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

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

We present reactions of size-selected free silicate, Mg_<i>l</i>SiO_<i>m</i>^–, and silica, Si_<i>n</i>O_<i>m</i>^–, cluster anions with a H₂O molecule focusing on H₂O adsorption. It was found that H₂O adsorption to Mg_<i>l</i>SiO_<i>m</i>^– with <i>l</i> = 2 and 3 (<i>m</i> = 4–6) is always followed by molecular oxygen release, whereas reactivity of the clusters with <i>l</i> = 1 (<i>m</i> = 3–5) was found to be much lower. On the contrary, in the reaction of Si_<i>n</i>O_<i>m</i>^– (<i>n</i> = 3–8, 2<i>n</i> – 1 ≤ <i>m</i> ≤ 2<i>n</i> + 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_<i>n</i>O_<i>m</i>^– 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_<i>n</i>O_<i>m</i>^–. The present findings give molecular-level insights into H₂O adsorption on silica and silicate species in the interstellar environment

Photoelectron Imaging Signature for Selective Formation of Icosahedral Anionic Silver Cages Encapsulating Group 5 Elements: M@Ag₁₂^– (M = V, Nb, and Ta)

An assembly of 13 atoms can form highly symmetric architectures like those belonging to D3h, Oh, D5h, and Ih point groups. Here, using photoelectron imaging spectroscopy in combination with density functional theory (DFT) calculations, we present a simple yet convincing experimental signature for the selective formation of icosahedral cages of anionic silver clusters encapsulating a dopant atom of group 5 elements: M@Ag12– (M = V, Nb, and Ta). Their photoelectron images obtained at 4 eV closely resemble one another: only a single ring is observed, which is assignable to photodetachment signals from a 5-fold degenerate superatomic 1D electronic shell in the 1S21P61D10 configuration of valence electrons. The perfect degeneracy represents an unambiguous fingerprint of an icosahedral symmetry, which would otherwise be lifted in all of the other structural isomers. DFT calculations confirm that Ih forms are the most stable and that D5h, Oh, and D3h structures are not found even in metastable states

Photoelectron Imaging Signature for Selective Formation of Icosahedral Anionic Silver Cages Encapsulating Group 5 Elements: M@Ag₁₂^– (M = V, Nb, and Ta)

An assembly of 13 atoms can form highly symmetric architectures like those belonging to D3h, Oh, D5h, and Ih point groups. Here, using photoelectron imaging spectroscopy in combination with density functional theory (DFT) calculations, we present a simple yet convincing experimental signature for the selective formation of icosahedral cages of anionic silver clusters encapsulating a dopant atom of group 5 elements: M@Ag12– (M = V, Nb, and Ta). Their photoelectron images obtained at 4 eV closely resemble one another: only a single ring is observed, which is assignable to photodetachment signals from a 5-fold degenerate superatomic 1D electronic shell in the 1S21P61D10 configuration of valence electrons. The perfect degeneracy represents an unambiguous fingerprint of an icosahedral symmetry, which would otherwise be lifted in all of the other structural isomers. DFT calculations confirm that Ih forms are the most stable and that D5h, Oh, and D3h structures are not found even in metastable states

Size-Dependent Ligand Quenching of Ferromagnetism in Co₃(benzene)_<i>n</i> ⁺ Clusters Studied with X‑ray Magnetic Circular Dichroism Spectroscopy

Cobalt–benzene cluster ions of the form Co₃(bz)_<i>n</i> ⁺ (<i>n</i> = 0–3) were produced in the gas phase, mass-selected, and cooled in a cryogenic ion trap held at 3–4 K. To explore ligand effects on cluster magnetic moments, these species were investigated with X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) spectroscopy. XMCD spectra yield both the spin and orbital angular momenta of these clusters. Co₃ ⁺ has a spin magnetic moment of μ_S = 6 μ_B and an orbital magnetic moment of μ_L = 3 μ_B. Co₃(bz)⁺ and Co₃(bz)₂ ⁺ complexes were found to have spin and orbital magnetic moments identical to the values for ligand-free Co₃ ⁺. However, coordination of the third benzene to form Co₃(bz)₃ ⁺ completely quenches the high spin state of the system. Density functional theory calculations elucidate the spin states of the Co₃(bz)_<i>n</i> ⁺ species as a function of the number of attached benzene ligands, explaining the transition from septet to singlet for <i>n</i> = 0 → 3