Theoretical investigation of the structure and spectra of donor–acceptor azobenzenes

Calculations are reported on the structure of 4-dimethylamino-4′-nitroazobenzene using the AM1, CNDO/2, STO-3G and 3-21G methods and the results are compared with crystallographic data on related structures. Subsequent spectroscopic calculations with a version of the CNDO/S method give transition energies which are highly dependent on the starting structure adopted. Similar trends are found with the calculated structure of the corresponding 4-dimethylamino-2′-nitroazobenzene, although here steric interactions are predicted by the AM1 and STO-3G methods to twist the aromatic rings and also force the nitro group into a non-planar conformation.