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

Abstract

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 (2)A 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 (2)B2 electronic state.