*4.3.5.1.3 Complex Cotton effect curve*

For compound with two or more different chromophores, its ORD curve may possess multiple peaks and troughs, which is called complex Cotton effect curve. Each ORD curve is the average effect of each chromophore in the molecule, and the contribution of each orientation and conformation of the molecule. Hence the Cotton effect curve is often complex.

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*Analytical Methods of Isolation and Identification DOI: http://dx.doi.org/10.5772/intechopen.88122*

*4.3.5.2 Circular dichroism (CD) spectrum*

*The ORD and CD spectrum of (+)-camphor.*

and Δє is as follows.

**Figure 11.**

**Figure 12.**

*The Cotton effect curves of* △*<sup>5</sup>*

*opposite absolute configuration (−) cotton.*

Optically active compounds have different molar absorption coefficients for left-circularly and right-circularly polarized light that make up plane polarized light, which is called circular dichroism (CD). The difference value between the two molar absorption coefficients (Δє = єL−єR) changes with the wavelength of the incident polarized light. With Δє as the ordinate, the wavelength as the abscissa, the spectrum obtained is called circular dichroism spectrum. Because the absolute value of Δє is very small, it is often replaced by molar ellipticity [*θ*]. The relationship between [*θ*]

*-cholestenone (A) natural cholesterone (+) cotton; (B) Cholesterone in the* 

[θ] = 3300Δє. (1)

more widely used than ORD in the study of chiral compounds.

Because Δє could be positive or negative, the circular dichroism curve also could be classified as positive and negative. In the CD spectrum showing positive Cotton effect, only a peak appears near the λmax of the chromophore in the molecule. Conversely, a trough appears in the CD spectrum showing negative Cotton effect. Therefore, CD spectra are simpler and easier to analyze than ORD spectra. For example, the ORD and CD spectra of (+)-camphor are shown in **Figure 12**. CD is

**Figure 10.** *ORD plain curves (A: Positive plain curve; B: Negative plain curve).*

*Analytical Methods of Isolation and Identification DOI: http://dx.doi.org/10.5772/intechopen.88122*

#### **Figure 11.**

*Phytochemicals in Human Health*

ORD curves are as follows.

*4.3.5.1.2 The cotton effect curve*

the peak. Cotton curves of △<sup>5</sup>

*4.3.5.1.3 Complex Cotton effect curve*

Cotton effect curve is often complex.

*ORD plain curves (A: Positive plain curve; B: Negative plain curve).*

*4.3.5.1.1 Plain curves*

other spectroscopic methods [28].

*4.3.5.1 Optical rotary dispersion (ORD) spectrum*

could provide information of absolute configurations, dominant conformations, and reaction mechanisms of chiral compounds, that cannot be replaced by any

The specific rotation [α] of a chiral compound depends upon the wavelength of the monochromatic light wave. The measurement of specific rotation as a function of wavelength is called optical rotator dispersion (ORD). The common types of

The ORD spectrum of an optically active compound with no chromophores is plain without peaks and troughs. An ORD curve of specific rotation increases with decrease of wavelength which is called positive plain curve, while in the case of negative plain curve, negative rotation increases with decrease of wavelength (see **Figure 10**).

If there is a simple chromophore in the molecule, the ORD curve is very different from plain curve. Near the absorption wavelength region of chromophore, a peak and a trough are exhibited, which is called the Cotton effect, and the spectrum drawn is called the Cotton effect curve. The spectrum with only one peak and one trough is called pure Cotton effect curve, while the spectrum with several peaks and troughs is called complex Cotton effect curve. The Cotton effect is called positive when the trough is observed at a shorter wavelength then peak. Conversely, the Cotton effect is called negative if the trough is observed at a longer wavelength than

and B possess the same structural formula, while different opposite configurations.

For compound with two or more different chromophores, its ORD curve may possess multiple peaks and troughs, which is called complex Cotton effect curve. Each ORD curve is the average effect of each chromophore in the molecule, and the contribution of each orientation and conformation of the molecule. Hence the


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**Figure 10.**

*The Cotton effect curves of* △*<sup>5</sup> -cholestenone (A) natural cholesterone (+) cotton; (B) Cholesterone in the opposite absolute configuration (−) cotton.*

**Figure 12.** *The ORD and CD spectrum of (+)-camphor.*

#### *4.3.5.2 Circular dichroism (CD) spectrum*

Optically active compounds have different molar absorption coefficients for left-circularly and right-circularly polarized light that make up plane polarized light, which is called circular dichroism (CD). The difference value between the two molar absorption coefficients (Δє = єL−єR) changes with the wavelength of the incident polarized light. With Δє as the ordinate, the wavelength as the abscissa, the spectrum obtained is called circular dichroism spectrum. Because the absolute value of Δє is very small, it is often replaced by molar ellipticity [*θ*]. The relationship between [*θ*] and Δє is as follows.

[θ] = 3300Δє. (1)

Because Δє could be positive or negative, the circular dichroism curve also could be classified as positive and negative. In the CD spectrum showing positive Cotton effect, only a peak appears near the λmax of the chromophore in the molecule. Conversely, a trough appears in the CD spectrum showing negative Cotton effect. Therefore, CD spectra are simpler and easier to analyze than ORD spectra. For example, the ORD and CD spectra of (+)-camphor are shown in **Figure 12**. CD is more widely used than ORD in the study of chiral compounds.

#### *4.3.6 Single crystal X-ray diffraction method*

Single crystal X-ray diffraction could be applied independently to analyze the structures, components, contents, configurations, conformations, solvents, and crystal forms of samples. It is widely used in the stereostructural study of natural compounds, synthetic compounds, peptides, proteins, etc. Therefore, X-ray diffraction analysis is a necessary physical method in the field of structure and function research of modern natural drugs.

Single crystal X-ray diffraction is a kind of quantitative analysis technology, which can provide three-dimensional structural information of molecules, including atomic coordinates, bond length, bond angles, dihedral angles, hydrogen bonds, salt bonds, coordinate bonds, and so on. In addition, it is also a reliable method to determine the absolute configuration of chiral drug molecules and the epimers in the stereochemical structures. For example, see [29].
