Reaction Sites of CO on Size-Selected Silicon Oxide Cluster Anions: A Model Study of Chemistry in the Interstellar Environment

We present reactions of size-selected free silicon oxide cluster anions, Si_<i>n</i>O_<i>m</i>^– (<i>n</i> = 3–7, 2<i>n</i> – 1 ≤ <i>m</i> ≤ 2<i>n</i> + 2), with a CO gas. Adsorption of CO on Si_<i>n</i>O_<i>m</i>^– is observed as a major reaction channel. The rate constant of the adsorption reaction is high for the oxygen-rich clusters with <i>m</i> ≥ 2<i>n</i> + 1, whereas almost no reaction product is observed for <i>m</i> ≤ 2<i>n</i>. DFT calculations revealed that a pair of dangling O atoms on 4-fold-coordinated Si atoms plays a key role, which is the adsorption site of CO on Si_<i>n</i>O_<i>m</i>^–. Bond formation between CO and one of the dangling O atoms is associated with electron transfer from the CO molecule to the other dangling O atom. The present findings give molecular-level insights into adsorption of CO molecules on silicates in the interstellar environment