Conformational Analysis

Abstract

Assembling a molecule with a modelling kit makes it already clear that rotations around single bonds can be easily carried out. The molecule will achieve a different shape, or as the chemists say, it is transformed into a different conformation. In a real molecule, rotations around these bonds are not fully free. They are subjected to a potential and the molecule adopts during the rotation particular, energetically favorable arrangements. n-Butane represents the simplest case (Fig. 16.1). The central torsion or dihedral angle determines the relative orientation of the two bonds to the methyl groups to one another. If n-butane is rotated out of the arrangement with the two bonds to the methyl groups in 180° orientation (trans), the methyl group at the “front” carbon and the hydrogen atom at the “back” carbon will directly coincide which each other at a rotation angle of 120° and 240° called “eclipsed”. In this geometry, they come closer to one another, therefore this arrangement is unfavorable for steric reasons. At a rotation angle of 60° and 300° the groups are again in a staggered geometry, which is an energetically more favorable situation. This arrangement is somewhat less favorable than the staggered trans orientation because of the spatial vicinity of the methyl groups, which are now said to be “gauche” to one another. Finally along the rotation path an orientation is adopted at 0° and 360° in which both methyl groups are exactly behind one another. This is an even less favorable orientation.