红外光谱Infrared spectroscopy

把化学键近似为弹簧，振动频率 ν̃ = (1/2πc)·√(k/μ)：力常数 k 越大（键越强）、折合质量 μ 越小，频率越高。只有使分子偶极矩发生变化的振动才是红外活性的（同核双原子 N₂、O₂ 无红外吸收）。振动方式包括伸缩振动（对称、不对称）和弯曲振动（剪式、摇摆、扭曲等）。非线性 N 原子分子有 3N − 6 个振动自由度。

Treating a bond as a spring, the frequency ν̃ = (1/2πc)·√(k/μ) rises with force constant k (bond strength) and falls with reduced mass μ. Only vibrations that change the dipole moment are IR-active, so homonuclear N₂ and O₂ do not absorb. Vibrations include symmetric and asymmetric stretches and bends such as scissoring, rocking and twisting, and a non-linear molecule of N atoms has 3N − 6 vibrational modes.

O–H 伸缩 3200–3650 cm⁻¹（醇宽峰；羧酸 2500–3300 极宽）；N–H 3300–3500（伯胺双峰）；C–H：sp³ 2850–2960，sp² 3000–3100，sp 约 3300，醛 C–H 约 2720 和 2820；C≡N 约 2250、C≡C 2100–2260；C=O 1650–1800（酮约 1715、醛约 1725、酯约 1740、酰氯约 1800、酰胺约 1650）；C=C 1620–1680；芳环 1450–1600。4000–1500 cm⁻¹ 称为官能团区，1500–400 cm⁻¹ 称为指纹区。

O–H stretches at 3200–3650 cm⁻¹ (broad for alcohols; very broad, 2500–3300, for carboxylic acids); N–H at 3300–3500 (two bands for primary amines); C–H at 2850–2960 for sp³, 3000–3100 for sp² and about 3300 for sp, with aldehyde C–H near 2720 and 2820; C≡N near 2250 and C≡C 2100–2260; C=O at 1650–1800 (ketones about 1715, aldehydes 1725, esters 1740, acid chlorides 1800, amides 1650); C=C 1620–1680; and aromatic rings 1450–1600. The 4000–1500 cm⁻¹ region is the functional-group region and 1500–400 cm⁻¹ the fingerprint region.

1800 年赫歇尔发现红外线。1905 年美国物理学家科布伦茨测量了上百种有机化合物的红外光谱，发现官能团与吸收频率的关系。1940 年代商用红外分光光度计出现。1960–70 年代傅里叶变换红外光谱（FTIR，用迈克尔逊干涉仪和快速傅里叶变换算法）取代色散型仪器，信噪比和扫描速度大幅提高。衰减全反射（ATR）附件使固体和液体无需制样即可直接测量。

Herschel discovered infrared in 1800. In 1905 the American physicist Coblentz measured IR spectra of over a hundred organic compounds and found the link between functional groups and absorption frequencies. Commercial IR spectrophotometers appeared in the 1940s, and in the 1960s and 1970s Fourier-transform IR, using a Michelson interferometer and the fast Fourier transform, replaced dispersive instruments with far better signal-to-noise and speed. Attenuated total reflection (ATR) accessories let solids and liquids be measured without preparation.