含义Meaning
把观察到的旋光角归一化到 1 dm 光程和 1 g/mL 浓度，是手性化合物的特征物理常数，常以钠 D 线（589 nm）、20 °C 测定。
Observed rotation normalised to 1 dm and 1 g/mL; a characteristic constant of a chiral compound, usually at the sodium D line (589 nm), 20 °C.
符号说明Symbols
| 符号Symbol | 含义 Meaning | 单位 Unit |
|---|---|---|
| \(\alpha\) | 观测旋光角observed rotation | ° |
| \(l\) | 样品管长度path length | dm |
| \(c\) | 浓度（纯液体用密度）concentration (density for neat liquids) | g·mL⁻¹ |
| \([\alpha]\) | 比旋光度（习惯省略单位 deg·mL·g⁻¹·dm⁻¹）specific rotation (units deg·mL·g⁻¹·dm⁻¹ usually omitted) | ° |
推导Derivation
毕奥（1815）发现旋光角与光程和浓度成正比：\(\alpha \propto lc\)。
Biot (1815): rotation is proportional to path length and concentration.
比例常数即比旋光度，与物质、波长、温度、溶剂有关。
The constant is the specific rotation, depending on compound, wavelength, temperature and solvent.
旋光色散：\([\alpha]\) 随波长变化（德鲁德方程 \([\alpha] \approx A/(\lambda^2 - \lambda_0^2)\)），接近吸收带时出现科顿效应。
Optical rotatory dispersion: \([\alpha]\) varies with λ (Drude \([\alpha] \approx A/(\lambda^2 - \lambda_0^2)\)), with Cotton effects near absorptions.
适用条件与局限Assumptions & limitations
必须注明波长、温度、溶剂和浓度；高浓度时可能非线性（缔合）。
Report λ, T, solvent and c; non-linear at high concentration (association).
旋光方向（+/−）与 R/S 构型没有简单对应关系。
The sign of rotation does not map simply onto R/S.
例题Worked example
蔗糖 [α]D²⁰ = +66.5°；c = 0.10 g/mL、l = 2 dm 时 α = +13.3°。
Sucrose [α]D²⁰ = +66.5°; at c = 0.10 g/mL, l = 2 dm, α = +13.3°.