Question

Why can the Drude equation not be applied in the range of wavelengths near maximum absorbance?

Answer 1

The dependence of the rotational strength optical active molecules on the wavelength is described by the Optical Rotation Dispersion (ORD). It is specific to the chiral molecule and differs in intensity, but in general, shows higher rotational strength for shorter wavelength in the so called plain curve. It is connected with Drude´s equation and describes the normal behaviour of the ORD in absence of chromophores or far away from absorbtion band.

Drude´s equation : [a]l = S Ai / (l2 -l i )

(Ai constant molecular characteristic)
(li constant wavelength)

Within or nearby strong absorbtion band, the ORD curve may become anormal and will not obey Drude´s equation anymore, showing Cotton effects and may even change sign of rotation. Due to this, the light spectra for chiral detection is restricted by strong absorbtion and Cotton effects for short wavelength in spite of high optical rotation. going to longer wavelength (IR), the optical rotation becomes extremely weak and therefore gets unfit for chiral detection .