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

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 H ₂ 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 H ₂ and OH ^– as above. The IR spectrum of ice-bound OH ^– is presented, based on nine clusters containing up to 31H ₂O 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 H ₂O to form formic acid (HCOOH) and the hydronium (H ₃O ⁺). When HCO ⁺ is deposited on a cluster containing OH ^–, the reaction proceeds in almost the same manner, but the H ₃O ⁺ and OH ^– charge centres migrate through the water network toward each other and tend to neutralize one another by forming H ₂O. 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.