Helicenes are benzologues of phenanthrene. With four or more rings (tetrahelicene,
or helicene) the compounds exist in two helical, enantiomeric forms, with C2 symmetry. Racemization
experiments, carried out in the seventies, gave surprising results: not only did racemization occur
at lower temperatures than expected from hardware models; the barriers even showed a tendency to level off for a
higher number of rings. Calculations confirmed that for - to helicene the racemization occurs via a
CS symmetric transition state (TS). For helicene shown in the animations on the left, the energy barrier to
racemization was measured as well as calculated to amount to about 175kJ/mol. Data and
transition state structures were taken from http://www.cmbi.kun.nl/~borkent/motm/deca.html.
For details see also R. H. Janke, G. Haufe, E.-U. Würthwein, and J. H. Borkent, J. Amer. Chem. Soc.1996, 118, 6031;
and S. Grimme and S. D. Peyerimhoff, Chem. Phys.1996, 204, 411.