High-resolution infrared spectra of (CO2)3 formed in a slit jet supersonic expansion are obtained via direct absorption of a tunable diode laser in the ν3 asymmetric stretch region of CO2. Over 100 distinct transitions are recorded in the trimer spectrum, which can be modeled as a perpendicular band of a planar symmetric top with C3h symmetry and no observable tunneling splittings. Results from the spectroscopic fit indicate that the complex is vibrationally averaged planar, with a carbon–carbon atom separation of RCC=4.0376(2) Å. An analysis of the vibrational blue shift for (CO2)3 of 2.5755(2) cm−1 via a resonant dipole–dipole interaction model yields an angular orientation for each CO2 axis of β=33.8(5)° away from a line tangent to the vertex and parallel to the opposite side of the equilateral triangle connecting the centers of mass of each CO2 monomer. Several model CO2–CO2 interaction potentials are tested against the vibrationally averaged structural parameters for (CO2)3. In particular, the potential of Murthy et al. [Mol. Phys. 50, 531 (1983)] reproduces RCC for the complex, but similar to all potentials tested, does not accurately predict the angular orientation β of the monomers within the trimer. Lastly, spectral evidence and model predictions suggest that there is an asymmetric top isomer of the trimer that is energetically comparable to the observed cyclic isomer.

Articles you may be interested inHigh-resolution microwave spectrum of the weakly bound helium-pyridine complex Quantum and classical calculations of transport and relaxation cross sections in He-CO mixturesThe infrared spectrum of the HeC0 2 van der Waals molecule is recorded in the region of the CO 2 V3 asymmetric stretch via direct absorption of a tunable Pb-salt diode laser. HeC0 2 is formed in a slit jet supersonic expansion; the slit valve and the stagnation gas must be precooled to -35°C before substantial formation of the complex is observed. Sixty-six rovibrational transitions are recorded by exciting the v3 asymmetric stretch of the CO 2 monomer within the complex. Forty-three of these transitions can be assigned using internally consistent combination differences as a b-type band of a T-shaped asymmetric rotor. There are several indications that large amplitude motion is significant in HeC0 2 , including the poor quality of the fit to an asymmetric rotor model and the large positive inertial defects of Ll=8.54 and 10.98 uA? in the ground and excited states, respectively. However, a hindered rotor analysis based on these inertial defects demonstrates that the CO 2 motion within the complex is far from the free rotor limit. No evidence of predissociation broadening is observed, indicating a lifetime for the complex of 7'>6 ns. Quantum close-coupling calculations which correctly treat both angular and radial degrees of freedom are carried out on the full 2D HeC0 2 potential energy surface of Beneventi et al. [J. Chern. Phys. 89, 4671 (1988)]. Comparison of this analysis with the experimental results demonstrates that the theoretical potential is too isotropic in the region of the potential minimum. Predicted spectra from this model potential, however, indicate that the remaining 17 much weaker HeC0 2 transitions are due to a "hot band" excitation out of the first intermolecular bending level, lying 9±2 cm-1 above the ground state. In sharp contrast to the ground vibrational state of HeC0 2 , an asymmetric rotor model fails qualitatively to characterize the rotational structure for the lowest excited bend. The simple physical reason for this is confirmed by inspection of the quantum wave functions; in the ground state tlle He atom is localized near the C atom in a T-shaped geometry, whereas in any of the excited bending states the He atom is largely delocalized around the CO 2 molecular framework.