optical activity


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optical activity,

the ability of asymmetric compounds to rotate the orientation of planar polarized light. Such compounds and their mirror images are know as enantiomers, or optical isomers. Although differing in geometric arrangement, enantiomers possess identical chemical and physical properties. Since each type of enantiomer affects polarized light differently, optical activity can be used to identify which enantiomer is present in a sample and its purity. Certain molecular groups, known as chromophores, possess high optical activity due to mobile electrons that interact with light and are responsible for the color of certain objects (e.g. chlorophyll chromophore). Optical activity is measured by two methods: optical rotation, which observes a sample's effect on the velocities of right and left circularly polarized light beams; and circular dichroism, which observes a sample's absorption of right and left polarized light. See also polarization of lightpolarization of light,
orientation of the vibration pattern of light waves in a singular plane. Characteristics of Polarization

Polarization is a phenomenon peculiar to transverse waves, i.e.
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.

Optical Activity

 

the ability of a medium to cause rotation of the plane of polarization of optical radiation (light) passing through it. Optical activity was first observed in quartz by D.-F. Arago in 1811. In 1815, J. B. Biot discovered the optical activity of pure liquids (turpentine), and later of solutions and vapors of many, mainly organic, substances. He also established that: (1) the angle of rotation Φ of the plane of polarization is linearly dependent on the thickness l of the layer of the active substance (or solution of the substance) and on the concentration c of the substance: Φ = [α]lc (the factor [a] is called the specific rotation); and (2) rotation in a given medium takes place clockwise (Φ > 0) or counterclockwise (Φ ≤ 0) if the path of the light rays is viewed head-on.

Optically active substances that display natural optical activity (optical activity not induced by the presence of external fields) are correspondingly divided into dextrorotatory, or right-handed (d), Φ > 0, and levorotatory, or left-handed (l), Φ ≤ 0. This conventional division holds for radiation over a wide range of wavelengths. It loses its meaning only near the fundamental (resonance) absorption band of the medium; in 1896 the French scientist A. Cotton observed that Φ has opposite signs in the same substance on opposite sides of a resonance absorption band.

Some substances, such as quartz and cinnabar, are optically active only in the crystalline state, so that their optical activity is a property of the crystal as a whole; their specific rotation is designated simply as a, and Biot’s formula is written as Φ = α l. Other substances are active in any state of aggregation; this means that their optical activity is determined by the properties of the individual molecules. The specific rotation depends not only on the type of substance but also on such factors as state of aggregation, temperature, pressure, and type of solvent. Typical values of [a] in degrees per decimeter per gram per cu cm (deg/dm • g/cm3) 66.473 + 0.0127c for a solution of sucrose in water; 14.83 —0.146c for tartaric acid in water; —3.068 + 0.08959c and —5.7 for malic acid in water and acetone, respectively; —37 for turpentine in water; and 40.9 + 0.135c for camphor in ethyl alcohol. Here c is the concentration of the dissolved substance in grams per 100 cm3 of solution. The first two variables apply to the range of concentration 0–50, [a] for camphor holds in the range 10–50, and the others are valid for any concentration, if in fact they are dependent on it. These values are given for standard conditions: wavelength of light 589.3 nm (the sodium D line) and a temperature of 20°C.

Artificial, or induced, optical activity, which is manifested only when an optically inactive substance is placed in a magnetic field (the Faraday effect), is distinguished from natural optical activity. The sign of the rotation in the Faraday effect depends both on the magnetic properties of the medium (whether it is paramagnetic, diamagnetic, or ferromagnetic) and on whether the propagation of the radiation is parallel or antiparallel to the field. This is related to the special nature of a magnetic field, the intensity of which is a pseudovector (axial vector). If linearly polarized light that has been transmitted through a layer of a substance with natural optical activity is reflected and then transmitted through the same layer in the opposite direction, the original polarization is restored, whereas in a medium with induced optical activity the angle of rotation is doubled.

A phenomenological (macroscopic) theory of optical activity was proposed in 1823 by A. J. Fresnel, who attributed optical activity to the difference between the indexes of refraction of a medium n+ and n- for right and left circularly polarized light waves. (A wave of linearly polarized light can always be represented as the sum of two left and right circularly polarized waves of equal intensity.) The expression obtained by Fresnel has the form Φ = πl/λ (n+ -n _), where λ is the wavelength of the radiation in a vacuum. Thus, Φ may be significant even when the difference between n+ and n- is very small if, as is usually the case, l is much greater than λ. This explains the extremely high sensitivity of methods based on the measurement of optical activity (for example, in determining differences in the index of refraction it is 104 times more accurate than the most precise measurements by means of interferometers).

The development of the theory of optical activity is closely associated with the study of its dispersion—the dependence of α or [a] on λ. Biot established that in the cases studied by him α became smaller as λ became larger (Φ ∼ λ-2). Such dispersion is characteristic of normal optical activity—far from the wavelengths λ0 at which resonance absorption takes place in an optically active substance. Aimé Cotton, who studied optical activity for radiation with λ close to λ0, detected anomalous optical activity (an increase in α with λ), and also a difference in the absorption indexes at these wavelengths for right and left circularly polarized rays, called circular dichroism, or the Cotton effect. Circular dichroism results not only in rotation of the plane of polarization of light near resonance absorption bands but also, at the same time, in conversion of the light into ellipti-cally polarized light.

Studies of optical activity have shown that consideration of the change in the field of a light wave over distances of the order of the dimensions α of a molecule or ion of the substance is essential in explaining optical activity. (This change may be disregarded in describing many other optical effects, since α/λ ∼-3, but it is this parameter that determines the difference between n+ and n-.) One of the decisive steps in elucidating the nature of optical activity was L. Pasteur’s discovery in 1848 of optical antipodes—substances that are indistinguishable in all physical and many chemical properties, except the direction of rotation of the plane of polarization (the specific optical activities of two antipodes are of unlike sign but equal in absolute value). It was found that optical antipodes (lattices in crystals; individual molecules, called optical isomers, in amorphous, liquid, and gaseous optically active substances) are mirror images of one another, so that even though the elements comprising them are completely identical, they cannot be superimposed in space by any displacements or rotations. Spatial asymmetry is characteristic of the molecules of every optical isomer—they do not have a plane of reflection symmetry or a center of inversion.

A theory of the optical activity of molecular vapors within the framework of classical electron theory was developed in 1915 by M. Born and independently by the Swedish physicist C. W. Oseen, who showed that the lack of phase synchronism of the microcurrents induced by the field of a light wave in different parts of a molecule (the smallness of a/λ notwithstanding) must be taken into account in addition to molecular asymmetry. A quantum theory of the optical activity of vapors was constructed in 1928 by the Belgian scientist L. Rosenfeld. Processes associated with the finite size of molecules (occurring at distances of approximately α) are also considered in this theory, which is more rigorous from the standpoint of modern science. To explain optical activity it was found to be necessary to take into account both the electric and the magnetic dipole moment induced in a molecule by the field of a transmitted wave. The theory of optical activity of molecular media that are active only in the crystal phase is closely associated with the theory of excitons, since the optical activity of the crystals is determined by the character of the polarization waves in them. (For the theory of induced optical activity, seeMAGNETO-OPTICSand FARADAY EFFECT)

Modern theories of optical activity describe the phenomenon in a qualitatively correct way, but the quantitative theory of the dispersion of optical activity encounters significant difficulties because of the complexity of the objects being studied.

Optical activity is displayed by broad classes of substances, especially organic substances. The nature of the dispersion of optical activity is extremely sensitive to the various factors that determine intramolecular and intermolecular interactions. Therefore, methods based on the measurement of optical activity are used extensively in physical, chemical, biological, and other research and in industry.

REFERENCES

Landsberg, G. S. Optika, 4th ed. Moscow, 1957. (Obshchii kurs fiziki, vol. 3.)
Born, M., and E. Wolf. Osnovy optiki, 2nd ed. Moscow, 1973. (Translated from English.)
Vol’kenshtein, M. V. Molekuliarnaia optika. Moscow-Leningrad, 1951.
Mathieu, J. P.“Activité optique naturelle.” In Encyclopedia of Physics (Handbuch der Physik), vol. 28. Berlin, 1957.

S. G. PRZHIBEL’SKII

optical activity

[′äp·tə·kəl ak′tiv·əd·ē]
(optics)
The behavior of substances which rotate the plane of polarization of plane-polarized light, as it passes through them. Also known as rotary polarization.
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