We present reactions of size-selected free silicate, MglSiOm-, and silica, SinOm-, cluster anions with a H2O molecule focusing on H2O adsorption. It was found that H2O adsorption to MglSiOm- 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 SinOm- (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 SinOm- 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 SinOm-. The present findings give molecular-level insights into H2O adsorption on silica and silicate species in the interstellar environment.
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