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 Cs 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 C2v symmetry with 2B2 electronic state.
The Journal of Chemical Physics
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American Institute of Physics
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Graduate School of Science