Bisignate CD curves

Correspondingly, Cotton effects are observed in the area of uv absorption maxima or shoulders. It has been stated that the most useful of these chro-mophoric derivatives are the 3-Npy derivatives since the 5-Npy derivatives show much higher absorption and hence much less favorable Aa/a ratios. The 3,5-Dnpy derivatives also have high uv absorption maxima and, in addition, could possibly present complications arising from the presence of bisignate CD curves. The latter could actually be a helpful analytical criterion in certain cases. 

Another way of looking into this problem of bisignated CD-curves is the following the a-band of benzene with D -symmetry is electrically as well as magnetically forbidden, the two branches (8,8 ) of the degenerate E -transi-tion are magnetically forbidden, but electrically allowed (and polarized in x-and y-direction). Addition to the two rings as in fig. 7 reduces symmetry to... 

Fig. 14 Bisignate CD-curves within the a-band of 3 related oxygensubstituted decahydrophenanthrenes, and CD of a related glycol allowing internal hydrogen bridging.

FIGURE 15. Typical bisignate exciton coupling CD spectra (upper) and bell-shaped UV (lower) spectra for chromophores with electronic transitions near 300 nm. The shape of the observed CD curve is due to overlapping, oppositely-signed positive and negative CD transitions from electronic excitation into (two) exciton states... 

Fig. 6 CD-curves of 6 and 7 with unusually pronounced bisignate finestructure.

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