Formation of hydroxide anions in amorphous astrophysical ices and recombination with protons: a quantum chemical study

ABSTRACT

Quantum chemical cluster calculations employing density functional theory and correlation consistent basis sets reveal the following pathways by which hydroxide anions (OH–) may form in amorphous astrophysical ices: (1) hydroxyl radicals (OH), which may arise in ice via ultraviolet photolysis, can capture electrons; (2) adsorbed hydrogen atoms can capture electrons to form H–, which reacts with water to yield H2 and OH–; (3) NaOH deposited on ice dissociates into Na+ and OH–; (4) NaH deposited on ice dissociates into Na+ and H–; H– then reacts with water to yield H2 and OH– as above. The IR spectrum of ice-bound OH– is presented, based on nine clusters containing up to 31H2O and 1–2 OH– anions. The interaction of OH– in ice with cations is also explored. Prior work shows that when HCO+ is deposited on pure amorphous water clusters, it reacts with H2O to form formic acid (HCOOH) and the hydronium (H3O+). When HCO+ is deposited on a cluster containing OH–, the reaction proceeds in almost the same manner, but the H3O+ and OH– charge centres migrate through the water network toward each other and tend to neutralize one another by forming H2O. This occurred in all but one of seven cases considered; migration occurred even when the oxygen atom attacked by HCO+ is over 10 Å from the oxygen atom in OH–. Cations and anions can interact in ice via pathways not present in the gas phase or incorporated in current models.