Polycyclic Aromatic Hydrocarbons (PAHs) in Interstellar Ices:A Computational Study into How the Ice Matrix Influences the Ionic State of PAH Photoproducts

Stephanie Ten Brinck, Celine Nieuwland, Angela Van Der Werf, Richard M.P. Veenboer, Harold Linnartz, F. Matthias Bickelhaupt*, Célia Fonseca Guerra

*Corresponding author for this work

Research output: Contribution to JournalArticleAcademicpeer-review


It has been experimentally observed that water-ice-embedded polycyclic aromatic hydrocarbons (PAHs) form radical cations when exposed to vacuum UV irradiation, whereas ammonia-embedded PAHs lead to the formation of radical anions. In this study, we explain this phenomenon by investigating the fundamental electronic differences between water and ammonia, the implications of these differences on the PAH-water and PAH-ammonia interaction, and the possible ionization pathways in these complexes using density functional theory (DFT) computations. In the framework of the Kohn-Sham molecular orbital (MO) theory, we show that the ionic state of the PAH photoproducts results from the degree of occupied-occupied MO mixing between the PAHs and the matrix molecules. When interacting with the PAH, the lone pair-type highest occupied molecular orbital (HOMO) of water has poor orbital overlap and is too low in energy to mix with the filled π-orbitals of the PAH. As the lone-pair HOMO of ammonia is significantly higher in energy and has better overlap with filled π-orbitals of the PAH, the subsequent Pauli repulsion leads to mixed MOs with both PAH and ammonia character. By time-dependent DFT calculations, we demonstrate that the formation of mixed PAH-ammonia MOs opens alternative charge-transfer excitation pathways as now electronic density from ammonia can be transferred to unoccupied PAH levels, yielding anionic PAHs. As this pathway is much less available for water-embedded PAHs, charge transfer mainly occurs from localized PAH MOs to mixed PAH-water virtual levels, leading to cationic PAHs.

Original languageEnglish
Pages (from-to)766-774
Number of pages9
JournalACS Earth and Space Chemistry
Issue number3
Early online date21 Feb 2022
Publication statusPublished - 17 Mar 2022

Bibliographical note

Funding Information:
The authors thank Louis Allamandola and A. (Xander) G. G. M. Tielens for the useful discussions, which contributed to the start of this research and the shaping of the research question. The authors thank the Netherlands Organization for Scientific Research (NWO) and the Dutch Astrochemistry Network (DAN) for financial support.

Publisher Copyright:
© 2022 American Chemical Society. All rights reserved.


  • astrochemistry
  • charge-transfer excitations
  • density functional theory
  • ice-matrix effects
  • photoproducts
  • polycyclic aromatic hydrocarbons (PAHs)


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