The infrared spectra of 3-X-2-methylpropenes (X = Cl, Br, I, NMe(2), NEt(2), OH, OMe, OEt, SH, SMe and SEt) have been recorded at room temperature in CCl(4) solution. The C=C 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 ν(C=C) 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 CCl(4) 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 C=C-C-X dihedral angle. Therefore, steric hindrance between X and =CH(2) 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.
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