Infrared and Raman study and ab initio calculations of internal rotation in methylisopropenyl ether

Abstract

The structure of methylisopropenyl ether and its rotational isomerism have been studied both by ab initio calculations and by IR and Raman spectra. The internal potential energy function shows the existence of two isomers; the more stable one has a planar s-cis structure, the second is predicted at 4.73 kcal mol −1 higher in a planar s-trans form with an interconversion barrier of about 7.5 kcal mol −1 at a torsional angle of 97°. The computed barrier heights to internal rotation of the two methyl groups are quite different: 0.9 kcal mol −1 for the methyl on carbon and 4.5 kcal mol −1 for the methyl on oxygen. Correlation of the orbital energies is made both with respect to methylvinyl ether and methane and to propene and methanol. The first two ionization potentials have also been measured at 8.97 and 11.7 eV. The vibrational spectra show that only the s-cis isomer can be found. An assignment of the complete spectra is proposed; the torsional frequency of the methyl on carbon could not be detected. V 3 for the torsion of the methyl on oxygen, derived from the torsional frequency, is 3.77 kcal mol −1. An estimate of the potential parameters for the isopropenyl-methoxy torsion yields V ∗ = 33.98 kcal mol −1 .