V. Buss et K. Kolster, ELECTRONIC-STRUCTURE CALCULATIONS ON HELICENES - CONCERNING THE CHIRALITY OF HELICALLY TWISTED AROMATIC SYSTEMS, Chemical physics, 203(3), 1996, pp. 309-316
CNDO/S calculations including configuration interaction of up to 300 s
ingly excited states have been performed on the MMP2 and PM3 optimized
geometries of [n]-helicenes, with n from 3 to 9. Oscillator and rotat
ory strengths have been calculated for all states down to 200 nm, with
generally good agreement in the long wavelength regions. The computed
data are analyzed on the basis of a four-orbital model yielding two f
orbidden L and two allowed B states. The latter ones including signs c
orrelate with two high intensity absorptions of opposite signs observe
d in the spectra of all helicenes except the smallest one. The descrip
tion in terms of tangential and radial transition dipoles provides a p
hysical picture of the excited states. It is shown that the -,+ patter
n of these bands is conclusive evidence for the M-helical absolute con
figuration of these compounds.