GEOMETRIC ISOMERISM IN CLUSTERS - HIGH-RESOLUTION INFRARED-SPECTROSCOPY OF A NONCYCLIC CO2 TRIMER

High resolution infrared spectra of a previously unidentified noncycli c isomer of (CO2)(3) have been obtained via direct absorption of a 4.3 mu m diode laser in a slit jet supersonic expansion. Two vibrational bands (labeled nu(I) and nu(III)) are observed, corresponding to the t wo most infrared active linear combinations of the three constituent C O2 monomer asymmetric stretches: nu(I) is redshifted -5.85 cm(-1) from the monomer vibrational origin and is predominately a c-type band of an. asymmetric top, while nu(III) is blueshifted +3.58 cm(-1) and is p redominately an a-type band. Transitions with K-a + K-c = odd (even) i n the ground (excited) state are explicitly absent from the spectra du e to the zero nuclear spin of CO2; this rigorously establishes that th e noncyclic isomer has a C-2 symmetry axis. The vibrational shifts and relative intensities of the bands are interpreted via a resonant dipo le interaction model between the high-frequency stretches of the CO2 m onomers. Rotational constants are determined by fits of transition fre quencies to an asymmetric top Hamiltonian. These results are used to d etermine vibrationally averaged structural parameters for the complex, which is found to be stacked asymmetric but with C-2 symmetry about t he b inertial axis. The structural parameters are then used to test se veral trial CO2-CO2 interaction potentials. (C) 1996 American Institut e of Physics.