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 v3 fundamental band of CS2 (6.5 mu m) and the v1 + v3 combination band (4.5 mu m), 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.