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) hav e been optimized by semiempirical AM1 and ab initio SCF methods. The r acemization barriers have been calculated as the energy difference bet ween the ground state minima possessing C-2 symmetry and a twisted tra nsition state having a C-s structure. A qualitatively correct dependen ce of the barrier heights with n in comparison with experimental data is obtained with both methods showing the reliability of the assumed r eaction path. The relative large overestimations in the ab initio SCF barriers (10-14 kcal/mol for n > 5) are attributed to neglect of corre lation energy which is larger in the more strained transition state th an in the ground state structures. Single-point density-functional cal culations yield barrier heights for the racemization of all helicenes accurate to +/-1 kcal/mol.