Can we use linear dichroism for non chiral molecules?
Circular dichroism id used for chiral molecules, but can we use chiral or non chiral molecules in linear dichroism and why?
Many thanks Dr. Padula, but the in-phase combination of R and L circularly polarized light passing through a chiral molecule gives also polarized light oriented in two perpendicular directions. So why the plots of LD is not identical or similar to that of ECD.
I am not sure about your nomenclature, but for me an "in-phase combination of R and L circularly polarized light" is nothing else but non-polarized light which is what you probably mean by "polarized light oriented in two perpendicular directions". There is no "passing through a chiral molecule" needed. In contrast, the chiral molecule would interact more with either R or L, therefore generating slightly circularly polarized light.
One word of caution concerning "linear dichroism": The so-called linear dichroism theory is an oversimplification of the actual situation, as it denies the existence of interfaces. Furthermore, it assumes that it is the absorbance that is proportional to the Cosinus squared of the angle between polarization direction and transition moment. This is humbug!
Please note this information from an article:
Optically active samples have distinct molar extinction coefficients
for left (EL) and right (ER) circularly polarized light (EL ≠
ER). This difference may be expressed as delta E. From
Lambert- Beer Law the difference in the absorbance of left
and right circularly polarized light delta A, can be given by
(Equation 1) delta A = AL – AR = delta E c l
where c is the concentration and l, the path length. When
AL and AR are absorbed to different extents, the resulting
radiation would be said to possess elliptical polarization.
This difference in absorbance, delta A, of the two
components, is a measure of CD, but it is generally
reported in terms of the ellipticity (Theta) in degrees, there
is a simple numerical relationship between delta A and
ellipticity in degrees given by equation 4
(Equation 2) CD = AL – AR
Not really! There you take either left or right circularly polarized light for a measurement and then you take the difference of the absorbance of both measurements. There is no "in-phase combination of R and L circularly polarized light"...
Any molecule (with a chromophore) is going to absorb R and L circularly polarised light (whose in-phase combination gives you plane polarised light). However, if a molecule is not chiral, such difference is going to be 0, thus you will observe no ECD signal.
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