Theoretical study of the structures and racemization barriers of [n]helicenes (n = 3–6, 8) Original Research Article

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 C2 symmetry and a twisted transition state having a Cs 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.