紫外–可见吸收光谱速查表UV–visible spectroscopy — transitions, chromophores and colour
紫外–可见吸收光谱记录分子在约 190–800 nm 范围内对光的吸收。吸收的能量使电子从较低的分子轨道跃迁到较高的分子轨道，对有机分子而言主要是 π 电子和非键（n）电子。横坐标为波长 λ，纵坐标为吸光度 A 或摩尔吸光系数 ε；吸收最强处的波长记作 λmax。
A UV–visible absorption spectrum records how strongly a molecule absorbs light between about 190 and 800 nm. The absorbed energy promotes an electron from a lower to a higher molecular orbital; in organic molecules these are mainly π and non-bonding (n) electrons. The horizontal axis is wavelength λ and the vertical axis is absorbance A or molar absorption coefficient ε; the wavelength of strongest absorption is written λmax.
读谱要点Reading a spectrum
朗伯–比尔定律：A = lg(I₀/I) = εcl。ε 为摩尔吸光系数（dm³ mol⁻¹ cm⁻¹），c 为浓度（mol dm⁻³），l 为光程（cm）。透光率 T = I/I₀，A = −lg T。
Beer–Lambert law: A = log₁₀(I₀/I) = εcl, with ε the molar absorption coefficient (dm³ mol⁻¹ cm⁻¹), c the concentration (mol dm⁻³) and l the path length (cm). Transmittance T = I/I₀ and A = −log₁₀ T.
定量测定时吸光度宜在约 0.1–1.0 之间；吸光度过高时应稀释样品后再测。
For quantitative work keep the absorbance between about 0.1 and 1.0; dilute the sample if it is higher.
共轭体系越长，HOMO 与 LUMO 的能量差越小，λmax 越长（向长波方向移动）。共轭双键约达到 8 个时，吸收进入可见区，化合物呈色。
The longer the conjugated system, the smaller the HOMO–LUMO gap and the longer λmax. With about eight conjugated double bonds the absorption reaches the visible region and the compound is coloured.
能量与波长：E = hc/λ。λ = 300 nm 的光子对应约 399 kJ mol⁻¹，λ = 600 nm 对应约 199 kJ mol⁻¹。
Energy and wavelength: E = hc/λ. Photons of 300 nm correspond to about 399 kJ mol⁻¹, and 600 nm to about 199 kJ mol⁻¹.
物质呈现的是被吸收光的互补色：吸收 500–560 nm 的绿光时呈紫红色，吸收 435–480 nm 的蓝光时呈黄色（见下方色轮表）。
A substance shows the complementary colour of the light it absorbs: absorbing green light (500–560 nm) makes it look purple, absorbing blue light (435–480 nm) makes it look yellow (see the colour table below).
| 术语 Term | English | 含义 Meaning |
|---|---|---|
| 发色团Chromophore | chromophore | 产生吸收的基团，如 C=C、C=O、N=N、芳环the group responsible for the absorption, e.g. C=C, C=O, N=N, aromatic rings |
| 助色团 | auxochrome | 本身在近紫外区不吸收、但连在发色团上使 λmax 变长、吸收增强的基团，如 –OH、–NH₂、–OR、卤素a group that does not absorb in the near UV itself but, attached to a chromophore, lengthens λmax and raises the intensity, e.g. –OH, –NH₂, –OR, halogens |
| 红移 | bathochromic shift (red shift) | λmax 向长波方向移动λmax moves to longer wavelength |
| 蓝移 | hypsochromic shift (blue shift) | λmax 向短波方向移动λmax moves to shorter wavelength |
| 增色效应 | hyperchromic effect | 吸收强度（ε）增大absorption intensity (ε) increases |
| 减色效应 | hypochromic effect | 吸收强度（ε）减小absorption intensity (ε) decreases |
电子跃迁类型Types of electronic transition
| 跃迁 Transition | 波长范围 Wavelength | ε / dm³ mol⁻¹ cm⁻¹ | 出现在 Found in |
|---|---|---|---|
| σ → σ* | < 150 nm | — | 饱和烷烃；位于真空紫外区，普通仪器测不到alkanes; in the vacuum UV, beyond ordinary instruments |
| n → σ* | 150–250 nm | 10²–10³ | 含 O、N、S、卤素的饱和化合物，如甲醇约 177 nm、甲胺约 215 nm、碘甲烷约 258 nmsaturated compounds with O, N, S or halogen, e.g. methanol ≈ 177 nm, methylamine ≈ 215 nm, iodomethane ≈ 258 nm |
| π → π* | 孤立双键约 170 nm；共轭体系 200–500 nm 以上 | 10³–10⁵ | 烯烃、共轭多烯、芳香化合物、烯酮；强吸收alkenes, conjugated polyenes, aromatics, enones; strong |
| n → π* | 250–350 nm | 10–10² | 醛、酮、硝基、偶氮化合物；跃迁禁阻，吸收弱；溶剂极性增大时蓝移aldehydes, ketones, nitro and azo compounds; forbidden and weak; blue-shifts in more polar solvents |
| d–d | 可见–近红外 | 1–10² | 过渡金属配合物（八面体配合物通常较弱，四面体较强）transition-metal complexes (octahedral usually weak, tetrahedral stronger) |
| 电荷转移 charge transfer | 紫外–可见 | 10³–10⁵ | MnO₄⁻、CrO₄²⁻、[Fe(SCN)(H₂O)₅]²⁺ 等深色物种intensely coloured species such as MnO₄⁻, CrO₄²⁻, [Fe(SCN)(H₂O)₅]²⁺ |
常见发色团的吸收Absorption of common chromophores
| 化合物 Compound | λmax / nm | ε | 跃迁 Transition | 说明 Note |
|---|---|---|---|---|
| 乙烯ethene | ≈ 170 | ≈ 15 000 | π → π* | 孤立 C=Cisolated C=C |
| 1,3-丁二烯buta-1,3-diene | 217 | 21 000 | π → π* | 两个共轭 C=Ctwo conjugated C=C |
| 1,3,5-己三烯hexa-1,3,5-triene | 258 | 35 000 | π → π* | 三个共轭 C=Cthree conjugated C=C |
| β-胡萝卜素β-carotene | ≈ 450 | ≈ 1.4 × 10⁵ | π → π* | 11 个共轭 C=C，吸收蓝光，呈橙色11 conjugated C=C; absorbs blue light and looks orange |
| 丙酮propanone (acetone) | 279 | 15 | n → π* | 孤立 C=O 的弱吸收weak band of an isolated C=O |
| 乙醛ethanal (acetaldehyde) | 290 | 17 | n → π* | |
| 丁-3-烯-2-酮but-3-en-2-one | 219 / 324 | 3 600 / 24 | π → π* / n → π* | C=C 与 C=O 共轭C=C conjugated with C=O |
| 苯benzene | 204 / 256 | 7 900 / 200 | π → π* | 256 nm 带有精细结构the 256 nm band shows fine structure |
| 苯酚phenol | 210 / 270 | 6 200 / 1 450 | π → π* | –OH 为助色团–OH acts as an auxochrome |
| 苯胺phenylamine (aniline) | 230 / 280 | 8 600 / 1 430 | π → π* | –NH₂ 为助色团–NH₂ acts as an auxochrome |
| 反式偶氮苯(E)-azobenzene | ≈ 320 / ≈ 440 | ≈ 2 × 10⁴ / ≈ 500 | π → π* / n → π* | 偶氮染料的母体，呈橙黄色parent of the azo dyes; orange-yellow |
数值为稀溶液（己烷或乙醇）中的典型文献值，随溶剂变化可相差数纳米。Typical literature values in dilute hexane or ethanol; they shift by a few nm with solvent.
吸收波长与呈现颜色Wavelength absorbed and colour seen
上：被吸收的光top: light absorbed下：物质呈现的颜色（互补色）bottom: colour seen (complementary)
常见无机显色物种Coloured inorganic species
| 物种 Species | 颜色 Colour | λmax / nm | 来源 Origin |
|---|---|---|---|
| [Ti(H₂O)₆]³⁺ | 紫色violet | ≈ 500 | d–d（d¹），ε 约 5d–d (d¹), ε about 5 |
| [Cu(H₂O)₆]²⁺ | 浅蓝pale blue | ≈ 800 | d–d，吸收红光到近红外d–d; absorbs red to near-infrared light |
| [Cu(NH₃)₄(H₂O)₂]²⁺ | 深蓝deep blue | ≈ 600 | d–d；NH₃ 场强较大，λ 比水合离子短d–d; NH₃ is a stronger-field ligand, so λ is shorter than for the aqua ion |
| MnO₄⁻ | 紫色purple | ≈ 525 | 电荷转移，ε 约 2 × 10³charge transfer, ε about 2 × 10³ |
| Cr₂O₇²⁻ | 橙色orange | — | 电荷转移charge transfer |
溶剂的紫外截止波长UV cut-off of common solvents
截止波长以下溶剂自身吸收过强（1 cm 光程吸光度约为 1），不能用来测样品。
Below the cut-off the solvent itself absorbs too strongly (absorbance about 1 in a 1 cm cell) to measure a sample.
数字为截止波长 λ / nm。Numbers are cut-off wavelengths in nm.
Woodward–Fieser 规则（进阶）Woodward–Fieser rules (advanced)
Woodward–Fieser 规则用经验加和估算共轭二烯和 α,β-不饱和羰基化合物 π → π* 吸收的 λmax：取母体基准值，再按取代基和结构逐项加上增量。估算值与实测值通常相差几纳米以内。
The Woodward–Fieser rules estimate λmax of the π → π* band of conjugated dienes and α,β-unsaturated carbonyl compounds by adding empirical increments to a parent value. Estimates are usually within a few nanometres of the observed value.
| 共轭二烯 Conjugated dienes | nm |
|---|---|
| 母体：链状（无环）二烯Parent: acyclic diene | 217 |
| 母体：异环二烯（两个双键分属两个环）Parent: heteroannular diene (double bonds in different rings) | 214 |
| 母体：同环二烯（两个双键在同一个环内）Parent: homoannular diene (both double bonds in one ring) | 253 |
| 每多一个共轭 C=Ceach additional conjugated C=C | +30 |
| 每个烷基取代基或环残基each alkyl substituent or ring residue | +5 |
| 每个环外双键each exocyclic C=C | +5 |
| –OR | +6 |
| –SR | +30 |
| –Cl, –Br | +5 |
| –NR₂ | +60 |
| –OC(O)R | 0 |
2,3-二甲基-1,3-丁二烯：链状二烯 217 + 两个烷基 2 × 5 = 227 nm（实测约 226 nm）。
2,3-Dimethylbuta-1,3-diene: acyclic diene 217 + two alkyl groups 2 × 5 = 227 nm (observed about 226 nm).
环己-2-烯-1-酮：六元环烯酮 215 + β 位环残基 12 = 227 nm（实测约 225 nm）。
Cyclohex-2-en-1-one: six-ring enone 215 + β ring residue 12 = 227 nm (observed about 225 nm).
4-甲基戊-3-烯-2-酮：链状烯酮 215 + β 位两个甲基 2 × 12 = 239 nm（乙醇中实测约 237 nm）。
4-Methylpent-3-en-2-one: acyclic enone 215 + two β-methyl groups 2 × 12 = 239 nm (observed about 237 nm in ethanol).
| α,β-不饱和羰基化合物 α,β-Unsaturated carbonyls | nm |
|---|---|
| 母体：链状或六元环 α,β-不饱和酮Parent: acyclic or six-ring α,β-unsaturated ketone | 215 |
| 母体：五元环 α,β-不饱和酮Parent: five-ring α,β-unsaturated ketone | 202 |
| 母体：α,β-不饱和醛Parent: α,β-unsaturated aldehyde | 210 |
| 母体：α,β-不饱和酸或酯Parent: α,β-unsaturated acid or ester | 195 |
| 每多一个共轭 C=Ceach additional conjugated C=C | +30 |
| 同环二烯组分homoannular diene component | +39 |
| 每个环外双键each exocyclic C=C | +5 |
| 烷基或环残基：α / β / γ 及更远alkyl or ring residue: α / β / γ and beyond | +10 / +12 / +18 |
| –OH: α / β / δ | +35 / +30 / +50 |
| –OR: α / β / γ / δ | +35 / +30 / +17 / +31 |
| –OC(O)CH₃（任意位置）–OC(O)CH₃ (any position) | +6 |
| –Cl: α / β | +15 / +12 |
| –Br: α / β | +25 / +30 |
| –NR₂: β | +95 |
溶剂校正（以乙醇中的数值为准）：水 −8，三氯甲烷 +1，乙醚 +7，己烷 +11 nm。Solvent correction (values refer to ethanol): water −8, chloroform +1, ether +7, hexane +11 nm.