| Abstract: Polycyclic aromatic nitrogen heterocycles (PANHs) are nitrogen substituted analogues of PAHs and are considered promising prebiotic molecules in the interstellar medium (ISM) due to their enhanced reactivity and structural similarity to nucleobases. However, simple PANHs are difficult to detect observationally because their high symmetry and negligible permanent dipole moments render them largely invisible to rotational spectroscopy, while their infrared features are often obscured by spectral overlap with abundant PAHs. In contrast, functionalized PANH derivatives are expected to be more readily detectable, as the addition of substituent groups breaks molecular symmetry and induces significant permanent dipole moments enhancing their rotational activity and produces distinct spectroscopic signatures, making them more favourable targets for astronomical detection than their parent molecules.
In this study, we investigate the reactivity of pyrrole with astrochemically relevant functional groups like cyano (CN), hydroxyl (OH), and amino (NH₂), under both gas phase and water ice surface conditions to simulate diverse interstellar environments. Multiple reaction pathways, including electrophilic substitution and hydrogen abstraction at different sites of the pyrrole ring, are explored. High level quantum chemical calculations are employed to map the potential energy surfaces and to determine whether these reactions proceed via barrierless or activated mechanisms. Transition states are located and validated through intrinsic reaction coordinate (IRC) calculations, allowing identification of the most stable products and dominant reaction channels under typical ISM conditions.
For the most favourable pathways, temperature dependent rate coefficients are calculated using the MESS master equation solver, with input parameters derived from DFT level thermochemical data. The resulting rate coefficients are subsequently fitted to the modified Arrhenius–Kooij expression for use in astrochemical models. In addition, the infrared and rotational spectroscopic properties of the stable pyrrole derivatives will be computed to assess their potential detectability in astronomical observations. |
