CH437 CLASS 21
CHIROPTICAL METHODS FOR THE DETERMINATION OF ORGANIC
STRUCTURES: CIRCULAR DICHROISM (CD) AND OPTICAL ROTATORY
DISPERSION (ORD)
Synopsis. Definitions of CD and ORD, with examples. Origins of CD and ORD:
interactions of electromagnetic radiation with chiral molecules.
Introduction and Definitions
Chiroptical properties are properties of chiral substances arising from their
nondestructive interaction with anisotropic radiation (polarized light). These
properties can differentiate between the two enantiomers of a chiral compound.
Spectroscopic methods that can utilize these properties are called chiroptical
methods. They are summarized below.
Method
Principle
Accessible chromophore
Polarimetry and optical
Refraction
No
Circular dichroism (CD)
Absorption
Yes
Circular polarization of
Emission
Yes
rotatory dispersion (ORD)
emission (CPE)
Circular dichroism (CD) describes the anisotropic absorption of circularly
polarized light (cpl) by chiral samples that have an excess of one
enantiomer.
This occurs only in the spectral regions in which absorption bands are found –
here, focus will be on the UV-visible region (electronic transitions), but CD also
occurs in the IR region (vibrational transitions).
CD spectra are usually plotted as molar ellipticity [] against wavelength, as
shown below. A comparison with the UV-visible absorption spectrum is shown on
the same diagrams. The positive CD curve is known as a “positive Cotton Effect”
and similarly for the negative CD curve. There is a parallel with ORD (see later).
Optical Rotatory Dispersion (ORD)
Optical activity (or optical rotatory power) results from refraction of left and right
circularly polarized light (cpl) to different extents by chiral molecules.
This differential refraction is also known as circular birefringence or anisotropic
refraction. As the light passes through the sample, n R  nL, where n is the
refractive index.
Optical rotatory dispersion describes the variation of optical rotatory power with
wavelength.
Usually, specific molar rotation [], rather than specific rotation [], is plotted
against wavelength.
Plain or Normal ORD Curves
When there is no chromophore present that absorbs strongly in the wavelength
range of study, “normal” or “plain” ORD curves are obtained that obey Fresnel’s
equation,
 =
_
1800 nL nR l
0
(in o)
l is pathlength in dm
 0 is wavelength of light in cm
Plain ORD curves are illustrated for sterol enantiomers, below.
Anomolous ORD Curves
When a chromophore is present that absorbs somewhere in the wavelength
range of study, “anomalous” ORD curves result, in which the plot passes through
[] = 0 (the “crossover point”) at a particular wavelength that corresponds to
absorption spectrum max (max). This effect is also known as the “Cotton Effect”.
The ORD spectrum of (+)-camphor, along with the corresponding UV-visible
absorption spectrum (with max for the n* transition at 292 nm) is shown
below. Various features associated with ORD curves are also shown in the
diagram.
The above is an example of a positive Cotton Effect, as the first peak
encountered on going from high to low wavelength is a positive peak. (-)Camphor would have a negative Cotton Effect ORD curve, which is virtually the
mirror image of the one above.
One complicating factor regarding ORD curves is the fact that other
chromophores that have absorptions near the range of wavelengths under study
contribute to the overall shape of the spectrum: in particular causing it to be less
symmetrical, as shown in the example below.
Diagram (a) shows the actual ORD spectrum of a steroid derivative, and (b)
shows the superposition of a negative Cotton Effect (background) due to a
chromophore (probably OH) at low wavelength, on a positive Cotton Effect due to
the absorption at 264 nm (the sulfide group).
Nowadays, CD spectra are more widely used than ORD spectra, largely because
the former are inherently easier to interpret, as illustrated for steroid ketone
below.
Origins of CD: Anisotropic Absorption
An electronic (or vibrational) transition associated with a chiroptic chromophore
causes left and right circularly polarized light (cpl) to be absorbed differentially.
This is called anisotropic absorption and gives rise to the Cotton Effect. A L  AR
and A = AL – AR ( = L – R) cl, is a measure of the CD, where L and R are the
molar absorption coefficients for left and right cpl, respectively. If L > R, then a
positive CD curve results, but if the reverse is true then a negative CD curve is
observed. The signs of the CD curve and the corresponding ORD curve in the
region of the anomaly are the same.
Elliptical polarization is the most general form of polarized light: linear (plane)
polarized light and circularly polarized light are special cases. The relationships
between these are shown below.
Linear
(plane)
polarized
light
Velocities of
transmission
Absorptions
Equal
Equal
Unequal
Equal
Rotated
plane
polarization
Elliptical
polarization
Equal
Unequal
Rotated
Elliptical
polarization
Unequal
Unequal
The eccentricity of the ellipse [(a – b)/a] is 1 for linear (plane) polarization and 0
for cpl (a = b). These points are illustrated in more detail below, showing elliptical
polarization resulting from unequally absorbed left and right cpl. Diagram (a)
shows displays polarization in a region of the electromagnetic spectrum where 
= 0, and shows the polarization in a region where  = a positive value, viewed
toward the light source.
The molar ellipticity [] is defined by the equations below.
[ ] =
[ ] =

cl
[] M
in 10-1 o cm2 g-1
in 10 o cm2 mol-1
100
 is in degrees (o)
l is in dm (10 cm)
c is in g cm-3
M is the molar mass
(g mol-1)
For spectroscopic transitions to occur,
(1) E (photon) = h = E (excited state) – E (ground state)
(2) The excitation must be accompanied by a migration of charge, thus setting
up a transitory electric transition moment, 
Now, CD requires that charge rotation must accompany the translation of the
excited electron: it must rotate or follow a helical path. Hence a magnetic
transition moment m is set up. The relative magnitudes and directions of  and m
can be estimated using MO theory, as illustrated for * and n* transitions
below.
 *
electric transition moment

X
+
_
+
_
_
=




~
Electron density
+
_
+
Charge distribution
 
(Sign inversion conforms
with 
L
R)
Quadrupole
The excited electron follows a linear path and so there is no magnetic transition moment
*
n
_
X
=
+
_
~
_
_
+
n



+

Electron density
+
Charge distribution
There is no electric transition moment
The excited electron follows a helical path (with rotation of electric charge), hence a
magnetic transition moment m exists (direction given by "right hand rule").
m
n

The rotational strength R (a measure of the “allowedness”) of the CD absorption
is given by
R = .m =  m cos ,
Where  is the angle between the  and m vectors. The nature of the CD curve
(sign and intensity) according to the angle  is summarized below.
The sense of chirality (the handedness or configuration) of an absorbing
molecule influences the sign of the Cotton Effect (the sign of both CD and
ORD curves) by influencing the angle between the electric and magnetic
transition moments.
This is most easily illustrated by hexahelicene, a molecule whose whole
chromophore is chiral (unlike simple alkenes or carbonyl compounds).
In Class 22, it will be shown that (semiempirical) sector and helicity rules are
available for correlating absolute stereochemistry (configuration or conformation)
of particular types of compounds with their chiroptical properties, especially the
signs of individual Cotton Effects, as manifested by CD and ORD curves.