Knowledge Bank

A complete structure of trans-1,2-difluoroethylene, which is a non-polar molecule, would assist in interpreling the surprisingly greater energy of this isomer in comparison with its cis $isomer.1$ We are in the process of obtaining high resolution ($0.004cm-1$) spectra of several bands of three isotopomers of this near-symmetric prolate top molecule with the goal of finding precise ground state rotational constants and a reliable $\gamma 0$ structure. Last year we reported an analysis of three bands in the spectrum of the $d0$ $species.2$ This year, we report on the preliminary analysis of two bands in the spectrum of the $d2$ species. From the A-type band at 1176 $cm-1$ which is due to antisymme""oe CF stretching. $v10$ $(bu)3$ we obtain initial values of $B\prime \prime$ and $C\prime \prime$ equal to 0.134021(5) $cm-1$ and 0.121108(5) $cm-1$. The higher K levels of this band are slightly perturbed, but a set of upper state constants can be obtained by fitting all observed transitions through K-6. The two ground state constants are in good agreement with values calculated from geometric parameters in which the CCF bond angle and CF bond length were fit to moments of inertia for the $d0$ species while fixing other parameters to those transferred from the cis isomer. In the trans isomer, the CCF bond angle is smaller, and the CF bond length is longer. In addition the A/B-hybrid band at 1331 $cm-1$ has been examined. This band, which is due to the combination tone, $V7(a\nu )+V8(bg)3$ is heavily perturbed. The A-type O-branch is split into two strong components separated by 1 $cm-1$.