REVISED VIBRATIONAL ASSIGNMENT FOR CYCLOBUTENE

Abstract

Lord and $Rea^{1}$ and Aleksanyan and $Garkusha^{2}$ observed the infrared and Raman spectra of cyclobutene and proposed assignments of its vibrational fundamentals. Baggott, Clase and $Mills^{3}$ contributed a higher resolution, FT-infrared study and reevaluated the assignments. For this molecule of $C_{2v}$ symmetry with 24 fundamentals, they left $v_{20}$ (in-plane ring bend) unassigned and gave provisional assignments for three of the five $a_{2}$ 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 $a_{2}$ modes of cyclobutene and about the previous characterizations of some of the $a_{2}$ and $b_{2}$ modes of this molecule. We have examined the Raman spectrum of liquid cyclobutene at - $100^{\circ} C$. We contirm $v_{10}(a_{2})$ at $1142 cm^{-1}$ and propose $V_{11}(a_{2}$ at $1011 cm^{-1}, v_{12}(a_{2}) 816 cm^{-1}$, and $v_{20}(b_{2}$) at $888 cm^{-1}$. We concur with Baggott, Clase, and Mills on all of the other fundamentals. $v_{12}(a_{2}$) and $v_{24}(b_{2}$) 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 $Krainov.^{4}$