Principles of CD/ORD (3): Features of ORD, CD measurements

November 1, 2019

⁃ Cotton effect

Fig. 6 schematically shows the relationship between the CD/ORD spectra and the UV spectrum in the absorption wavelength region. Within the UV spectral range, anomalous dispersion of the ORD spectrum and a peak in the CD are observed. These two phenomena are called the Cotton effect, and provide clues to the configuration and conformation of optically active substances. Three items of information can be obtained from the Cotton effect:
1: Sign (±): CD (+), ORD (long-wavelength maximum)

2: Magnitude: CD peak value, ORD peak, valley amplitude

3: Position: CD peak wavelength = ORD inflection point    

Fig. 6 Typical CD, ORD, and UV spectra

Fig. 7 shows the UV, ORD and CD spectra of (1S)-(+)-10-camphorsulfonic acid ammonium salt. The Cotton effect is seen in the wavelength range of 220 to 350 nm. When the maximum ORD value is at a long wavelength as in this case, this is called a positive Cotton effect. When the maximum is at a short wavelength , it is called a negative Cotton effect. The (1R)-(-)-10-camphorsulfonic acid ammonium salt, which is an optical isomer of (1S)-(+)-10-camphorsulfonic acid ammonium salt, has an ORD maximum at a short wavelength. By measuring the ORD and CD spectra, it can be determined whether the D- or L-form is present.

Fig. 7 Cotton effect in (1S)-(+)-10-camphorsulfonic acid ammonium salt

 

⁃ Characteristics of CD/ORD method

CD and ORD measurements are suitable for detecting conformational changes and molecular interactions, rather than analyzing the detailed structure of a substance. In addition, CD measurement is superior to ORD for ease of analysis and measurement of the Cotton effect.

 

Characteristics ORD CD
Cotton effect profile Complicated Simple and good separation
Ease of application Difficult * Easy
Quantitative analysis Possible Possible
Identification of sample Suitable Difficult
Measurement target Optically active substances Compounds with unequal absorption

*sensitive to turbidity and birefringence