Knowledge Bank

Lord and $Rea1$ and Aleksanyan and $Garkusha2$ observed the infrared and Raman spectra of cyclobutene and proposed assignments of its vibrational fundamentals. Baggott, Clase and $Mills3$ contributed a higher resolution, FT-infrared study and reevaluated the assignments. For this molecule of $C2v$ symmetry with 24 fundamentals, they left $v20$ (in-plane ring bend) unassigned and gave provisional assignments for three of the five $a2$ modes based on indirect evidence. Our assignments of the out-of-plane modes for 1-chlorocyclobutene and 1-fluorocyclobutene raised doubts about some of the assignments for the $a2$ modes of cyclobutene and about the previous characterizations of some of the $a2$ and $b2$ modes of this molecule. We have examined the Raman spectrum of liquid cyclobutene at - $100\circ C$. We contirm $v10(a2)$ at $1142cm-1$ and propose $V11(a2$ at $1011cm-1,v12(a2)816cm-1$, and $v20(b2$) at $888cm-1$. We concur with Baggott, Clase, and Mills on all of the other fundamentals. $v12(a2$) and $v24(b2$) are more appropriately characterized as out-of-plane modes of vinylic CH bonds than as CH2 modes, in accord with the normal coordinate calculations of Sverdlov and $Krainov.4$