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

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.

Flipping Kinetics of the Water Trimer on Acenaphthylene: Persistence of a Highly Dipolar ddd Configuration at Interstellar Temperatures

“…We have theoretically investigated complexes formed between a cyclic water trimer and a tricyclic PAH molecule (acenaphthylene) via O–H···π bonds, displaying distinct spatial configurations. These systems are similar to probable candidates to be found in cold extraterrestrial environments . However, there exist intrinsic difficulties in determining infrared emission bands of pure PAHs .…”

“…These systems are similar to probable candidates to be found in cold extraterrestrial environments. 65 However, there exist intrinsic difficulties in determining infrared emission bands of pure PAHs. 66 Then, we have focused on the ddu ↔ ddd flipping process of the trimer anchored on the aromatic plane, which may favor a stable and highly dipolar structure of the resulting complex at interstellar temperatures.…”

Photoabsorption of Microhydrated Naphthalene and Its Cyano-Substituted Derivatives: Probing Prereactive Models for Photodissociation in Molecular Clouds

“…Considering now diffuse clouds with highradiation field to support our prereactive models for photodissociation of these complexes, it is necessary to take a closer look at the electronic structure between their moieties. A recent study 25 has reported an MO mixing analysis that allows rationalizing the experimental observation of PAHs trapped in ice matrices. In general, PAHs become cationic when exposed to UV radiation because of the smaller superposition involving the lone pair HOMO of the water molecules with π-filled MOs of a PAH.…”

“…Regarding the physicochemical processes in the ISM, the predissociative stage may be modeled by a microhydrated PAH molecule, and a related phenomenon consists of photoionization of the PAH in the presence of the Lyman-α field. − As noticed for the ionic forms at temperatures above 120 K, it is expected that the OH – ion reacts with the PAH + , and H + diffuses through solvation shells, mainly when exposed to VUV irradiation. , Theoretical investigations on the reactivity of these processes are scarce in the literature. Recently, ten Brinck et al have employed time-dependent density functional theory (TD-DFT) calculations to rationalize the reactivity of ice-embedded PAHs. Their results are in line with the experimental observation that, in the presence of water, PAHs become cationic when exposed to VUV irradiation.…”

“…23,24 Theoretical investigations on the reactivity of these processes are scarce in the literature. Recently, ten Brinck et al 25 have employed time-dependent density functional theory (TD-DFT) calculations to rationalize the reactivity of ice-embedded PAHs. Their results are in line with the experimental observation that, in the presence of water, PAHs become cationic when exposed to VUV irradiation.…”