chemistry · coordination complexes 3. optical rotatory dispersion (ord) 3.1 circular birefringence...

CHEMISTRY

Paper 7: Inorganic Chemistry-II (Metal-Ligand Bonding, Electronic Spectra

and Magnetic Properties of Transition Metal Complexes)

Module 25: Spectroscopic methods for determination of Absolute

Configuration of Coordination Complexes

Subject Chemistry

Paper No and Title Paper 7: Inorganic Chemistry-II (Metal-Ligand Bonding,

Electronic Spectra and Magnetic Properties of Transition

Metal Complexes)

Module No and Title 25: Spectroscopic methods for determination of Absolute


Module Tag CHE_P7_M25

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TABLE OF CONTENTS

1. Learning Outcomes

2. Spectroscopic Methods for determination of Absolute Configuration of

Coordination Complexes

3. Optical Rotatory Dispersion (ORD)

3.1 Circular Birefringence

3.2 Optical Rotatory Dispersion (ORD) curves

3.2.1 Plain curves

3.2.2. Anomalous/Cotton effect curves

4. Circular Dichroism (CD)

5. Summary

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1. Learning Outcomes

After studying this module, you shall be able to

Know about spectroscopic methods for determination of the absolute configuration of

coordination complexes

Learn the spectroscopic methods: Optical Rotatory Dispersion (ORD) and Circular

Dichroism (CD)

Learn about Circular Birefringence and Cotton Effect.

Identification of different types of curves obtained from these techniques

Learn the difference between Optical Rotatory Dispersion (ORD) and Circular Dichroism

(CD)

2. Spectroscopic Methods for determination of Absolute Configuration

of Coordination Complexes

As we are aware that lowest d-d transitions in the hexaminecobalt(III) complex is 1A1g1T1g.

Thus, for the complex [Co(en)3]3+, a very similar electronic transition and visible spectrum will

be observed. There are two phenomenon associated with these d-d transitions namely Optical

Rotatory Dispersion (ORD) and Circular Dichroism (CD). These are very useful in determining

the absolute configuration of complexes. There are two spectroscopic methods for the

determination of the absolute configuration.

(i) Optical Rotatory Dispersion (ORD)

(ii) Circular Dichroism (CD)

These methods depend on the behavior of polarized light passing through a solution (usually) of

the optically active compound. The results have to be interpreted by comparison with a similar

compound of known absolute configuration. The principles of these techniques are discussed in

following sections.

3. Optical Rotatory Dispersion (ORD)

The rate of change of specific rotation with respect to wavelength is known as optical rotatory

dispersion (ORD). As we already know that plane polarized light can be regarded as and

experimentally broken down into two circularly polarized components, equal in amplitude but

opposite in rotation. An optically inactive substance will retard the speeds of two circularly

polarized components to the same extent, with no net rotation. However, the speed of the

circularly polarized components are retarded by an optically active substance to different extent

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resulting in the rotation of plane of polarization. The rotation of plane of polarization by an

optically active medium is the result of unequal angles which EL and ER make with Z axis.

Rotation of electric field component E is depicted in figure 1. It has been shown that since the

speed of light in a medium is manifested in the refractive index of the medium, essential property

of an optically active substance is that it has different refractive indices for the left and right

circularly polarized light, nL and nR, respectively. This is also the reason why an optically active

substance is said to be "circularly birefringent."

Figure 1

3.1 Circular Birefringence

The difference in indices of refraction for right circularly polarized light (RCPL) and left

circularly polarized light (LCPL) is known as circular birefringence. Thus, on passing plane

polarized light (PPL) through optically active compound results in an unequal rate of propagation

of right and left circularly polarized rays due to circular birefringence. This unequal rate of

propagation for both right and left circularly polarized light deviate the PPL from its original

direction and it is called optical rotation. Optical rotation is caused by compound changes with

the wavelength of PPL that means circular birefringence. Optical rotation is measured by

polarimeter. Measuring optical rotation as a function of wavelength is termed as optical rotatory

dispersion (ORD) spectroscopy. Circular birefringence or optical rotation can be calculated

quantitatively by using the following equations;

Angle of rotation ‘’ per unit length expressed in degrees () is given by:

=𝜋

(𝑛𝐿 − 𝑛𝑅)

Where, is the wavelength of incident light, nL and nR are the refractive indices for left and right

circularly polarized light, and 1 is the path length of the medium.

Multiplying equation 1 by 1800/ converts to the rotation (/ dm)

Optical rotation = 1800/

EREL

Z

ER

EL

Z

Incident Light

Optically Active Complex

Exiting Light

X

E= ER + EL E= ER + EL

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Spectropolarimeters are the polarimeters used to make measurements at a variety of wavelength;

they record ‘’ as a function of at a specific temperature. At a specified wavelength; optical

rotation ‘’ is called as specific rotation, []T, occasionally we use the term molar rotation [M]

Specific rotation, [] is given by:

[𝜶]𝑻 =

𝜶

𝒍 × 𝒄

Where is the observed rotation at wavelength in degrees, l is the light path in decimeters, c is

concentration of the optically active substance in grams per ml. For ORD we commonly we use

molar rotation [M] (unit: cm2/ dmol) which is defined as:

[𝑀] = 𝑀

100 𝑙 𝑐

Where is the observed rotation at wavelength in degrees, l is the light path in decimeters, c is

the concentration of the optically active substance in grams per ml, and M is the molecular

weight in grams per mol. A curve showing the wavelength dependence of optical rotation is

called an Optical Rotatory Dispersion (ORD) spectrum. ORD curve is a plot of specific rotation,

[] vs or molar rotation [M] vs .

The following animation (Figure 2) represents what happens when a plane-polarized (the plane of

polarization is vertical here) light wave (indicated by light blue color) traverses through a

medium that does not slow down the left circularly polarized component (this is the circular wave

shown in red) of the wave at all, but slows down the right circularly polarized component (this is

the circular wave shown in green) to some extent. For the latter component, the refraction index

of the material is n =1.05.

Figure 2

We placed an intersecting plane (Figure 3) before and after the piece of material and we show the

field vectors at the point of intersection of the light beam and the intersecting planes. The

following animation shows intersecting planes from the front. On the left, the plane before

material, on the right, plane after the material. As one can see, the exiting wave continues to be a

plane-polarized wave, but its plane of polarization is no longer vertical, the polarization plane has

rotated by 36°.

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Figure 3

3.2 Optical Rotatory Dispersion (ORD) curves

ORD curve is plotted between change in the optical rotation and wavelength. According to

Djerassi & Klyne ORD curves can be classified into following two categories:

1. Plain curves

2. Anomalous/Cotton effect curves

3.2.1 Plain curves

These curves don’t contain any peak or inflections (maximum or minimum) and these don’t cross

the zero rotation line. The plain curves are obtained for chiral compounds which are wanting any

chromophore. It means that the chiral compounds do not show any absorption at the wavelength

which are being examined. e.g. hydrocarbons, alcohols etc.

Usually two types of plain curves are observed (Figure 4).

(i) Plain positive ORD: Graph A (Figure 4) represents a plain positive ORD curve. Plain

positive ORD curves are obtained when specific rotation increases with decreasing wavelength.

In other words, clockwise rotation is plotted positively.

(ii) Plain negative ORD: Graph B (Figure 4) represents plain negative ORD curve. Plain

negative ORD curves are obtained when specific rotation decreases with decreasing wavelength.

In other words, counter clockwise rotation is plotted negatively.

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Figure 4

3.2.2. Anomalous/Cotton effect curves

To understand these curves we have to have an idea about which presents characteristic change in

optical rotatory dispersion in the vicinity of an absorption band for a particular substance. In a

wavelength region where the light is absorbed, absolute magnitude of the optical rotation at first

varies rapidly with wavelength, crosses zero at absorption maxima and then again varies rapidly

with wavelength but in opposite direction. The each Cotton effect consists of two extremes, a

geometric maximum called a "peak" and a geometric minimum called a "trough". The Cotton

effect is called positive (Figure 5) if the optical rotation first increases as the wavelength

decreases that means positive Cotton effect curve has its peak in the longer wavelength region,

while it is called negative if optical rotation first decreases as the wavelength decreases. Negative

Cotton effect curve is defined as having its trough appearing at the longer wavelength. Optically

pure enantiomers always display opposite Cotton effect ORD curves of identical magnitude.

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Figure 5

Anomalous/Cotton effect curves are obtained for optically active compounds having

chromophores absorbing in the near UV region. They show peaks and troughs depending on the

absorbing groups present in the system and thus these are called an anamalous dispersion of

optical rotation. These are again divided into two categories;

(i) Single Cotton effect curves

(ii) Multiple Cotton effect curves

(i) Single Cotton effect curves: These are anomalous dispersion curves which show maximum

and minimum both occurring in the region of maximum absorption. So there appears only one

peak and one trough in the Single Cotton effect curves. The vertical distance between peak and

trough is called amplitude “a”. Amplitude is measured in degrees.

Molecular amplitude, 𝒂 =𝟏−𝟐

𝟏𝟎𝟎

Where 1 is molar rotation of trough or peak from shorter wavelength and 2 is the molar

rotation of extreme peak or trough from higher wavelength.

(ii) Multiple Cotton effect curves: These are a little different from the single Cotton effect

curves. They contain more than one peak and one trough (Figure 6).

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Figure 6

4. Circular Dichroism (CD)

The differential absorption of left and right circularly polarized light is referred to Circular

dichroism (CD). Optically active compounds possess this special property of absorption of the

left-handed circularly polarized light to a different extent than the right-handed circularly

polarized light. This phenomenon is called the circular dichroism. The electric field of a light

beam causes linear displacement of charge upon interaction with a molecule, whereas its

magnetic field causes a circulation of charge. These two motions collectively results in a helical

displacement when light impinges on an optically active molecule (both field vectors in the same

place are of same direction but at different moments of time).

Figure 7

EL

ER

Optically Active Material

ER + EL

ER - EL= + + =

EL

ER

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Circular dichroism occurs as the wavelength of incident light approaches that of the absorption

band. In this case oscillation of charges in the material is damped as energy is removed from the

field by absorption process. If the absorption is different for right and left handed circular-

polarized light then the linearly polarized light will become elliptically polarized. The elliptical

polarized light (Purple) is shown in figure 7, which is composed of unequal contributions of left

(red) and right (green) circular polarized light. The ellipticity () of light is defined by the arc

tangent of the ratio of major axis to the minor axis of transmitted light. Usually, actual absorption

of each component of light is measured and the difference in absorption is called the circular

dichroism (CD).

The following figure 8 shows what happens when a plane-polarized (the plane of polarization is

vertical here) light wave (indicated by light blue color) traverses through a medium that does not

absorb left circularly polarized component of the wave at all (this is the circular wave shown in

red) but highly absorbs right circularly polarized component (this is the circular wave shown in

green).We placed an intersecting plane before and after the piece of material and show the field

vectors at the point of intersection of light beam and intersecting planes. The figure 9 shows

intersecting planes from the front. On the left side of figure 9 plane before the material, on the

right, the plane after the material.

Figure 8

Figure 9

The component shown in red traverses the medium unchanged and the one shown in green gets

fainter: its intensity decreases to about 36% of the original value. The superposition of the two

components is no longer a linearly polarized wave: the resulting field vector does not oscillate

along a straight line but it rotates along an elliptical ath. Such a light wave is called

an elliptically polarized light.

Take a closer look and notice that the major axis of the ellipse is parallel to the polarization plane

of the original light wave. This is always the case, regardless of which circular component is

absorbed to greater extent by the medium. But the direction of rotation of elliptically polarized

light (or, more exactly, its field vector) is determined by the circular component that remains

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stronger after traversing the material. In this case, the field vector of elliptically polarized light

rotates in the same direction as that of the component shown in red color, which traverses

through the medium without losing intensity. Of course, it is very unusual for a material not to

absorb one circular component at all. Real materials usually absorb both components, to a

different extent. It is only for the sake of simplicity that we presented a situation where material

is completely transparent to one of the circularly polarized components.

The quantities in CD are (the difference in molar absorptivity) and the angle of ellipticity

per unit length

∆𝜀 = 𝜀𝐿 − 𝜀𝑅 ≠ 0

The angle ofellipticity is defined as:

𝜃 =𝜋(κ𝐿 − κ𝑅)

Where tan 𝜃 =𝐸𝑅−𝐸𝐿

𝐸𝑅+𝐸𝐿

Where k is the absorption index, it is related to absorption coefficient k by equation k= (4/) κ,

and has the units- radians/cm. The equation has now same form as the equations for ORD so

can be converted from radians to degree by multiplying by 1800/ and k= (4/) κ

𝜃 =1800(κ𝐿 − κ𝑅)

=

1800(𝑘𝐿 − 𝑘𝑅)

4𝜋

Now the molar ellipticity of the sample is given by

[𝜃] = (𝜃

𝑐𝑙) (

𝑀

100)

Where c is the concentration in moles/liter, l is path length in cm, and is the measured

ellipticity. In terms of extinction coefficients where, 𝑘 = 2.303𝜖𝑐, the equation can be rewritten

as:

[𝜃] =2.303 × 4500(𝜀𝐿 − 𝜀𝑅)

𝜋= 3300(∆𝜀)

𝐶𝑖𝑟𝑐𝑢𝑙𝑎𝑟 𝐷𝑖𝑐ℎ𝑟𝑜𝑖𝑠𝑚 = ∆𝜀 = 𝜀𝐿 − 𝜀𝑅

All optically active compounds exhibit CD in the region of appropriate absorption band. CD is

plotted as 𝜀𝐿 − 𝜀𝑅 vs . The difference between absorption of left (blue) and right (red) handed

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circularly-polarized light is measured as a function of wavelength (Figure 10). CD is measured as

a quantity called mean residue ellipticity [], whose units are degrees-cm2/dmol.

Figure 10

CD spectrometers have an optical system resembling to UV-visible spectrophotometers with the

addition of a crystal of ammonium phosphate mounted to allow imposition of a large electrostatic

field on it. When the field is imposed, crystal allows only circularly polarized light to pass

through, changing the direction of field rapidly providing alternating left & right circularly

polarized light. The light received by detector is compared electronically and presented as the

difference between absorbances 𝜀𝐿 − 𝜀𝑅. An ordinary spectrophotometer can also be used to

measure CD. It is only necessary to provide some means of production of right and left circularly

polarized radiation. The spectrum obtained in CD (Figure 11) is almost identical to an absorption

spectrum except that the peaks can be both positive and negative. These positive and negative

deflections in CD spectrum depends on the sign of or [] and also corresponds to the sign of

the Cotton effect. Maximum of the CD occurs at the absorption max.

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Figure 11

5.Summary

In this module, we discussed that

There are two spectroscopic methods for the determination of the absolute configuration;

Optical Rotatory Dispersion (ORD) and Circular Dichroism (CD)

ORD spectra are dispersive whereas circular dichroism spectra are absorptive. The two

phenomena are related by the so-called König-Kramers transforms.

CD plots are Gaussian; however ORD plots are S-shaped.

Positive or negative deflections depend on the sign of or [] and corresponds to the

sign of the Cotton effect

Maximum CD occurs at the absorption max.

For more than one overlapping Cotton effect, the CD may be easier to interpret than the

ORD with overlapping S-shaped bands.

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chemistry · coordination complexes 3. optical rotatory dispersion (ord) 3.1 circular birefringence...

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