IR photodissociation spectroscopy of (OCS)_n⁺ and (OCS)_n⁻ cluster ions: Similarity and dissimilarity in the structure of CO₂, OCS, and CS₂ cluster ions

Infrared photodissociation (IRPD) spectra of (OCS)_n⁺ and (OCS)_n⁻ (n = 2–6) cluster ions are measured in the 1000–2300 cm⁻¹ region; these clusters show strong CO stretching vibrations in this region. For (OCS)₂⁺ and (OCS)₂⁻, we utilize the messenger technique by attaching an Ar atom to measure their IR spectra. The IRPD spectrum of (OCS)₂⁺Ar shows two bands at 2095 and 2120 cm⁻¹. On the basis of quantum chemical calculations, these bands are assigned to a C₂ isomer of (OCS)₂⁺, in which an intermolecular semi-covalent bond is formed between the sulfur ends of the two OCS components by the charge resonance interaction, and the positive charge is delocalized over the dimer. The (OCS)_n⁺ (n = 3–6) cluster ions show a few bands assignable to “solvent” OCS molecules in the 2000–2080 cm⁻¹ region, in addition to the bands due to the (OCS)₂⁺ ion core at ∼2090 and ∼2120 cm⁻¹, suggesting that the dimer ion core is kept in (OCS)_3–6⁺. For the (OCS)_n⁻ cluster anions, the IRPD spectra indicate the coexistence of a few isomers with an OCS⁻ or (OCS)₂⁻ anion core over the cluster range of n = 2–6. The (OCS)₂⁻Ar anion displays two strong bands at 1674 and 1994 cm⁻¹. These bands can be assigned to a C_s isomer with an OCS⁻ anion core. For the n = 2–4 anions, this OCS⁻ anion core form is dominant. In addition to the bands of the OCS⁻ core isomer, we found another band at ∼1740 cm⁻¹, which can be assigned to isomers having an (OCS)₂⁻ ion core; this dimer core has C₂ symmetry and ²A electronic state. The IRPD spectra of the n = 3–6 anions show two IR bands at ∼1660 and ∼2020 cm⁻¹. The intensity of the latter component relative to that of the former one becomes stronger and stronger with increasing the size from n = 2 to 4, which corresponds to the increase of “solvent” OCS molecules attached to the OCS⁻ ion core, but it suddenly decreases at n = 5 and 6. These IR spectral features of the n = 5 and 6 anions are ascribed to the formation of another (OCS)₂⁻ ion core having C_2v symmetry with ²B₂ electronic state