Conformational reactions of D2-symmetric twisted acenes

Computational analyses of the longitudinally twisted polycyclic aromatic hydrocarbons 9,18-diphenyltetrabenz[a,c,h,j]anthracene (1), 9,10,19,20-tetraphenyltetrabenzo[a,c,j,l]naphthacene (2), and 9,10,11,20,21,22-hexaphenyltetrabenzo[a,c,l,n]pentacene (3) were performed at the B3LYP/6-31G(d) level of theory. Ground state and transition state structures were located for two classes of conformational reactions in these molecules: twist inversions (enantiomerization or racemization reactions) and phenyl rotations, and the free energies of activation for these processes were calculated. Where possible, the computational results were compared with both new and existing experimental data for the rates of these conformational reactions. Multiple pathways were identified for some processes, but the low-energy transition states were often quite different from those that common chemical intuition might suggest.