Differences Of Circular Dichroism (CD) Fundamentals

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Circular Dichroism (CD) fundamentals.
CD is defined as the difference between absorptions of left (Al) and right (Ar) circularly polarized light beams,
For each absorption band, the CD of two enantiomers are always exactly opposite.
For each electronic transition one can define an electric (µij≠0) and a magnetic (mij≠0) transition dipole. Both can lead to absorption of radiation, the intensity of which is directly related to the oscillator strength f, fij≈| µij |2+ | mij |2
In CD spectroscopy, in contrast to an electronic absorption (where electronic dipole >> magnetic dipole) spectroscopy, both transition dipole moments are important. Therefore, an integral of CD (Rij) band is proportional to the scalar product of 2 dipoles:
Rij≈ µij * mij (both should be vectors)
Rij is defined as rotational strength, which is opposite in sign for 2 chiral molecules. Integral of CD over the whole EM spectrum is zero.
Chromophore is a molecular moiety, responsible for electronic transitions in UV or in the visible range. Chromophores can be classified in terms of chiral spheres. Intrinsically chiral
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If a molecule contains a chromophore of complex or not well understood nature, then 2 chromophores with absorbance shifted to the red are introduced. Chrom group (p.922, Fig. 9) of chromophores are suitable for -OH group derivatization and protonated Shiff bases are suitable for amino groups. Porphyrins possess a very intense sharp and narrow Soret band, therefore can be used as versatile chromohores, providing an unsurpassed exciton couplet amplitude of A=570 and allowing CD coupling over the large distance of 40-50 Å. In complex molecules with multiple chromophores, matrix method or computational methods can be

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