Cotton effect

(analytical chemistry) The characteristic wavelength dependence of the optical rotatory dispersion curve or the circular dichroism curve or both in the vicinity of an absorption band.

The characteristic wavelength dependence of the optical rotatory dispersion curve or the circular dichroism curve or both in the vicinity of an absorption band.

When an initially plane-polarized light wave traverses an optically active medium, two principal effects are manifested: a change from planar to elliptic polarization, and a rotation of the major axis of the ellipse through an angle relative to the initial direction of polarization. Both effects are wavelength dependent. The first effect is known as circular dichroism, and a plot of its wavelength (or frequency) dependence is referred to as a circular dichroism (CD) curve. The second effect is called optical rotation and, when plotted as a function of wavelength, is known as an optical rotatory dispersion (ORD) curve. In the vicinity of absorption bands, both curves take on characteristic shapes, and this behavior is known as the Cotton effect, which may be either positive or negative (see illustration). There is a Cotton effect associated with each absorption process, and hence a partial CD curve or partial ORD curve is associated with each particular absorption band or process. See also Optical rotatory dispersion.

Behavior of the ORD and CD curves in the vicinity of an absorption band at wavelength λ₀ (idealized). (a) Positive Cotton effect. (b) Negative Cotton effect.

The rotational strengths actually observed vary over quite a few orders of magnitude; this variation in magnitude is useful in stereochemical interpretation of molecular structure.

The introduction to this article provides insufficient context for those unfamiliar with the subject. Please help improve the article with a good introductory style. (October 2009)

The Cotton effect is the characteristic change in optical rotatory dispersion and/or circular dichroism in the vicinity of an absorption band of a substance. In a wavelength region where the light is absorbed, the 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. This phenomenon was discovered in 1895 by the French physicist Aimé Cotton (1869-1951).

The Cotton effect is called positive if the optical rotation first increases as the wavelength decreases (as first observed by Cotton), and negative if the rotation first decreases.^[1] Protein structure like beta sheet shows positive Cotton Effect.

See also

Cotton-Mouton effect

References

1. ^ Ernest L. Eliel and Samuel H. Wilen, "Stereochemistry of Organic Compounds" (Wiley, 1994), p.1000

This atomic, molecular, and optical physics-related article is a stub. You can help Wikipedia by expanding it. v • d • e

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)