Efficient stabilization of cyanonaphthalene by fast radiative cooling and implications for the resilience of small PAHs in interstellar clouds

Abstract

After decades of searching, astronomers have recently identified specific Polycyclic Aromatic Hydrocarbons (PAHs) in space. Remarkably, the observed abundance of cyanonaphthalene (CNN, C10H7CN) in the Taurus Molecular Cloud (TMC-1) is six orders of magnitude higher than expected from astrophysical modeling. Here, we report unimolecular dissociation and radiative cooling rate coefficients of the 1-CNN isomer in its cationic form. These results are based on measurements of the time-dependent neutral product emission rate and kinetic energy release distributions produced from an ensemble of internally excited 1-CNN+ studied in an environment similar to that in interstellar clouds. We find that Recurrent Fluorescence – radiative relaxation via thermally populated electronic excited states – efficiently stabilizes 1-CNN+, owing to a large enhancement of the electronic transition probability by vibronic coupling. Our results help explain the anomalous abundance of CNN in TMC-1 and challenge the widely accepted picture of rapid destruction of small PAHs in space.

Experimental radiative cooling rates of a polycyclic aromatic hydrocarbon cation

“…29,30 The methods used here have largely been described previously. 6,16 Briefly, 1-CNN (Sigma-Aldrich, >96%) was sublimed from powder in a resistively heated oven coupled to an electron cyclotron resonance (ECR) ion source (Pantechnik Monogan) using helium as a support gas. Cations extracted from the source were accelerated to 34 keV kinetic energy.…”

“…We adopt the parameters from our earlier study, 6 which found a rather small reverse barrier height ∆E = 7.6(4) meV and a transition state frequency hω = 350(20) cm −1 from simultaneous fits to the KER distributions for spontaneously decaying 1-CNN up to 20 ms after ionization. That study also found the dissociation rate coefficient to follow a modified Arrhenius expression:…”

“…The orange symbols E m (t) are the energies of spontaneously decaying 1-CNN + determined previously. 6 lated at the B3LYP/6-31G(d,p) level of Density Functional Theory (DFT) as implemented in Gaussian 16 38 . Finally, the energy E m of the precurser ion includes a finite heat bath correction due to the energy required to reach the transition state 39 :…”

“…where a nominal value of the pre-exponential factor A diss 1000K = k B [1000K]/h = 2 × 10 13 s −1 was adopted. 6…”

“…1 Their model, which performs well for smaller nitriles, 5 underestimates the observed abundance of CNN by six orders of magnitude. 1 In a forthcoming report, 6 we address this gap by elucidating the main destruction pathway for one of the two CNN isomers, which are expected to behave similarly and indeed have comparable abundances in TMC-1. 1 We find, contrary to the explicit assumption of McGuire et al, that 1-cyanonaphthalene (1-CNN) is efficiently stabilized by rapid radiative cooling following ionization, closing off some of the reaction channels assumed to deplete CNN from TMC-1.…”