AN EXPERIMENTAL AND THEORETICAL-STUDY OF THE STEREOCHEMICAL PROPERTIES OF 2-TERT-BUTYL-1,3-BUTADIENE AND 2,3-DI-TERT-BUTYL-1,3-BUTADIENE

The molecular structure and conformation of 2-tert-butyl-1,3-butadiene (1) and of 2,3-di-tert-butyl-1,3-butadiene (2) have been studied by the gas electron diffraction method. The monosubstituted compound is observed to have a gauche orientation (phi = 32-degrees) of the conjugated C=C bonds. The forces determining the preferred conformation of this molecule (syn or gauche) appear to be similar to those operative in the case of the high-energy conformer of unsubstituted 1,3-butadiene. The disubstiluted compound has an approximately perpendicular orientation of the C=C-C=C system (phi = 101.5-degrees), and in this case the preferred orientation is described as a result of minimalization of the nonbonded interactions. - Theoretical calculations of the structure and conformation of the two molecules have been carried out by molecular mechanics (MM3) and by ab initio (HF/3-21G) methods. The results from both sets of calculations are in good agreement with the observed structures of the two compounds.