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

TL;DR

This work reports the first microwave rotational spectra of two aromatic imines, 9-iminofluorene and benzophenone imine, yielding precise rotational, distortion, and 14N quadrupole constants to guide astronomical searches. Using CP-FTMW spectroscopy (6–18 GHz) and high-level quantum chemistry (B3LYP-D3(BJ)/6-311++G(d,p); M06-2X-D3/def2TZVPP; DLPNO-CCSD(T)/aug-cc-pVTZ), the authors characterize the laboratory spectra and assess plausible formation pathways via PES calculations. The interstellar search toward TMC-1 with GOTHAM/GBT data finds no detections for either ketimine or the simple phenylmethanimine isomers, and places stringent upper limits on their column densities; kinetic inhibition due to barriers in the proposed formation mechanisms provides a compelling explanation. Collectively, the results deliver essential spectroscopic parameters for future searches and highlight how reaction kinetics at 7 K in cold clouds shapes the presence of aromatic imines, informing astrochemical models and guiding observational strategy.

Abstract

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.

Figures (5)

• Figure 1: The different views of B3LYP-D3(BJ)/6-311++G(d,p) level optimized geometric structures for 9-iminofluorene (left) and benzophenone imine (right). The arrow shows C9 position of iminofluorene.
• Figure 2: The proposed two possible formation routes for 9-iminofluorene, while two important intermediates are benzophenone imine and 2-cyanobiphenyl.
• Figure 3: Parts of experimental spectra (black) overlaid with simulated fitting spectra (blue for imines, red for ketones) for (a) 9-Iminofluorene/Fluorenone and (b) Benzophenone imine/Benzophenone. The insets show zoomed-in views to highlight the resolved hyperfine splitting. For instance, in Figure a, the hyperfine components of $J_{K_a, K_c} = 4_{2,2} - 3_{1,3}$ transition from 9-iminofluorene is presented, with the changes in the fourth quantum number resulting from $^{14}$N quadrupole coupling clearly labeled. For the simulation lines, it has arbitrary units, and the intensity is scaled to simulate the experimental lines. The grey dotted line in the background marked the frequency as multiples of 250 MHz, and all strong lines at these frequencies could come from the clock as a reference in the spectrometer.
• Figure 4: Possible pathways to form 9-iminofluorene from benzonitrile that are discussed in this work. Apart from the column density of benzonitrile reported by DR5.3, all other upper limits are reported in this work using the same derivation method.
• Figure 5: (a) The calculated energy (relative to the separated reactants, including H2) for transition states and intermediates involved in the formation routes for benzophenone imine (blue) and cyanobiphenyl (red) under DLPNO-CCSD(T)/aug-cc-pVTZ//M06-2X-D3/def2-TZVPP level. (b) Calculated reaction rate coefficients obtained from transition state theory with tunneling corrections included using the Kinetic and Statistical Thermodynamical Package (KiSThelP, KiSThelP). The shaded regions represent the propagated uncertainty in the barrier heights of TS1 and TS2 (±$2.5\,\mathrm{kJ\,mol^{-1}}$), with $+2.5\,\mathrm{kJ\,mol^{-1}}$ corresponding to the lower red and blue dashed lines, and $-2.5\,\mathrm{kJ\,mol^{-1}}$ to the upper dashed lines.