Knowledge Bank

The cis isomer of 3,4-difluorocyclobutene is formed as a minor product in the reaction of gaseous cis-3,4-dichlorocyclobutene with solid $AgF21.$ On a sample of only 0.1 mmol of cis-3,4-difluorocyclobutene gas-phase IR spectra and liquid-phase Raman spectra were recorded. For this molecule of $Cs$ symmetry, $\kappa =0.49$, and the axis of the least moment of inertia, $Ia$, is perpendicular to the symmetry plane. Thus, $a\prime \prime$ modes correspond to bands of A-type shape in the gas-phase infrared spectrum, whereas $a\prime$ modes give bands of hybrid B/C shape. The assigned fundamentals are, in $cm-1:(a\prime )$ 3102 (ave. of FR doublet), 2878, 1560, 1350, 1234, 1170, 1050, 998, 837, 769, 408 and 243; $(a\prime \prime )$ 3078, 2956, 1343, 1307, 1180, 1092, 1057, 954, 923, 588, 466 and 160. Frequencies in italics are liquid-phase observations; others are gas-phase values. Overall, the assignments correlate reasonably well with the assignments for the trans isomer and for cis-3,4-$dichlorocyclobutene.1,2$