Infrared spectrum of the CS2 trimer: observation of a structure with D3 symmetry

Abstract

Infrared spectra of a carbon disulfide trimer formed in a pulsed supersonic slit-jet expansion are obtained via direct absorption of a tuneable diode laser in the region of the CS₂ν₃ fundamental (∼1535 cm⁻¹). This is the first high-resolution spectroscopic observation of (CS₂)₃. Two bands sharing the same lower state are assigned to (¹²C³²S₂)₃. These correspond to the two infrared active trimer vibrations (a parallel and a perpendicular band) of the constituent CS₂ monomer asymmetric stretches. The weaker perpendicular band is centered at 1524.613 cm⁻¹, shifted by −10.74 cm⁻¹ with respect to the free CS₂ monomer. The parallel band is centered at 1545.669 cm⁻¹, a vibrational shift of +10.31 cm⁻¹. Transitions with K ≠ 3n and those with K = 0, J = odd in the ground state are absent, establishing that this trimer has D₃ symmetry. The two parameters required to define this structure are determined to be 3.811 Å for the C−C bond distance and 61.8° for the angle between a monomer axis and the plane containing the C atoms. In addition, a parallel band arising from trimers with a single ³⁴S substitution is observed around 1544.46 cm⁻¹. Together with the recently observed cross-shaped CS₂ dimer, these results indicate a tendency for CS₂ to form highly symmetric clusters.