Theoretical and infrared spectroscopy study of the conformational preferences for some 3-Monosubstituted-2-Methylpropenes

Abstract

The infrared spectra of 3-X-2-methylpropenes (X = Cl, Br, I, NMe2, NEt2, OH, OMe, OEt, SH, SMe and SEt) have been recorded at room temperature in CCl4 solution. The CC stretching mode was analyzed and compared to theoretically calculated data to give insight about the conformational isomerism of these compounds. A combination band systematically appears in all spectra (except for amines); the remaining νCC band and the corresponding intensities were used to obtain the conformer populations. For second-period atoms bonded to C-3, two or more conformers are observed in CCl4 solution, but when substitution by heavier atoms takes place, only one conformer is observed under the tested conditions, i.e., the gauche form with respect to the CCCX dihedral angle. Therefore, steric hindrance between X and CH2 in the s-cis form is strongly dependent on the heteroatom size, while the effect of the alkyl chain bonded to the heteroatom seems to be of secondary importance.