Optical Rotatory Dispersion - Senkoubunsan

Abbreviation: ORD. A phenomenon in which the optical rotation of an optically active substance changes depending on the wavelength. Linearly polarized light or plane polarized light is expressed as a combination of left and right circularly polarized light. Optically active substances have different refractive indices for left and right circularly polarized light, so when plane polarized light passes through the substance, a phase difference occurs between the two circularly polarized light, and the plane of polarization rotates (optical rotation). Also, when the absorption of an optically active substance differs between left and right circularly polarized light, the magnitude of the electric vector of the two polarized light changes, so when plane polarized light passes through the substance, it becomes elliptical polarized light (circular dichroism). Optical rotatory dispersion generally appears when these two phenomena overlap. This is displayed by an optical rotatory dispersion curve as shown in the figure. The vertical axis is the molecular optical rotation [φ], and the horizontal axis is the wavelength. When [φ] simply increases or decreases from long wavelengths to short wavelengths in the wavelength range of the measured light, it is called a simple curve. In this case, an increase on the positive side is called positive, and an increase on the negative side is called negative. Dispersion that shows a simple curve is called normal dispersion. Even in this case, anomalous dispersion will appear if the shorter wavelength region is measured. Simple curves are approximated by the Drude dispersion formula.

A dispersion curve that changes in a complex manner with maxima and minima is called an anomalous dispersion curve. A curve that has one maximum and one minimum is called a single Cotton effect curve, and when there is a maximum on the long wavelength side and a minimum on the short wavelength side it shows a positive Cotton effect, and vice versa it shows a negative Cotton effect. When multiple Cotton effects appear on an anomalous dispersion curve, it is called a compound Cotton effect curve.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

略称ORD．光学活性物質の旋光度が波長によってかわる現象．直線偏光または平面偏光は，左右円偏光の合成によって表される．光学活性物質は左円偏光と右円偏光に対する屈折率が異なるため，平面偏光がこの物質を通過すると，両円偏光に位相差を生じて偏光面が回転される(旋光性)．また，光学活性物質の吸収が左円偏光と右円偏光に対して異なるときは，両偏光の電気ベクトルの大きさが変化するため，平面偏光がこの物質を通過するとだ円偏光になる(円二色性)．旋光分散は，一般にこれら二つの現象が重なって現れる．これは図のような旋光分散曲線によって表示される．縦軸に分子旋光度[φ]をとり，横軸に波長をとる．測定光の波長範囲において，長波長から短波長に向かって，[φ]が単純に増加または減少する場合を単純曲線という，この場合，正の側に増大するものを正とよび，負の側に増大していくものを負という．単純曲線を示す分散を正常分散という．この場合でもさらに短波長領域を測定すれば異常分散が現れる．単純曲線はドルーデの分散式で近似される．

分散曲線が極大，極小をもって複雑に変化するものを異常分散曲線という．1個ずつの極大と極小を有する曲線を単一コットン効果曲線といい，長波長側に極大，短波長側に極小を有するときを正のコットン効果，この逆のときを負のコットン効果を示すという．異常分散曲線に複数のコットン効果が現れる場合を，複合コットン効果曲線という．

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