Knowledge Bank

The rotational spectrum of the CH$2$F$_2 \cdots $CO$_2$ 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 $\mu c$ dipole moment component. A Pickett-type coupled Hamiltonian has been used to analyze the spectra, giving an energy difference ($\Delta E$) 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 ($Gb=7.740(6)$ 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 $Cs$ symmetry structure in which the $C2$ axis of CH$2$F$2$ makes an angle of roughly $23irc$ with the axis of the CO$2$, with the fluorine atoms of CH$2$F$2$ straddling the CO$2$ carbon atom. The spectra of five additional isotopologues were analyzed, providing detailed structural information, and all except the mixed C$\mathrm{<mrow\; data-mjx-texclass="ORD">18</mrow>}$O$\mathrm{<mrow\; data-mjx-texclass="ORD">16</mrow>}$O species showed inversion splittings similar to the normal species. The observation of unsplit spectra for two distinct C$\mathrm{<mrow\; data-mjx-texclass="ORD">18</mrow>}$O$\mathrm{<mrow\; data-mjx-texclass="ORD">16</mrow>}$O isotopologues confirms that the internal motion involves movement of the CH$2$F$2$ subunit between the two ends of the CO$2$ molecule.