Circular dichroism and ORD: reading Cotton effect curves
Learn to read ORD and CD curves, fix the sign of a Cotton effect and use the octant rule for ketones, with two worked practice questions and a study plan.
This post shows you how to read an ORD curve and a CD band, and how to turn the sign into an absolute configuration.
You will also solve two practice questions and get a short plan for revision.
What ORD and CD actually measure
Ordinary light vibrates in one plane. Circularly polarized light is different: its electric vector rotates around the direction of travel, like a corkscrew. It can rotate to the left (LCP) or to the right (RCP).
A chiral molecule treats the two kinds differently. This property is called optical activity. It shows up in two ways.
- Optical rotatory dispersion (ORD): the optical rotation is measured at many wavelengths and plotted against wavelength. Rotation arises because the two circular components travel at different speeds in the sample.
- Circular dichroism (CD): the sample absorbs LCP and RCP light to different extents. The CD value is Δε = εL − εR, where ε is the molar absorptivity (how strongly the sample absorbs).
The two techniques are linked. CD is seen only where the molecule absorbs light. ORD can be seen even far from an absorption band. That is why ORD shows a long tail on both sides of a band, while CD shows a band only at the absorption.
Reading a Cotton effect curve
Far from any absorption, an ORD curve rises or falls smoothly. This is a plain curve. It tells you the sign of rotation but gives little structural information.
Near an absorbing chromophore (the part of the molecule that absorbs light), the curve changes sharply. It shows a peak and a trough, and it crosses zero close to the absorption maximum. This S-shaped feature is the Cotton effect.
The sign follows a simple rule. Read the curve from long wavelength to short wavelength:
- If you meet the peak first, then the trough, the Cotton effect is positive.
- If you meet the trough first, then the peak, the Cotton effect is negative.
In CD, the sign is easier. A positive Cotton effect gives a positive CD band (εL > εR). A negative Cotton effect gives a negative CD band. The CD band sits at the absorption maximum, where the ORD curve crosses zero.
| Feature | ORD | CD |
|---|---|---|
| What is plotted | Rotation vs wavelength | Δε vs wavelength |
| Shape near a chromophore | Peak and trough (S-shaped) | Single band |
| Positive Cotton effect | Peak at longer wavelength, trough at shorter | Positive band |
| Negative Cotton effect | Trough at longer wavelength, peak at shorter | Negative band |
| Seen away from absorption? | Yes, as a tail | No |
| Enantiomer | Mirror-image curve | Mirror-image band |
The last row matters in exams. Enantiomers give curves of equal size and opposite sign. So if you know the sign for one enantiomer, you know it for the other.
The octant rule for saturated ketones
A saturated ketone has a weak n→π* transition. An electron moves from a lone pair on oxygen to the π* orbital of C=O. This is the band whose Cotton effect we study. The octant rule tells you its sign.
Here is how to apply it, step by step.
- Draw the ketone so that you look along the C=O bond from the oxygen end toward the carbon. The carbonyl then points away from you.
- Divide space into eight octants using three planes. Two planes cut through the carbonyl group, and the third is perpendicular to the C=O axis near the carbonyl carbon.
- Most of the molecule lies behind the carbonyl carbon, in the four rear octants. Mark each substituent in its octant.
- Give each substituent the sign of its octant. In the rear, upper-left and lower-right are positive. Upper-right and lower-left are negative. Front octants have the opposite signs.
- Substituents lying on a plane contribute nothing. Substituents in opposite-sign octants cancel if they are equal in size.
- The net sign is the sign of the Cotton effect.
Hydrogen atoms are usually ignored. Look for the largest atoms or groups that are not on a plane. In a rigid ring such as a cyclohexanone chair, the ring carbons often cancel in pairs. The substituent that breaks the symmetry decides the sign.
The octant rule is for saturated ketones. Do not apply it blindly to conjugated ketones or to other chromophores. Those have their own empirical rules.
Exam angle: how this topic is asked
Questions on this topic usually test one of three skills.
- Naming the Cotton effect as positive or negative from a drawn ORD curve.
- Converting between ORD and CD, including the mirror-image logic for enantiomers.
- Placing a substituent in an octant and stating the sign for a given ketone, or working backwards to the configuration.
Depth varies from one university to another. Some syllabi stop at definitions of ORD, CD and the Cotton effect. Others expect the octant rule with applications. Before you decide how deep to go, open the chemistry syllabus page on the ABC Chemistry site. It lists official syllabus links for universities. You can search by university, city or state, and there is a filter to show only those with a syllabus link. The links were checked on 24 Oct 2026, so always confirm the current session on your university's own page.
Two practice questions, fully worked
These are practice questions written for this post. They are not past-paper questions.
Practice question 1. A chiral ketone is studied by CD at its n→π* band. At the absorption maximum, εL = 12.3 and εR = 11.8. (a) Find Δε and state the sign of the Cotton effect. (b) Describe the ORD curve. (c) What would its enantiomer show?
Solution.
(a) Δε = εL − εR = 12.3 − 11.8 = +0.5. The CD band is positive, so the Cotton effect is positive.
(b) A positive Cotton effect means the ORD curve shows a peak at longer wavelength and a trough at shorter wavelength. It crosses zero near the absorption maximum.
(c) The enantiomer shows the mirror image. Its Δε is −0.5 at the same wavelength, so the Cotton effect is negative. Its ORD curve has the trough first and the peak second.
Practice question 2. An optically active cyclohexanone shows an ORD curve with a trough at longer wavelength, a peak at shorter wavelength, and a zero crossing at the n→π* absorption maximum between them. Looking from oxygen toward carbon, the only substituent that does not cancel is a methyl group. It is either in the rear upper-left octant or in the rear lower-left octant. Which one is it? What does the CD curve look like?
Solution.
Step 1. Read the curve from long to short wavelength. The trough comes first and the peak comes second. The Cotton effect is negative.
Step 2. The CD band is therefore negative. It is centred where the ORD curve crosses zero.
Step 3. Match the sign to the octant. Rear upper-left is positive, and rear lower-left is negative. A negative net sign means the methyl group lies in the rear lower-left octant.
Step 4. If the methyl group had been in the rear upper-left octant, the Cotton effect would be positive and the compound would be the mirror-image arrangement. Once you know the octant, use the drawn structure and the priority rules to assign R or S in the usual way.
A short study plan
Spread this topic over one week. Keep each session short and active.
- Day 1: Write the definitions of ORD, CD, Δε and the Cotton effect from memory. Check them.
- Day 2: Sketch positive and negative ORD curves and the matching CD bands several times. Label peak, trough and zero crossing.
- Day 3: Practise drawing the three planes and the eight octants. Mark the four rear signs without looking.
- Day 4: Place substituents on drawn cyclohexanones. Say aloud which ones cancel.
- Day 5: Solve questions that go backwards, from the sign to the octant.
- Day 6: Revise the limits of the octant rule and the difference between CD and ORD.
- Day 7: Redo both practice questions above without notes.
Key takeaways
- CD is Δε = εL − εR. ORD plots rotation against wavelength. Both are signs of optical activity.
- Read an ORD curve from long to short wavelength. Peak first means positive. Trough first means negative.
- Enantiomers give mirror-image ORD and CD curves.
- For saturated ketones, view from oxygen toward carbon, mark substituents in the rear octants and add their signs.
- Check your own university's syllabus for how deep the octant rule goes.
Frequently asked questions
Why is CD easier to read than ORD?
CD gives one band at the absorption maximum, and its sign is the sign of the Cotton effect. ORD gives a peak and a trough, so you must read the order of the two.
Can ORD be seen where the molecule does not absorb?
Yes. ORD shows a tail even far from an absorption band. CD appears only at wavelengths where the sample absorbs.
Does the octant rule work for every chromophore?
No. It is meant for the n→π* band of saturated ketones. Other chromophores need other rules, so check which one your question uses.
What does a zero in an octant mean?
A group lying exactly on one of the three planes contributes nothing. Equal groups in octants of opposite sign also cancel each other.
Is there a technique that works in the infrared?
Yes. Vibrational circular dichroism extends CD into the infrared. It is used with calculated spectra to find the absolute configuration of small organic molecules in solution.
