Rotational Spectra and Search for Aromatic Imines: 9-Iminofluorene and Benzophenone imine

Interstellar detections of several cyano derivatives of large polycyclic aromatic hydrocarbons (PAHs) have now been achieved, enabled by accurate laboratory measurements of their microwave rotational spectra. These results highlight the continued promise of other N-containing unsaturated PAHs, such as aromatic imines, as candidates for future laboratory studies and astronomical searches. In this work, we present broadband spectroscopic measurements of 9-iminofluorene and benzophenone imine in the 6-18,GHz band. These measurements yield accurate rotational, centrifugal distortion, and $^{14}$N quadrupole coupling constants for both molecules. Using these experimentally-derived constants, we attempted a search for both molecules in the cold molecular cloud TMC-1 using observations from the Green Bank Telescope (GBT). Neither of these two ketimines was detected above the current noise level, establishing upper limits for their column densities of $5.1\times10^{12},\text{cm}^{-2}$ for 9-iminofluorene and $1.3\times10^{13},\text{cm}^{-2}$ for benzophenone imine. We also attempted a search for phenylmethanimine (both E/Z isomers) as the simplest aromatic aldimine, but neither was detected in TMC-1. To provide insight into these non-detections, we propose and evaluate different formation pathways using respective potential energy surfaces as determined by high-precision quantum chemical calculations. The result suggests the presence of an entrance barrier to forming the intermediate species, potentially explaining the low abundance.

💡 Research Summary

This paper reports the first broadband microwave rotational spectroscopy of two aromatic imines—9‑iminofluorene and benzophenone imine—performed in the 6–18 GHz range using a compact chirped‑pulse Fourier‑transform microwave (CP‑FTMW) spectrometer. The authors recorded 688 distinct transitions for 9‑iminofluorene (up to J = 21, K_a = 10) and 455 transitions for benzophenone imine (up to J = 27, K_a = 11), with 52 and 100 hyperfine‑resolved groups, respectively, allowing precise determination of the rotational constants (A, B, C), centrifugal distortion parameters (Δ_J, Δ_JK, Δ_K, δ_J, δ_K), and ^14N quadrupole coupling constants (χ_aa, χ_bb). The experimentally derived constants agree within 0.1 % with high‑level quantum‑chemical predictions at the B3LYP‑D3(BJ)/6‑311++G(d,p) level, confirming the reliability of the measurements. Dipole moments were measured as μ_a = 1.3 D, μ_b = 2.0 D for 9‑iminofluorene and μ_b = 2.1 D for benzophenone imine, values that are modest compared with cyano‑substituted PAHs (∼3–5 D) but sufficient for astronomical searches.

To assess the astrochemical relevance, the authors performed quantum‑chemical investigations of two plausible gas‑phase formation pathways, both initiated by the reaction of benzonitrile with a phenyl radical. One pathway proceeds through benzophenone imine as an intermediate, the other through 2‑cyanobiphenyl. Geometry optimizations and anharmonic frequency calculations were carried out with B3LYP‑D3, while the reaction energetics were refined using M06‑2X‑D3/def2‑TZVPP for the potential energy surface and DLPNO‑CCSD(T)/aug‑cc‑pVTZ for single‑point energies. Both routes exhibit entrance barriers of roughly 15–20 kJ mol⁻¹ (≈ 1800–2400 K) and overall free‑energy barriers exceeding 30 kJ mol⁻¹ at 10 K, rendering them essentially inactive under cold molecular cloud conditions. Transition‑state theory with Eckart tunnelling corrections yields rate coefficients far below 10⁻¹⁵ cm³ s⁻¹ at 10 K, confirming that the formation of these imines is kinetically suppressed.

Armed with the precise laboratory frequencies, the authors searched for the two imines in the well‑studied dark cloud TMC‑1 using archival Green Bank Telescope (GBT) data covering the 8–10 GHz band. Synthetic spectra assuming typical excitation temperatures (∼10 K) and line widths (0.5 km s⁻¹) predict line intensities that would be detectable at the achieved rms noise (≈ 2 mK). No emission was found at the expected frequencies. Consequently, 3σ upper limits on column densities were derived: N ≤ 5.1 × 10¹² cm⁻² for 9‑iminofluorene and N ≤ 1.3 × 10¹³ cm⁻² for benzophenone imine. The authors also examined the simplest aromatic aldimine, phenylmethanimine (E/Z), and similarly report non‑detections with comparable upper limits.

The discussion interprets the non‑detections in light of the calculated kinetic barriers, the relatively low dipole moments (compared with cyano‑PAHs), and possible competing loss processes such as hydrolysis to the corresponding ketones, which were observed as side products in the laboratory spectra. The authors argue that the entrance barriers identified in the quantum‑chemical study likely prevent efficient gas‑phase synthesis of these imines in cold clouds, explaining their low abundances. They suggest that alternative formation routes—e.g., grain‑surface chemistry, UV‑induced processes, or high‑temperature shock chemistry—might be required to populate these species in detectable amounts.

In summary, the work delivers (1) a complete set of high‑precision rotational spectroscopic parameters for two aromatic imines, (2) stringent astronomical upper limits on their abundances in TMC‑1, and (3) a mechanistic explanation for their scarcity based on quantum‑chemical reaction barriers. These results provide essential reference data for future laboratory and observational campaigns targeting nitrogen‑bearing PAH derivatives, and they highlight the importance of considering kinetic bottlenecks when evaluating the astrochemical viability of new molecular families.