Spectroscopic observation and ab initio calculations of a new isomer of the CS2 trimer

We report spectroscopic observation and theoretical calculations of a new isomer of (CS₂)₃ as observed in the regions of the ν₃ fundamental band of CS₂ (6.5 μm) and the ν₁ + ν₃ combination band (4.5 μm), using tunable laser sources and a pulsed supersonic slit-jet. The previously observed CS₂ trimer has a barrel-shaped structure with three equivalent monomers and D₃ symmetry. The new isomer consists of a staggered parallel “dimer pair” of equivalent CS₂ monomers with a third CS₂ monomer sitting “on top”, similar to the known non-cyclic CO₂ trimer. This structure has C₂ rotational symmetry corresponding to the b inertial axis of the trimer, as proven by observed nuclear spin statistics. Ab initio calculations correctly give the two observed isomer structures and indicate that they lie very close in binding energy.