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

Abstract

We present reactions of size-selected free silicon oxide cluster anions, SinOm(-) (n = 3-7, 2n - 1 ≤ m ≤ 2n + 2), with a CO gas. Adsorption of CO on SinOm(-) is observed as a major reaction channel. The rate constant of the adsorption reaction is high for the oxygen-rich clusters with m ≥ 2n + 1, whereas almost no reaction product is observed for m ≤ 2n. 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 SinOm(-). 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.

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

“…No H 2 O adduct was observed for Si 3 O 5 – , Si 3 O 8 – , Si 4 O 7 – , and Si 4 O 10 – , which are represented by white pixels. Larger and oxygen-rich clusters tend to exhibit higher reactivity, although this trend is not so clear as that found in our previous study on the reaction with a CO molecule, where the rate constants of CO adsorption were remarkably high in clusters with m ≥ 2 n + 1 . The rate constants for reactive clusters are on the order of 10 –10 to 10 –11 cm 3 s –1 ; it is 2 orders of magnitude larger than those of CO adsorption …”

“…By reference to the isosurfaces of the SOMOs previously reported, the present results suggest that the location of the SOMO plays a role in the site selectivity, as in the case of CO adsorption. For clusters with m ≤ 2 n (Si 4 O 8 – , Si 5 O 9 – , and Si 5 O 10 – ), OH is adsorbed on the triangular-pyramidal 3-fold-coordinated Si site, where the SOMO is localized, and H is on one of the dangling O atoms in a SiO 4 tetrahedron; for clusters with m = 2 n + 1 (Si 4 O 9 – and Si 5 O 11 – ), H is adsorbed on one of the dangling O atoms, where the SOMO is localized, and OH is on a planar 3-fold-coordinated Si site.…”

“…The dissociation barrier of an O−H bond could affect the reactivity if the barrier is high. Although the dissociation barrier on Si n O m − has not been reported, it is reported for Mn 4 O 4+ that the barrier of water splitting is low: 0.15 eV for a ring-shaped isomer and <0.05 eV for a cubic isomer.27 Because these values are much lower than the collision energy of the present experiments (e.g., 0.5 eV for Si 4 O 8 − in the center-of-mass frame), it is plausible that the contribution of the dissociation barrier to the reactivity is small.By reference to the isosurfaces of the SOMOs previously reported,14 the present results suggest that the location of the SOMO plays a role in the site selectivity, as in the case of CO adsorption. For clusters with m ≤ 2n (Si 4 O is adsorbed on the triangular-pyramidal 3-foldcoordinated Si site, where the SOMO is localized, and H is on one of the dangling O atoms in a SiO 4 tetrahedron; for clusters with m = 2n + 1 (Si 4 O 9 − and Si 5 O 11 −…”

“…Larger and oxygen-rich clusters tend to exhibit higher reactivity, although this trend is not so clear as that found in our previous study on the reaction with a CO molecule, where the rate constants of CO adsorption were remarkably high in clusters with m ≥ 2n + 1. 14 The rate constants for reactive clusters are on the order of 10 −10 to 10 −11 cm 3 s −1 ; it is 2 orders of magnitude larger than those of CO adsorption. ), H is adsorbed on one of the dangling O atoms, where the SOMO is localized, and OH is on a planar 3-fold-coordinated Si site.…”

“…Ion–molecule reactions of ions consisting of Si, O, Mg, and Fe with gas molecules, for example, H 2 O, H 2 , and CO, might contribute to chemistry in the interstellar environment such as molecular evolution and delivery of water to the Earth. In this regard, we have reported the formation of clusters with mineral compositions and reaction of silicon-oxide cluster anions, Si n O m – , with a CO molecule because gas-phase clusters provide good models of free radicals and ions. Agglomeration of silicon-oxide clusters has been investigated to understand the growth process of silicate and silica particles in space. , Reactions of silicate and silica clusters with H 2 O, however, have not been reported; here we examine clusters consisting of Si, O, and Mg as silicate clusters and silicon-oxide clusters (Si n O m – with m / n ≈ 2) as silica clusters.…”