Knowledge Bank

In 1974 Suzuki and Nibler observed the vibrational spectra of cis-3,4-dichlrocyclobutene and proposed an assignment of the fundamentals of this $molecule.1$ At the time the trans Isomer was unknown. In the meanime. Hoberg and Frolich obtained the trans Isomer by alumnium-chloride-promoted Isomerization of the cis $Isomer.2$ As part of a continuing study of the vibrational spectra of halogen-substituled cyclobutenes and related cations, we have prepared the trans Isomer and investigated its infared and Raman spectra. This molecule has $C2$ symmelry, and the $fb$ principal moment-of-inertia axis coincides with the two-fold symmetry axis. We propose the following assignment of fundamentals. For the 13 modes of a symmetry (type-B bands in the intrarod, polarized Raman bands): $3107,2984,1555,1284,1248,1126,1010,955,920,822,452,226,132cm-1$ For the 11 modes of b symmetry (hybrid type-B/C infrared bands, depolarized Raman bands): 3087, 2998, 1291, 1220, 956, 889, 782, ---, 606, 329, 252 $cm-1$. Most of these assignments correlate with those of the cis isomer. An important exception is the frequency of $132cm-1$ for the ring puckering mode in the trans isomer. The assignment of $380cm-1$ for ring puckering in the cis $isomer1$ does not fit the pattern that has emerged for other halogen-substituted $cyclobutenes.3$ A reassignment to $145cm-1$ is proposed.