红外光谱IR.ppt
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1、江南大学超值-划算-购物推荐群: 302284607,Charpter 3 Infrared Spectroscopy,Vibrational portion,Fig. 3.1 the Relationship of the infrared region to others included in the electromagnetic Spectrum,红外光,Table 3.1 IR Range,Fig. 3.2 The Infrared Spectrum of polystyrene,3.1 The Infrared Absorption Process,Molecules are e
2、xcited to a higher energy state when they absorb IR radiation. 8-40 kJ/mole, selectively Radiation in this energy range corresponds to the range encompassing the stretching and bending vibrational frequencies of the bonds in most covalent molecules. Increase the amplitude of the vibrational motion.
3、Only those bonds which have a dipole moment that changes as a function of time are capable of absorbing IR radiation. Symmetric bonds, such as those of H2 or Cl2, do not absorb infrared radiation.,Symmetric or pseudosymmetric,A bond must present an electrical dipole which is changing at the same fre
4、quency as the incoming radiation in order for energy to be transferred. The changing electrical dipole of the bond can then couple with the sinusoidally changing electromagnetic field of the incoming radiation. A symmetric bond which has identical or nearly identical groups on each end will not abso
5、rb in the infrared.,3.2 Uses of the Infrared Spectrum,The infrared spectrum can be used for molecules much as a fingerprint can be used for humans. The infrared spectrum is to determine structural information about a molecule.,Fig. 3.3 The approximate regions where various common types of bonds abso
6、rb,the Modes of Stretching and Bending,Symmetric Stretch Asymmetric Stretch,Complicated Spectrum?,Fundamental absorptions: the ground state to the lowest-energy state.,Overtones: 2 , 3 , ,Combination bands:,Difference bands:,Fermi resonance: a fundamental vibration couples with an overtone or combin
7、ation band. Carbonyl compounds,Rotational coupling may lead to a considerable amount of unresolved fine structure.,3.4 Bond Properties and Absorption Trends,c- the speed of light K- the force constant - reduced mass,Hooks law:,Stronger bonds have a larger force constant K and vibrate at higher frequ
8、encies than weaker bonds. C-C C=C CC 1200cm-1 1650cm-1 2150cm-1 increasing K,Bending motions occur at lower energy (lower v) than the typical stretching motions because of the lower value for K. CH stretching CH bending 3000cm-1 1340cm-1,Hybridization affects the force constant K, spsp2sp3 sp sp2 sp
9、3 CH = CH CH 3300cm-1 3100cm-1 2900cm-1,Resonance has the effect of reducing the force constant K, and the absorption moves to a lower frequency. C=O (normal) C=O (conjugated) 1715cm-1 1675 1680cm-1,3.5 the Infrared Spectrometer,FT-IR (Fourier Transform Infrared Spectrometer) Greater speed Greater s
10、ensitivity Better signal-to noise,Fig. 3.4 A schematic diagram of a FT-IR Spectrometer,Michelson Interferometer,3.6 Preparation of Samples for Infrared Spectroscopy,Liquids. Salt plate KBr, 4000-400cm-1; NaCl, 4000-650cm-1 Solids. KBr pellet Nujol mull (mineral oil) Solution CCl4 The region around 7
11、85cm-1 is often obscured by the strong C-Cl stretch that occurs there.,Fig. 3.5,Solids: KBr pellet,The main disadvantage of this method is that KBr absorb water. Nujol mull method: grind the compound with mineral oil (Nujol) to create a suspension of the finely ground sample dispersed in the mineral
12、 oil. Nujol bands appear at 2924, 1462, and 1377 cm-1.,Fig. 3.6,Gas,Fig. 3.7 the Gas Cell,3.7 What to Look for When Examining Infrared Spectra,Fig. 3.8 The infrared spectrum of 3-methyl-2-butanone (neat liguid, KBr plates).,Characteristic the position (wavenumbers) the intensity the shape,C=O at 171
13、5cm-1 strong absorption broad peak,1,2,Fig. 3.9 A Comparison of the Intensities of the C=O and C=C Absorption Bands,C=O 1850-1630cm-1 C=C 1680-1620cm-1,Fig. 3.10 A Comparison of the Shapes of the Absorption Bands for the O-H and N-H Groups,O-H 3650-3200cm-1 N-H 3500-3300cm-1,3.8 Correlation Charts a
14、nd Tables,Table 3.2 A Simplified Correlation Chart,Fig. 3.11 Absorption bands of bonds,Table 3.3 Base Values for Absorptions of Bonds,3.9 How to Approach the Analysis of a Spectrum (or what you can tell at a glance),The C=O, O-H, N-H, C-O, C=C, CC, CN, and NO2 peaks are the most conspicuous and give
15、 immediate structural information if they are present. Do not try to make a detailed analysis of the C-H absorptions near 3000cm-1; almost all compounds have these absorptions.,The important gross features,1. Is a carbonyl group present? 1820-1660cm-1, often the strongest absorption and of medium wi
16、dth. If C=O is present: Broad absorption near 3400-2400cm-1O-Hacids Medium absorption near 3400cm-1(or a double peak) N-Hamides Strong-intensity absorptions near 1300-1000cm-1C-Oesters Two C=O absorptions near 1810 and 1760 cm-1anhydrides Two weak absorptions near 2850 and 2750 cm-1aldehydes The pre
17、ceding five choices have been eliminated ketones If C=O is absent: Broad absorption near 3400-3300cm-1O-H, confirm this by finding C-O near 1300-1000cm-1alcohols, phenols Medium absorption(s) near 3400cm-1N-Hamines Check for C-O near 1300-1000cm-1(and absence of O-H near 3400cm-1 ethers,2. Double bo
18、nds and/or aromatic rings,C=C is a weak absorption near 1650cm-1. Medium to strong absorptions in the region 1600-1450cm-1; these often imply an aromatic ring. Confirm the double bond or aromatic ring by consulting the C-H region; aromatic and vinyl C-H occurs to the left of 3000cm-1 (aliphatic C-H
19、occurs to the right of this value). 3. Triple bonds CN is a medium, sharp absorption near 2250cm-1. CC is a weak, sharp absorption near 2150cm-1. Check also for acetylenic C-H near 3300cm-1. 4. Nitro groups Two strong absorptions at 1600-1530cm-1 and 1390-1300cm-1. 5. Hydrocarbons None of the preced
20、ing are found. Major absorptions are in C-H region near 3000cm-1. Very simple spectrum; the only other absorptions appear near 1460 and 1375cm-1.,A. Alkanes,C-H stretch,around 3000cm-1,sp3 C-H 3000 cm-1 (vinylic, aromatic, acetylenic, or cyclopropyl),3.10 Hydrocarbons: Alkane, alkenes, and Alkynes,C
21、-H bending,CH2 1465cm-1(m) CH3 1450cm-1, 1375cm-1(m) A long-chain band (CH2)n bending (rocking), n 4, 720cm-1,C-C stretch not interpretatively useful; many weak peaks,Fig. 3.12 the Infrared Spectrum of Decane (癸烷),Fig. 3.13 the Infrared Spectrum of mineral oil,Fig. 3.14 the Infrared Spectrum of Cycl
22、ohexane,B. Alkenes,=C-H stretch (sp2) 3000cm-1(m) (3095-3010cm-1) out-of-plane (oop) bending 1000-650cm-1(s),C=C stretch 1660-1600cm-1(m-w),conjugation,moves C=C stretch to the lower frequencies increases the intensity,substituted,symmetrically substituted bonds no absorption symmetrically disubstit
23、uted double bonds trans vanishingly weak cis stronger,Fig. 3.15 the Infrared Spectrum of 1-Hexene,Fig. 3.16 the Infrared Spectrum of Cyclohexene,Fig. 3.17 the Infrared Spectrum of cis-2-Pentene,Fig. 3.18 the Infrared Spectrum of trans-2-Pentene,C-H (sp) stretch near 3300cm-1(s) CC stretch near 2150c
24、m-1(m-w),C. Alkynes,conjugation moves to lower frequency. disubstituted or symmetrically substituted no absorption or weak absorption,Fig. 3.20 the Infrared Spectrum of 4-octyne,Fig. 3.19 the Infrared Spectrum of 1-octyne,Fig. 2.21 the C-H Stretch Region,C-H Stretch Region 3300-2750cm-1,Table 3.4 St
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