Adsorption and Subsequent Reaction of a Water Molecule on Silicate and Silica Cluster Anions

We present reactions of size-selected free silicate, Mg_lSiO_m^-, and silica, Si_nO_m^-, cluster anions with a H₂O molecule focusing on H₂O adsorption. It was found that H₂O adsorption to Mg_lSiO_m^- with l = 2 and 3 (m = 4-6) is always followed by molecular oxygen release, whereas reactivity of the clusters with l = 1 (m = 3-5) was found to be much lower. On the contrary, in the reaction of Si_nO_m^- (n = 3-8, 2n - 1 ≤ m ≤ 2n + 2), a H₂O adduct is observed as a major reaction product. Larger and oxygen-rich clusters tend to exhibit higher reactivity; the rate constants of the adsorption reaction are 2 orders of magnitude larger than those of CO adsorption previously reported. DFT calculations revealed that H₂O is dissociatively adsorbed on Si_nO_m^- to form two SiO₃(OH) tetrahedra. The site selectivity of H₂O adsorption is governed by the location of the singly occupied molecular orbital (SOMO) on Si_nO_m^-. The present findings give molecular-level insights into H₂O adsorption on silica and silicate species in the interstellar environment.