Internal rotation effects in the pulsed jet rotational spectrum of the trifluoromethane-carbon dioxide dimer

The rotational spectrum of the normal isotopic species of the HCF3-CO2 weakly bound complex has been measured by Fourier-transform microwave (FTMW) spectroscopy. All transitions are split into A and E states by internal rotation of the trifluoromethane subunit. A global fit of these states gives rotational constants that are consistent with a structure predicted by an MP2/6-311++G(2d,2p) ab initio calculation in which the axes of the monomers are coplanar, with the hydrogen atom of the trifluoromethane angled toward one of the oxygen atoms of the CO2. Measured dipole moment components (μa = 0.431(6) D, μb = 0, μc = 1.436(6) D, μtotal = 1.499(6) D) confirm the ab initio prediction of an ac plane of symmetry; however, the very near-prolate nature of the complex (κ = −0.997), combined with the relatively high barrier to internal rotation (∼30 cm−1) leads to asymmetry splittings and internal rotation splittings of similar magnitude, resulting in the observation of dipole forbidden b-type E-state transitions in addition to the expected a- and c-type lines. Although this effect has been observed previously in several monomer spectra, this appears to be one of few examples for a weakly bound complex.