optical rotatory dispersion and circular...
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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.