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East Anglia, NorwichThe absorption and c.d. spectra of [7]-helicene are reported, together with a re-examination of the electronic spectrum of [6]-helicene. The transition energies and dipole and rotational strengths of [6]-and [7]-helicene have been evaluated in the n-SCF approximation, and theoretical absorption and c.d. spectra are derived on the assumption that the absorption bands have Gaussian forms. It is found that the degree of correspondence between the experimental and theoretical spectra declines with increasing molecular size over the series [5]-, [6]-, and [7] -helicene, and the significance of u-n interaction and of n-electron exchange between the nearest-neighbour non-bonded carbon atoms in the terminal rings of the higher helicenes are discussed. It is concluded that the (+)-isomers of [4] -, [5] -, [6] -, and [7]helicene have a right-handed helical configuration. THE sixth and seventh members of the helicene series, now extending to [13]-helicene,l are of particular electronic and stereochemical interest in that [6]helicene completes one turn of the general helicene structure and [7]-helicene3 is the first member of the series in which the terminal benzene rings facially overlap. The enantiomers of these overcrowded aromatic hydrocarbons have large optical activities, [a]D26 3750 for [6]-helicene and [ U . ]~~O 6200 for [7]-helicene,6 and the rotatory power of the former, representing the classical example of an inherently dissymmetric chromophore, has attracted considerable attention.Fitts and Kirkwood? by using the polarisability theory of optical activity,' showed that six benzene rings disposed in the form of a right-handed helix with a pitch of 3-80A gives a positive rotation, [a4, +3010. The 0.r.d. curve of [6]-helicene was calculated in the Huckel n-approximation from the one-electron theory of optical activity by Moscowitz,Q who obtained a quantitative agreement between the theoretical and experimental curves over the first two absorption bands and to lower frequencies. A more approximate 0.r.d. curve of [6]helicene was calculated by Tinoco and Woody,l0 by the free-electron method and with a simpler structural model of the hydrocarbon, which consisted of a singlestrand helix with radius 2-50 and pitch 3.80 A. It was found lo that in the free-electron model, right-handed helical x-systems are generally dextrorotatory at wavelengths longer than those of the absorption bands of the system. The same general conclusion emerges from the bond-polarisability helix model of Brewster, who has calculated for [6]-helicene a molecular rotation of the order of magnitude observed.llThe polarised excitation and partial c.d. spectrum of [6]-helicene has been recorded by Weigang and cot Present address .

The absorption and circular dichroism spectra of (-)D-l,12-dimethylbenzo[c]phenanthrene-5-acetic acid are reported, together with the polarized excitation and fluorescence spectra of the parent hydrocarbon, [4]-helicene. The transition energies of 1,12-dimethyl-[4]-helicene calculated in the ~r-SCF approximation, and the corresponding dipole and rotational strengths obtained by the dipole-velocity method, show that the optical isomer studied has the M-configuration with the stereochemical form of a segment of a left-handed helix. Analogous calculations in the INDO approximation using both the dipole-velocity and the dipole-length procedure with separate consideration of the one-centre and the two-centre transition moment integrals, indicate that the more satisfactory dipole and rotational strengths are obtained in the two-centre dipole--velocity formulation. The appearance of a~zt* transitions at higher energies limits the application of the 1r-SCF procedure, but the INDO method is found to place these transitions at too low an energy.

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