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Chapter 8 Spectroscopic methods of structure determination

Chapter 8 Spectroscopic methods of structure determination. 结构测定的波谱方法. Introduction Ultraviolet Spectroscopy (UV) 紫外光谱 ---- 测定有机物中是否存在共轭双键和芳香族化合物 Infrared Spectroscopy (IR) 红外光谱 ---- 测定有机物中官能团。

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Chapter 8 Spectroscopic methods of structure determination

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  1. Chapter 8Spectroscopic methods of structure determination 结构测定的波谱方法

  2. Introduction Ultraviolet Spectroscopy (UV) 紫外光谱----测定有机物中是否存在共轭双键和芳香族化合物 Infrared Spectroscopy (IR)红外光谱----测定有机物中官能团。 Nuclear Magnetic Resonance Spectroscopy (NMR) 核磁共振谱----测定有机物中不同类型的氢或碳的数目和位置。（1H NMR and 13C NMR) Mass Spectrometry (MS)质谱----测定有机物的分子量。

  3. The characteristics of spectroscopic determination Microscale sample (1 – 5 mg, 样品少) It need short time to determine sample(时间短) Identify structure very fast.(结构确认快而准) Most are nondestructive (多不破坏样品) 重点:了解简单原理, 简单识谱

  4. The electromagnetic spectrum 电磁波谱

  5. Ultraviolet spectroscopy(紫外光谱, 15-13, p 666) • UV spectoscopy observes electronic transitions and provides information on the electronic bonding in the sample.

  6. σ* σ→ σ* π* n→ σ* ΔE E n π→ π* UV π n→ π* σ 1) Spectral region of UV: 200-400nm 2) Ultraviolet light and electronic transition (紫外光与电子跃迁)

  7. π→ π*：K带, 吸收峰强, 共轭双键增加, 向长波方向移动, B带, 苯的吸收带, 宽 230~270 nm, 中心254 nm E带, 把苯环看成乙烯键或共轭乙烯键跃迁引起的吸收带. n→ π*:R带, > 270 nm, 吸收峰较弱, C=O, C=N, -NO2

  8. Absorptance π→ π*：K带 n→ π*:R带 Wavelength

  9. 增大 A compounds that contain a longer chain of conjugated double bonds absorbs light at a longer wavelength. 共轭体系增加, 吸收波长增大, 即吸收“红移”

  10. β – Carotene 胡罗卜素 β – Carotene 胡罗卜素 Lycopene 番茄红素

  11. 酸式结构, 无色 碱式结构, 红色 酚酞

  12. 3) Obtaining an UV spectrum

  13. reference cell source 光源 Ir detector 检测仪 A λ Is recorder 记录仪 monochromator 单色器 sample cell 样品池 UV测定常用溶剂: CH3OH, CH3CH2OH

  14. Lambert-Beer Rule 朗伯－比尔定律 UV principle A----absorbance 吸光度 ε--- molar extinction coefficient 摩尔消光系数 c----molar concentration of sample，mol/L l-------the length, cm 液层厚度 Is--- 透过样品的光强度 Ir ---透过空白样的光强度

  15. 4) Interpreting UV spectra (解谱) (1) 判断有无共轭体系存在，或有无芳烃、醛、酮、羧酸、芳胺等有机物。 每增加一个共轭双键，波长增加 30~40 nm; 每增加一个烷基取代基, 波长增加约 5 nm. See: Table 15-2 (p 671) (problem: 15-3, 15-22)

  16. (2) 共轭链连有未共用电子对基团如：-NH2, -NR2, -OH, -OR, -SR, -Cl, -Br, -I,可产生p~π共轭，使化合物颜色加深，λmax 向长波方向移动-----把这样的基团称为助色基． 发色基； 记录格式 2,5-Dimethyl-2,4-hexadiene

  17. logε ε λ/nm λ/nm 苯在己烷中的吸收光谱 联苯类化合物的紫外光谱图

  18. 280nm (ε13500); 295nm (ε27000) 顺式： 由于位阻，共轭不是太好，故波长短，ε小些 反式： 共轭效果好，故波长增加，ε增大

  19. Assignment：T-1, 15-28; T-2， 8-2

  20. 2. Infrared spectroscopy (IR, 红外光谱, 12-1~12, p 490-519) • IR spectroscopy observes the vibrations of the bonds and provides evidence of the functional groups present.

  21. Spectral region of IR: • 中红外: λ2.5-25 μm (波数4000~400 cm-1)

  22. 2) Molecular vibrations and IR spectroscopy Stretching vibration 伸缩振动 symmetric stretching antisymmetric stretching Bending vibration 弯曲振动 Rocking Twisting Scissoring wagging

  23. IR principle like a spring ν：振动频率 ｋ：化学键的力常数 μ：折合质量 Frequency: decreases with increasing atomic weight; increase with bonding energy. 键能越大，折合质量越小，频率越高，峰出现在高波数区．

  24. Bond stretching frequency (Table 12-1)

  25. Frequency range of functional groups 1500-3600, functional group region 官能团区 <1500, fingerprint region 指纹区

  26. IR-active and IR-inactive vibration • A polar bond is usually IR-active. • A nonpolar bond in a symmetrical molecule will absorb weakly or not at all. 　　只有分子振动时偶极矩（dipole moment）发生变化的振动（Δμ≠0）才有红外吸收。偶极矩μ=δ×ｄ 偶极矩变化越大，红外吸收越强． 　偶极矩变化与以下因素有关: 电负性(电负性差别大，吸收强)、 振动方式(不对称伸缩>对称伸缩>弯曲振动) 分子的对称性(对称性差，吸收峰强) 氢键(使吸收峰变宽变强) problem 12-2

  27. 3) Measurement of the IR spectrum Infrared spectrometer (红外光谱仪) 红外测定方法： 1. KBr disc (KBr压片法) 2. 液膜法（NaCl盐片) 3 石腊油法 A Nicolet 800 FT-IR, 400 to 4000 cm-1）

  28. Characteristic absorptions of common functional groups(常见官能团的特征吸收) hydrocarbons (烃): stretching frequency(cm-1)

  29. Alkane CH3(CH2)6CH3 CH3: 1380

  30. Alkene CH3(CH2)3CH=CH2

  31. Alkyne

  32. benzene =C-H toluene Aromatic compounds

  33. Alcohols and amines (醇和胺):

  34. Alcohol

  35. Amine

  36. Ethers C－O－C 1150-1070 [strong] 1120

  37. carbonyl compounds (羰基化合物): Usually, it’s the strongest IR signal.

  38. Ketone

  39. Aldehyde

  40. Carboxylic acid

  41. Amide Acetamide, 乙酰胺 3348, 3173 N-H 1681 C=O

  42. Carbon - Nitrogen Stretching • C - N 1200 cm-1. • C = N 1660 cm-1 and is much stronger than the C = C absorption in the same region. • C  N absorbs strongly just above 2200 cm-1. The alkyne C  C signal is much weaker and is just below 2200 cm-1 .

  43. A Nitrile IR Spectrum =>

  44. Summary of IR Absorptions

  45. Interpretation of the IR spectra (红外光谱解析) 红外光谱主要是获得有机物官能团信息． (1) 吸收峰的位置、强度和形状。 (2)先特征峰，后一般峰。先强峰，后次强峰，再中强峰，同时注意峰形，宽，尖，单峰或双峰。相关峰。 与标准图谱对照可以确认化合物。

  46. C5H10O 1716 3-pentanone CH3CH2COCH2CH3 Diethyl ketone

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