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

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.