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2.5.10 Optical rotation

Natural optical activity

A number of crystals, liquids, solutions and vapours rotate the electric vector of linearly polarized light passing through them, this property being known as optical activity. The rotation is proportional to the thickness of the medium traversed and its sense is related to the direction of the light beam. Rotations that are clockwise to an observer looking against the direction of the light beam are said to be right handed or dextrorotatory and are considered positive; anticlockwise rotations are said to be left handed or laevorotatory.

In many chemical compounds, both organic and inorganic, the optical activity is due to asymmetry of the structure of the molecule and is retained in solution form. However, some optically active crystals lose this property on fusion or in solution, the optical activity being due not to the individual molecules but to their orientation within the crystal (e.g. crystalline quartz). At wavelengths well away from an absorption band the optical rotation varies approximately with wavelength λ as 1/λ².

The specific rotation of a solid is defined as the optical rotation in degrees produced by a 1 mm thickness of the solid. For a compound in solution the specific rotation [α] is given by

   [α] = 10⁴α/dc

where α is the measured rotation in degrees, d the path length of the solution in mm and c the concentration in g/100 cm³.

Specific rotation (degrees) at 20 °C

    ^† Sucrose concentration: 26 g/100 cm³ in water.

Specific rotation values from Proceedings of the 18th Session of ICUMSA, 1982 (The International Commission for Uniform Methods of Sugar Analysis) and ICUMSA News, May 1992.

Magnetic rotation of polarized light

Polarized light passing through a medium in which there is a magnetic field H parallel to the direction of propagation suffers a rotation of the plane of vibration α = rHl, where l is the length of path and r is a constant for the material known as Verdet’s constant. r may be conveniently expressed in terms of rotation per ampere turn. It is considered positive if, as is usually the case, the rotation produced by passage through a solenoid with a core of the material is in the same sense as the circulating current. As a rough guide r may be taken to vary with wavelength λ as 1/λ².

Verdet constants in the literature are still frequently expressed in terms of min/oersted/cm; these must be multiplied by the factor 1.2566 to give values in min/A, i.e. as given in the tables below.

Liquids^†

     ^† For λ = 589.3 nm.
    * Schott low stress optical coefficient glass.

R.J.King / K.W.Raine

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