Spectroscopic methods unique to optically active molecules

4 Spectroscopic methods unique to opticaiiy active moiecuies 

If a weight at the end of a piece of string is set in motion so that it traverses a circular path, this motion may be regarded as composed of two mutually 

Consider what happens when each of the circularly polarized components is propagated along the threefold axis of the optically active ion [Co(en)3] . From Fig. 12.9 it is evident that one of the circularly polarized components tends to pass along the backbone of the ethylenediamine ligands whilst the other tends to cut the backbone perpendicularly. This description is inaccurate in that the wavelength of visible light (and it is in this that we shall be interested) is of the order of a thousand times greater than the molecular dimensions. Nevertheless, the vital point remains—the two 

9 Two circularly polarized beams of light passing through a molecule of [Co(en)3], the threefold axis of the latter being coincident with the axis of propagation of the light. One circularly polarized beam (the top one) tends to cut the backbone of an ethylenediamine molecule, the other to pass along it. 

It is well known that the absorption of light by a molecule is frequently anisotropic—and this anisotropic absorption occurs simply because molecules are frequently anisotropic. Thus, a light wave incident on a molecule in one direction may excite a transition but, light of the same wavelength, incident 

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Spectroscopic methods unique to optically active molecules 277... 

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