The ethylene-carbon dioxide complex and the double rotor model

The infrared spectrum of the weakly-bound C2H4-CO2 complex is investigated in the region of the nu3 fundamental band of CO2 (~2350 cm-1), using a tunable OPO laser source to probe a pulsed supersonic slit jet expansion. The spacing of the various K-subbands in this perpendicular spectrum is very irregular, and the pattern of irregularity is quite different from that observed previously in another C2H4-CO2 band by Bemish et al. [J. Chem. Phys. 103, 7788 (1995)]. But by allowing for the different symmetry of the nu3 (CO2) upper vibrational state, both results can be strikingly well explained using the 'double internal rotor' model as described by Bemish et al.