Rotational Isomers of Vinyl Ethers. Studies of the Vibrational Spectra and Dipole Moments of cis- and trans-β-Chlorovinyl Methyl Ethers

Abstract

Abstract Raman and infrared spectra of cis- and trans-β-chlorovinyl methyl ethers (ClCHCHOCH3) were measured in the vapor and liquid states, and the infrared spectra in the solid state were also measured for the trans compound. The dipole moments were measured for both of the compounds in a benzene solution. From the observed value of the dipole moments, it was decided that the compound having the lower boiling point had a trans configuration and the other, a cis configuration. On the basis of these configuration, interpretation of the vibrational spectra of both compounds was undertaken. As for the trans compound, the existence of the rotational isomers arising from the internal rotation around the C–O axis was verified by the fact that some of the infrared bands disappeared on solidification of the samples. The spectra in the liquid and vapor states were interpreted as a mixture of those of two rotational isomers, and tentative assignments were given for one of the isomers by reference to the spectra of methyl vinyl ether. The most probable form of the molecule was supposed as the planar zigzag form. On the contrary, it was established that there existed no rotational isomer in the cis compound in any state. The spectra of the cis compound did not resemble those of the trans compound or the methyl vinyl ether and were interpreted as a superposition of group frequencies of the ether linkage and of the –CH=CH- group. Consequently, a non-planar model was proposed, and the angle of internal rotation was determined to be about 90° from the observed value of the dipole moment. In order to explain the difference in the spacial configuration between the cis and the trans compounds, the electrostatic effect, in addition to the resonance effect and the steric repulsion, was considered. From the results of the calculation of the dipole-dipole interaction, it was shown that this electrostatic force played a very significant role in the internal rotation.