The gas-phase nitridation processes of large, astronomically relevant polycyclic aromatic hydrocarbons cations in the interstellar medium

¹ Hunan Key Laboratory for Stellar and Interstellar Physics and School of Physics and Optoelectronics, Xiangtan University, Xiangtan, Hunan 411105, China
² National Key Laboratory of Deep Space Exploration/School of Earth and Space Sciences, University of Science and Technology of China, Anhui, Hefei 230026, China

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Received: 4 December 2025
Accepted: 27 January 2026

Abstract

Atomic nitrogen reacts efficiently with polycyclic aromatic hydrocarbons species, and potentially generates a diverse array of complex organic molecules in the interstellar medium. In this work, the gas-phase chemical evolution of large, astronomically relevant PAH, tetra-benzo-pero-pyrene (TBPP, C₃₆H₁₆) cations under N-atom bombardment is investigated experimentally and theoretically. A series of nitridated TBPP cations, including [C₃₆H₁₆N_n]⁺ (n=1–5) and dehydrogenated species [C₃₆H₁₀₋₁₅N_n]⁺ (n=1–5), are effectively formed. Additionally, we identify denitridation pathways of nitridated TBPP cations that, through loss of a CN or HCN/HNC unit, lead to the formation of smaller PAH cations (with odd carbon numbers and five-membered carbon cycles, e.g., [C₃₅H₁₅]⁺). We investigated the structures and the vibrational infrared spectra of newly formed nitridated TBPP cations and the bonding energy for the reaction pathways using theoretical calculations, which were based on density functional theory with the hybrid density functional B3LYP/6-311++G(d, p). The reaction energy is relatively high, which indicates that the addition of N atoms to the carbon skeleton is a random and independent event, i.e., there is no carbon-edge structural effect. Furthermore, theoretical analyses confirm that denitridation pathways involving the loss of CN or HCN/HNC units are energetically favorable, which underscores their potential role in the top-down evolution of large PAHs. The obtained results highlight the importance of PAH cations evolution under N-atom bombardment, and demonstrate the formation of complex organic species containing nitrogen functionalities, such as C=N, C–N–H, and C–N–C groups. Furthermore, these findings provide insights into the molecular diversity and chemical evolution of PAHs, where ion-atom collisions drive both the functionalization and structural truncation of large molecules in astrophysical environments.