Chiroptical Properties of Some Monoazapentahelicenes

Three closely related previously synthesized monoaza[5]helicenes have been resolved into their enantiomers via enantioselective HPLC using a cellulose-derivative Chiralcel OD column. Circular dichroism (CD) spectra of the enantiomerically enriched samples have been recorded and assigned. The spectra were analyzed as a function of time, and different rate constants were found in the kinetics of racemization for the three molecules. Ab initio DFT calculations for the ground electronic states were employed to determine minima and saddle point structures and to understand the racemization process. The theoretical geometries compared well with those from X-ray structures. CD spectra were calculated by TD-DFT ab initio methods, and compared with experimental data.