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,F Matthias Bickelhaupt,Angela Van Der Werf,Richard M P Veenboer,Harold Linnartz,Célia Fonseca Guerra

doi:10.1021/acsearthspacechem.1c00433

Stephanie Ten Brinck, Celine Nieuwland + Show 5 more

Open Access

https://doi.org/10.1021/acsearthspacechem.1c00433

Copy DOI

Abstract

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.

Highlights

The presence of polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium has been proven through the presence of strong infrared (IR) emission bands between 3.3 and 11.3 μm.[1]
By time-dependent density functional theory (DFT) calculations, we demonstrate that the formation of mixed PAH−ammonia molecular orbital (MO) opens alternative charge-transfer excitation pathways as electronic density from ammonia can be transferred to unoccupied PAH levels, yielding anionic PAHs
We focus on the highest occupied MOs (HOMOs) and lowest unoccupied MOs (LUMOs) in particular since these levels are most likely involved in electronic excitations

Summary

Introduction

The presence of polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium has been proven through the presence of strong infrared (IR) emission bands between 3.3 and 11.3 μm.[1]. In the 80s, these features were hypothesized to originate from various stretching and bending modes of vibrationally excited hydrocarbon systems, such as PAHs, after excitation by photons from the interstellar radiation field.1b−d,g,2 Nowadays, the existence of interstellar PAHs is generally accepted, after the recent unambiguous astronomical identification of a number of aromatic species.[3] Organic molecules such as PAHs play an important role in interstellar chemistry. They are formed in the outflows of dying stars, contribute upon fragmentation to the molecular inventory in space, and are expected to freeze out onto cold dust grains. The vacuum UV (VUV) irradiation and particle bombardment of these interstellar ices produces complex organic molecules that possibly played a vital role in the origin of life.[7]

Objectives

Methods

Results

Conclusion