REPORT C D curves, w h i l e the discreteness of C D curves is p a r t i c u l a r l y useful i n i d e n t i f y i n g overlapping or hidden o p t i c a l l y active transitions. CD measurements are also to be preferred where studies of r o t a t i o n a l strengths of i n d i v i d u a l transitions are of importance. W h e n expressed i n terms of e l l i p t i c i t y , the m a g nitude of peak C D due t o a single o p t i c a l l y active t r a n s i t i o n is app r o x i m a t e l y equal to the peak t o t r o u g h magnitude of the corresponding O R D curve and, i n p r i n ciple at least, the u l t i m a t e sensit i v i t i e s of systems measuring C D or O R D are v e r y nearly the same. Theories of Optical
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I n 1848 Pasteur (2J+) presented the first molecular theory of o p t i c a l a c t i v i t y ; he proposed t h a t molecules, like geometrical figures, m a y be d i v i d e d i n t o t w o classes: those w i t h superposable m i r r o r images and those w i t h nonsuperposable images. Pasteur postulated t h a t molecules belonging to the second class exhibit optical r o t a t o r y power. T h e t e r m "molecular d i s s y m m e t r y " was introduced to characterize t h i s class. F u r t h e r , Pasteur speculated t h a t molecular d i s s y m m e t r y m a y be a t t r i b u t e d to some f o r m of dissymmetric grouping of atoms i n a h e l i cal or tetrahedral configuration. I n 1874, L e B e l adopted the t e t r a hedral model of the carbon a t o m and associated o p t i c a l a c t i v i t y w i t h centers of a s y m m e t r y i n the active compound (24) • V a n ' t Hoff, i n 1874, distinguished between t w o types of o p t i c a l l y active compounds and a t t r i b u t e d optical a c t i v i t y to either of t w o properties: the presence of an asymmetric center i n the molecule or an o\ r erall dissymmetric structure containing no asymmetric centers. I n 1893, W e r n e r used available data on o p t i c a l r o t a t i o n of m e t a l complexes to establish octahedral s t r u c t u r a l models e x h i b i t i n g dissymmetric configurations (24). D r u d e formulated, i n 1896, a physical theory of optical a c t i v i t y based on the assumption t h a t i n a dissymmetric structure, charged particles are constrained t o move along helical paths (18). T h e t i m e dependent electric and magnetic fields associated w i t h an incident wave of electromagnetic r a d i a t i o n induce electronic motions w h i c h i n
