Racemization Barriers of Helicenes:  A Computational Study¹

The racemization barriers of pentahelicene up to nonahelicene have been computed with AM1, MNDO, and PM3. All methods lead to C_s transition states which have lower energy than those with C_2v symmetry. The barriers calculated by AM1 match the experimental values best for all helicenes. The reliability of the results has been confirmed by ab initio methods using the B3LYP functional with the 3-21G basis set as implemented in the GAUSSIAN94 package. Furthermore, 12 methyl-substituted helicenes have been computed with the AM1 method. The racemization barriers of 1-methyl-substituted penta- and hexahelicene are at least as high as that of the next higher unsubstituted helicene. A second methyl group in the 1‘-position increases the barrier further, while methyl groups in the 2-position do not have a severe influence on the racemization barrier.