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

Abstract

We investigate the photoionization pathways of naphthalene, 1-cyanonaphthalene, and 2-cyanonaphthalene upon complexation with the water dimer, aiming to understand the photodissociation process under conditions of the interstellar medium (ISM). We analyze the intermolecular bonding pattern, equilibrium rotational properties, energy complexation, far-IR spectra, and ionic trends of the possible photoproducts using dispersion-corrected density functional theory (DFT-D) and timedependent DFT (TD-DFT). For the different configurations, we evaluate the possible charge-transfer (CT) excitations near the photoionization limit. Our results indicate that, in high-radiation regions of the ISM (>8.0 eV), CT excitations occur from localized occupied molecular orbitals (MOs) in the aromatic molecules to mixed unoccupied MOs in the complexes, favoring cationic aromatic species in these conditions. We notice that the photoabsorption spectra depend on the type of intermolecular interaction (H-bonds or O−H•••π bonds) in the complexes, as well as the presence and position (1 or 2) of the cyano-functional group in naphthalene. For hydrated naphthalene, the O−H•••π complexes assume a more relevant role for photodissociation. In the case of the cyano-substituted derivatives, the H-bonded structures are more favorable to be considered as prereactive models. However, the cyano group at position 2 indicates that CT excitations toward the water dimer are more likely to occur.

Regioselectivity in the Nitration of Eugenol Is Independent of Inorganic Reagents: An Experimental and Theoretical Investigation with Synthetic and Mechanistic Implications

“…Considering now in these model systems that electron transfer occurs via the smallest energy gap, we obtain 1d as the cation more likely to occur. Using the MO mixing approach, we obtain the average mixing between the LUMO of NO 2 + and the HOMO of complexes 1d or 1e , as well as between the HOMO of the neutral eugenol–water complex and the LUMO of 1d or 1e , also indicate the cation more likely to occur during the reaction. For example, the average HOMO contribution of 1d for the MO mixing is ∼65%, while that for intermediate 1e is ∼62%.…”