Optical Rotatory Dispersion and Circular

DichroismDR DARSHANA MEHTA

SCHOOL OF STUDIES IN CHEMISTRY AND BIOCHEMISTRY, VIKRAM UNIVERSITY, UJJAIN – 456 010, MADHYA PRADESH, INDIA

❑ Light is an electromagnetic phenomenon. A beam of light consists of two mutually perpendicular

oscillating fields: an oscillating electric field and an oscillating magnetic field.

❑ If we were to view a beam of ordinary light from one end, and if we could actually see the planes in

which the electrical oscillations were occurring, we would find that oscillations of the electric field were

occurring in all possible planes perpendicular to the direction of propagation.

❑ When ordinary light is passed through a polarizer, the polarizer interacts with the electric field so that

the electric field of the light that emerges from the polarizer (and the magnetic field perpendicular to

it) is oscillating only in one plane. Such light is called plane-polarized light

The device that is used for measuring the effect of optically active compounds on plane-

polarized light is a polarimeter.The principal working parts of a polarimeter are

(1) a light source (usually a sodium lamp)

(2) a polarizer

(3) a cell for holding the optically active substance (or solution) in the light beam

(4) an analyzer

(5) a scale for measuring the angle (in degrees) that the plane of polarized light has been

rotated.

The analyzer of a polarimeter is nothing more than another polarizer. If the cell of the

polarimeter is empty or if an optically inactive substance is present, the axes of the plane-

polarized light and the analyzer will be exactly parallel when the instrument reads 08, and the

observer will detect the maximum amount of light passing through.

❑ If, by contrast, the cell contains an optically active substance, a solution of one

enantiomer, for example, the plane of polarization of the light will be rotated as it

passes through the cell.

❑ In order to detect the maximum brightness of light, the observer will have to rotate the

axis of the analyzer in either a clockwise or counterclockwise direction.

❑ If the analyzer is rotated in a clockwise direction, the rotation, a (measured in degrees),

is said to be positive (+). If the rotation is counterclockwise, the rotation is said to be

negative (-).

❑ A substance that rotates plane-polarized light in the clockwise direction is also said to

be dextrorotatory, and one that rotates plane-polarized light in a counterclockwise

direction is said to be levorotatory (Latin: dexter, right, and laevus, left).

❑ Optical activity is measured by the degree of

rotation of plane-polarized light passing through a

chiral medium.

❑ The theoretical explanation for the origin of optical

activity requires consideration of circularly-polarized

light, however, and its interaction with chiral

molecules.

❑ While it is not possible to provide a full theoretical

explanation for the origin of optical activity here, the

following explanation will suffice.

❑ A beam of plane-polarized light (Fig. a) can be

described in terms of circularly-polarized light. A

beam of circularly-polarized light rotating in one

direction is shown in Fig. b.

Light behaves as though it consists of waves vibrating in all

directions around the direction of wave propagation. For polarized

light, the propagation can be regarded as a vector, which can be

resolved into two circular vectors. If there is no rotation of the

plane, it is expected that motion along each vector is equivalent so

that each vector traverses an equal distance around the circle

If polarized light passes through a medium that exhibits optical

rotation, the motion along one of the circular vectors is slower

than that of the other. The resultant vector is thus displaced from

the original vector by some angle, φ. Figure shows the vector

model in which the phase difference is φ and α is defined as one-

half of the phase difference.

The vector sum of two counterrotating in-phase circularly-polarized beams is a beam of

plane-polarized light (Fig. c). The optical activity of chiral molecules results from the

fact that the two counter rotating circularly-polarized beams travel with different

velocities through the chiral medium. As the difference between the two circularly-

polarized beams propagates through the sample, their vector sum describes a plane

that is progressively rotated (Fig. d). What we measure when light emerges from the

sample is the net rotation of the plane-polarized light caused by differences in velocity

of the circularly-polarized beam components. The origin of the differing velocities as

ultimately to do with interactions between electrons in the chiral molecule and light.

Achiral molecules in solution cause no difference in velocity of the two circularly polarized

beams; hence there is no rotation of the plane of polarized light described by their vector

sum. Randomly-oriented achiral molecules, therefore, are not optically active.

(However, oriented achiral molecules and crystals having specific symmetric

characteristics can rotate plane-polarized light.)

Polarized light can be either circularly polarized or plane polarized. When circularly

polarized, the electric or magnetic vector rotates (right-handed if clockwise rotation when

viewed facing the source, left-handed if counterclockwise) with a frequency related to the

frequency of the light. Plane-polarized light is made up of both right- and lefthanded

components; when combined, the vectors reinforce each other at 0° and 180" and cancel at

90° and 270°, leaving a planar motion of the vector

❑ One enantiomer of a chiral compound

rotates the plane of polarized light through a

characteristic angle, the instrument used to

measure this rotation is called a polarimeter.

❑ The direction indicated (a clockwise rotation

as we view the light as it emerges from the

polarimeter) is designated as +αo. The other

enantiomer of the same compound would

rotate the plane of polarized light through an

angle -αo.

The cis-[Co(NH3)4Cl2]Cl isomer (violeo) is violet,

while the trans isomer (praseo) is green.

Geometrical isomers of the square planar

[Pt(NH3)2Cl2] compound. The cis isomer is better

known as the anticancer drug cisplatin.

Geometric isomers have the same type and number of each kind of ligand,

but differ in the geometrical arrangement of those ligands around the

central metal ion. The most common set of geometrical isomers are cis

and trans, which occur in both octahedral and square planar molecular

geometries.

For octahedral compounds having the general formula MA3B3, two other geometrical

isomers exist. The facial (fac) isomer of [Co(NH3)3Cl3] is shown at the left, where the

three chloro ligands all lie along one of the six triangular faces of the octahedron, giving

this isomer its name. The meridional (mer) isomer, on the other hand, has all three chloro

ligands lying in the same plane, along the meridian of the molecule.

Optical isomers are molecules that have the same number and type of each kind of

ligand, but they are non-superimposable mirror images of each other. As such, optical

isomers are chiral and will rotate plane-polarized light in opposite directions.

Optical isomers are non-superimposable mirror

images. The dashed line represents the mirror

plane in which the two isomers are reflected.

The optical isomers of cis-[CoCl(NH3)(en)2]2+ first isolated by Werner and King.

Octahedral coordination compounds having the general formula ML2X2, where L is a

bidentate ligand and X is a halide, will also exhibit optical isomerism. Werner and his

students were among the first chemists to synthesize an optically-active coordination

compound. Specifically, Werner was able to resolve the optical isomers of cis-

[CoCl(NH3)(en)2]2+

Enantiomers are a pair of stereoisomers that are nonsuperposable

mirror images.

A chiral molecule and its mirror image partner are called enantiomers

(from the Greek word for ‘both parts’). Chiral molecules that do not

interconvert rapidly between enantiomeric forms are optically active in

the sense that they can rotate the plane of polarized light.

Enantiomeric pairs of molecules rotate the polarization of light to equal

extents, but in opposite directions, the dextrorotatory (d) enantiomer to

the right and the laevorotatory (l) to the left.

MABCD COMPLEX MA2B2C2 COMPLEX

[M(acac)3] COMPLEX [Co(en)2Cl2] COMPLEX

Diastereomers are isomeric compounds that contain two chiral centres,

one being of the same absolute configuration in both components and

the other being enantiomeric between the two components. An

example of diastereomers is provided by the two salts of an

enantiomeric pair of cations, A, with an optically pure anion, B, and

hence of composition [Δ-A][Δ-B] and [Λ-A][Δ-B]. Because

diastereomers differ in physical properties (such as solubility), they are

separable by conventional techniques.

(+) and (-) prefixes:

The specific rotation of enantiomers is equal and opposite, and a useful

means of distinguishing between enantiomers is to denote the sign of

[α]D. Thus, if two enantiomers of a compound A have [α]D values of +12o

and -12o, they are labelled (+)-A and (-)-A.

d and l prefixes:

Sometimes (+) and (-) are denoted by dextro- and laevo- (derived from

the Latin for right and left) and these refer to right- and left-handed

rotation of the plane of polarized light respectively; dextro and laevo are

generally abbreviated to d and l. The +/- or d/l notation is not a direct

descriptor of the absolute configuration of an enantiomer (the

arrangement of the substituents or ligands) for which the following

prefixes are used.

Octahedral compounds having the general formula ML3, where L is a

bidentate ligand, will also be optically active.

Δ and Λ prefixes:

Enantiomers of octahedral complexes containing three equivalent

didentate ligands (tris-chelate complexes) are among those which are

distinguished using (delta) and (lambda) prefixes. The octahedron is

viewed down a three-fold axis, and the chelates then define either a

right- or left-handed helix. The enantiomer with right-handedness is

labelled Δ , and that with left-handedness is Λ .

δ and λ prefixes:

The situation with chelating ligands is often

more complicated. Consider the chelation

of 1,2-diaminoethane to a metal centre. The

5-membered ring so formed is not planar

but adopts an envelope conformation. This

is most easily seen by taking a Newman

projection along the C-C bond of the

ligand; two enantiomers are possible and

are distinguished by the prefixes δ and λ.

Octahedral compounds having the general formula ML3, where L is a bidentate ligand, will also

be optically active. Thus, for example, the strongly luminescent [Ru(bpy)3]2+ compound, which

phosphoresces a bright orange color in fluid solution, has the optical isomers. The Δ-isomer has

the appropriate geometry to fit into the groove of right-handed B-DNA, while the Λ-isomer

cannot.

Optical isomers of [Ru(bpy)3]2+, showing how rotation of the second isomer by 180∘ does not

lead to a superimposable mirror image to the original.

Chiral molecules occur in pairs related by a symmetry plane; their

mirror images cannot be superimposed (enantiomers). Such molecules

exhibit optical activity, that is, they transmit left and right circularly

polarized light in a different manner.

The difference in the refraction indices for left and right circularly

polarized light and is called optical rotatory dispersion (ORD), the

corresponding difference in absorption coefficients is called circular

dichroism (CD).

ORD and CD can be related to each other by Kramers–Kronig

transformation.

Optical Rotatory Dispersion (ORD)

Pairs of chiral molecules transmit left and right circularly polarized light

by a different velocity. The two forms of chiral molecules have an

asymmetric distribution of electrons, and hence they interact with right

and left polarized light in opposite ways.

If the index of refraction for right polarized light is larger than for left

polarized light, the plane of polarization will be rotated toward left, and

viceversa

Optical rotatory dispersion:

Linearly polarized light can be considered as superposition of opposite circularly

polarized light of equal amplitude and phase. Different velocities of left and right

circularly polarized light lead to optical rotation of the polarization plane of the

transmitted light.

The rotation, α, may be measured in an instrument called a polarimeter. In

practice, the amount of rotation depends upon the wavelength of the light,

temperature and the concentration of compound present in solution.

The angle of rotation α at the wavelength λ is directly proportional to the

concentration c

where l is the path length of the sample cell.

The rotation is related to the molar mass,

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

c is the concentration of the optically active substance in grams per ml, and M0 is the

molecular weight in grams per mol

The obtained molar rotation [m] may be influenced by the refractive index n of the solvent.

The corrected molar rotation is defined by

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.

Circular Birefringence

Circular birefringence or optical rotation can be calculated

quantitatively by using the following equations;

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

Where, λ 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/π converts ϕ λ to the rotation α(o/dm)

Circular Dichroism (CD)

Similar to that in ORD, linearly

polarized light can be considered

as superposition of circularly

polarized light of opposite

direction of rotation but equal

amplitude and phase. Differences

in absorption of left and right

polarized light lead to elliptic

polarization of the transmitted

light.

Circular dichroism (CD) is a second chiroptical phenomenon, most frequently applied for the

assignment of the absolute stereostructure. When the linearly polarized light passes an optically

active medium in a spectral region where absorption takes place, left- and right-circularly

polarized rays do not only propagate with different velocities, but they are also absorbed by a

chiral sample to a different extent (i.e., AlCpl ≠ArCpl). Thereupon, the incident Lpl is converted

into elliptically polarized light (Epl), and, consequently, the resulting electric field vector sum

traces an ellipse (Figure a), which is characterized by the major and minor axes, a and b,

respectively, and by the angle ψ, defined as the ellipticity (Figure b).

In analogy with the quantities related to optical rotation, a specific

ellipticity and a molar ellipticity can be defined for characterization of CD.

Nowadays, however, circular dichroism is usually obtained by measuring

the difference in the molar extinction coefficients of left- εlCpl and right-

εrCpl circularly polarized components of light

Δε is also called molar CD, and it directly correlates with the molar

ellipticity

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 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 Δε or [ψ] and also corresponds to the

sign of the Cotton effect. Maximum of the CD

occurs at the absorption λmax.

Similarly as the ORD curve can be obtained, the measurement of the Δε

value as a function of the wavelength λ leads to a CD spectrum.

In general, the measured CD effect is directly associated with the ORD

anomaly, since they both reflect the interaction of the polarized light with

the same chiroptical chromophore.

The maximum of the CD curve coincides with the wavelength of

anomalous ORD crossover, and the sign of COTTON effect in ORD

spectrum corresponds to that of CD.

Furthermore, if one of the two curves is known over the entire

spectral range, the other can be calculated by using the KRONIG-

KRAMERS equations.

Although both phenomena afford complementary information, CD

spectroscopy has now completely replaced the ORD technique,

because it provides better discrimination between overlapping bands,

whereas the ORD curves often possess a fine structure due to

molecular vibrations, which complicates the interpretation of the

spectra.

Circular dichroism is usually observed only in the

vicinity of an absorption band, a positive Cotton

effect showing a positive peak at the absorption

maximum and a negative effect showing a

negative peak. This simple spectrum makes CD

more selective and easier to interpret than ORD.

With improvements in instrumentation, it has

become the method of choice for studying chiral

complexes.

Compounds having the same optical configuration show similar Cotton

effects. If the absolute configuration is known (for example, from x-ray

diffraction) for one optically active compound, a similar Cotton effect

exhibited by another compound indicates that it has the same optical

configuration as the known. In other words, if two compounds give

electronic transitions that show Cotton effects that are the same (either

both positive or both negative), the compounds have the same chirality

or optical configuration. Although other methods for studying the

absolute configuration of complexes exist, the methods described here

have been widely used and are historically important.

Cotton effects

CD and ORD spectra describing (a) the positive and (b) negative CEs of a single electronic transition.