures. The scale-bridging strategy is quantitatively justified via collision kinetics: for carbon-chain cyclization ($E_a = 23$ kJ/mol), the half-life at realistic shock densities spans $10^3$–$10^6$ yr, but drops to roughly 3 ns at the simulated density of $n \sim 2.5\times10^{19}$ cm$^{-3}$ and 3500 K, matching the MD timescale. The authors candidly note the caveat: high density suppresses radiative cooling and may inflate the yield of unclosed chains and non-hexagonal defects relative to low-density shocks, although kinetically trapped configurations remain physically accessible local minima.

The products are neither planar PAHs nor closed fullerenes: they exhibit curved, warped skeletons with interspersed 5-6-7 ring motifs and short chain appendages. Hydrogen-poor runs trend toward fullerene-like closure; hydrogen-rich runs yield open curved structures passivated by residual hydrogen. This intermediate morphological regime has received little attention in astronomical spectral modeling.

Diagnostic infrared signatures

DFT harmonic spectra (B3LYP/4-31G with Grimme D3 dispersion and Bauschlicher scaling factors) were computed for 48 products and 41 precursors in neutral, anion, and cation states, benchmarked against 735 hydrocarbon structures in PAHdb v4.0 (mean $N_C \sim 77$ versus $\sim 84$ for the products). Three findings stand out:

Reduced charge sensitivity: unlike canonical planar PAHs, whose ionized spectra show dramatic C–C stretching enhancements driven by Jahn-Teller-like distortions, the shock-formed products show weak dependence on charge state across comparable sizes. The authors attribute this to isotropic $\pi$ delocalization in 3D curved skeletons and to broken symmetry producing large intrinsic dipole moments even when neutral. If confirmed observationally, charge-state diagnostics calibrated on planar PAHs would be systematically biased in regions dominated by defective carriers.

A 4.6–5.5 µm carbon-chain band: normal-mode analysis assigns features here to stretching vibrations of linear $sp^2$0-hybridized chains attached to the PAH periphery — modes with virtually no counterpart in ideal PAHs. These chains persist as kinetic traps even after high-temperature annealing. The region carries observational confounders (CO ice at 4.68 µm, OCN$sp^2$1 at 4.62 µm, PAH overtones), which the paper acknowledges.

Enhanced 14.5–20 µm skeletal emission: where planar symmetric PAHs show only very weak out-of-plane activity, the curved products display strong, broad emission from out-of-plane puckering modes activated by dipole-moment changes. This parallels the physics of the known C$sp^2$2 bands at 17.4 and 18.9 µm, suggesting a shared curvature-based origin. Test calculations with B3LYP/6-31G(d,p) on curved structures give post-scaling wavelength shifts of 0.104–0.189 µm and intensity differences of 0.53%–3.68%, supporting the reliability of the smaller basis set, though the authors concede that missing polarization functions introduce some uncertainty for non-planar geometries.

Confrontation with JWST observations

The paper presents JWST/MIRI-MRS spectra of NGC 7023 (ten 0.5″ apertures across the northwest PDR) and Mrk 1066 (six apertures along the AGN-driven radio jet). Both sources show a distinct emission peak near 5.2 µm. The authors explicitly challenge the standard CH overtone/combination assignment of Boersma et al., arguing that second-order processes should be intrinsically weak and spectrally broad, whereas the observed feature is sharp and relatively intense; fundamental carbon-chain stretching requires no anharmonic intensity borrowing. This is a direct contradiction of prior interpretation, and one that rests partly on qualitative profile arguments rather than quantitative band modeling — a point a reader should weigh.

In NGC 7023, the broad 15–18 µm envelope (part of the 17 µm complex with sub-components at 16.4 and 17.4 µm) persists across all apertures. While the 17.4 µm component is conventionally attributed to neutral C$sp^2$3, the authors propose that curved defect-rich networks contribute substantially to the 15–18 µm excess via enhanced puckering modes. The spatially extended nature of both features supports a distributed carrier population consistent with an intermediate stage in photochemical evolution toward fullerenes. In Mrk 1066, the 5.25 µm feature appears along the outflow — plausibly indicating shock synthesis — but the 15–20 µm continuum is comparatively weak, which the authors attribute to destruction or ionization of large curved structures in the harsher AGN radiation field; a detailed analysis is deferred to a follow-up paper.

The authors also address carrier survival: bare $sp^2$4-chains photodissociate readily, but attachment to a large aromatic scaffold enables internal conversion and energy redistribution, while photolytic fragmentation and grain resupply can maintain a steady-state population of chain-bearing PAHs in PDRs like NGC 7023.

Limitations and open questions

Several limitations bear directly on the strength of the conclusions. The graphene dust surrogate neglects chemisorption and grain-composition diversity. The elevated simulation temperature and compressed density, though kinetically justified, may overproduce unclosed chains and topological defects; the absolute defect yields should therefore be treated as upper limits. The DFT level (B3LYP/4-31G) lacks polarization functions, a recognized concern for non-planar structures despite the benchmark tests reported. Most importantly, the observational case rests on two sources, and the 15–18 µm attribution cannot yet disentangle defective PAHs from fullerenes within the blended 17 µm complex; the paper itself concedes that decoupling these contributions requires a broader spectroscopic survey. Whether the 5.2 µm feature's profile and strength uniquely select carbon chains over alternative carriers also remains untested against detailed excitation and radiative-transfer modeling.

Conclusion

This work demonstrates computationally that shock-driven synthesis naturally produces PAHs with curved, fullerene-like skeletons and peripheral carbon chains whose IR spectra contain diagnostic bands absent from idealized templates: a 4.6–5.5 µm carbon-chain stretching window and strongly enhanced 14.5–20 µm out-of-plane skeletal emission. Matching JWST detections near 5.2 µm and 15–18 µm in NGC 7023 and Mrk 1066 provide observational support for a population of defective, shock-formed PAHs, and imply that structural parameters inferred with perfect-molecule templates may carry systematic biases wherever such populations dominate. Establishing the prevalence of these carriers, and their contribution relative to fullerenes in the 17 µm complex, remains an open observational problem.