Theoretical study of the structures and racemization barriers of [n]helicenes (n=3-6,8)

The minima and saddlepoint structures of [n]helicenes (n = 3-6, 8) have been optimized by semiempirical AM1 and ab initio SCF methods. The racemization barriers have been calculated as the energy difference between the ground state minima possessing C-2 symmetry and a twisted transition state having a C-s structure. A qualitatively correct dependence of the barrier heights with n in comparison with experimental data is obtained with both methods showing the reliability of the assumed reaction path. The relative large overestimations in the ab initio SCF barriers (10-14 kcal/mol for n > 5) are attributed to neglect of correlation energy which is larger in the more strained transition state than in the ground state structures. Single-point density-functional calculations yield barrier heights for the racemization of all helicenes accurate to +/-1 kcal/mol.