15min:

REBECCA A. PEEBLES, AMELIA J. THOMAS, MICHAL M. SERAFIN AND SEAN A. PEEBLES, Department of Chemistry, Eastern Illinois University, 600 Lincoln Ave., Charleston, IL 61920.

The rotational spectrum of the CH₂F₂ CO₂ dimer has been measured using chirped-pulse and resonant cavity Fourier-transform microwave (FTMW) spectroscopy, with the broadband spectrum playing an essential role in allowing identification of the tunneling splittings. The observed a-type transitions were doubled by a few megahertz, while c-type transitions were split by around 200 MHz, suggesting a tunneling motion that inverts the µ_c dipole moment component. A Pickett-type coupled Hamiltonian has been used to analyze the spectra, giving an energy difference ( E) between the tunneling states for the normal isotopologue of 115.140(2) MHz, and rotational constants of A₀ = 5567.8604(27) MHz, B₀ = 1832.9676(5) MHz, C₀ = 1828.6132(5) MHz, A₁ = 5567.8540(26) MHz, B₁ = 1831.7711(4) MHz, C₁ = 1828.6106(4) MHz. It was also necessary to include a Coriolis coupling term (G_b = 7.740(6) MHz) as well as fourth and sixth order centrifugal distortion constants to obtain a satisfactory spectroscopic fit. The rotational constants and planar moments are consistent with a C_s symmetry structure in which the C₂ axis of CH₂F₂ makes an angle of roughly 23^o with the axis of the CO₂, with the fluorine atoms of CH₂F₂ straddling the CO₂ carbon atom. The spectra of five additional isotopologues were analyzed, providing detailed structural information, and all except the mixed C¹⁸O¹⁶O species showed inversion splittings similar to the normal species. The observation of unsplit spectra for two distinct C¹⁸O¹⁶O isotopologues confirms that the internal motion involves movement of the CH₂F₂ subunit between the two ends of the CO₂ molecule.