2.1.1 Infrared region

Transcription

1 Chapter 2 Infrared Spectroscopy

2 What is IR looks like?

3 2.1 Introduction Infrared region The structure of new compounds that are isolated from natural sources or prepared din the lab must be determined (and/or verified). Chemical analysis Spectroscopy Spectroscopic techniques are non-destructive and generally require small amounts of sample

4 Infrared spectroscopy uses light from the infrared region of the electromagnetic spectrum. The absorption of IR radiation leads to absorption bands in the IR spectrum. The position of a band is reported in wavenumbers ( v) the number of wavelengths per cm Directly proportional to energy

5 Wavenumbers vs wavelengths = = = 4000cm λ ν ν 2 = = = 625cm λ

6 The term "Infrared" covers the range of the electromagnetic spectrum between 0.78 and 1000 mm. In the context of infrared spectroscopy, wavelength is measured in "wavenumbers", which have the units cm-1. It is useful to divide the infrared region into three sections Wavelength range Wave number range Region (μm) (cm -1 ) Near Middle Far

7 Infrared spectroscopy is a type of absorption spectroscopy Any technique that measures the amount of light absorbed by (or transmitted through) a compound as a function of the wavelength of light. Sample is irradiated by a light source Amount of light transmitted (or absorbed) at various wavelengths is measured by a detector A spectrum is obtained. Graph of light transmitted (or absorbed) as a function of wavelength

8 principle i of finfrared spectra Molecular vibrations The atoms in a molecule are in constant motion. The covalent bond between two atoms acts like a spring, allowing the atoms to vibrate (stretch and bend) relative to each other.

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10 2.2.2 The frequency and intensity of absorption The absorption of IR radiation increases the amplitude of the bond vibrations. Since energy is quantized, covalent bonds can vibrate/stretch only at certain allowed frequencies. The position of an absorption band correlates with the type of chemical bond.

11 The frequency and intensity of an absorption band in an IR spectrum depends primarily on: Type of vibration Stretching vibrations require more energy than bending, absorption band at higher frequency Masses of the atoms in a bond Strength of the bond/bond order Difference in electro negativity between the atoms

12 The stretching frequency depends on atomic weight and bond energy/bond order. Frequency decreases with increasing atomic weight. Frequency increases with bond energy/bond order.

13 Bond polarity has little affect on the frequency of an IR band but it does significantly influence the intensity of the peak. Vibrations that cause a significant change in the dipole moment of a chemical bond lead to strong absorption bands. Symmetrical bonds lead to very weak or no absorption band.

14 Infrared Spectrometer

15 可逆电机 记录仪 衰减器 M 1 M 3 M 2 光源 样品池 M 5 M 6 M 4 切光器 波长扫描电动机光栅检测器 滤波器 调制器 放大器 相敏检波器 前置放大器 双光束光学自动平衡式红外光谱仪原理图

16 Fourier Transform Infrared Spectrometer (FTIR) Uses an interferometer Interferogram simultaneously contains data for all frequencies Has better sensitivity. Less energy is needed dfrom source. Completes a scan in 1-2 seconds. Takes several scans and averages them. Has a laser beam that keeps the instrument accurately calibrated.

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18 2.3 Functional Group Absorptions Carbon-Carbon Bonds O-H and N-H Bonds Carbonyls Fingerprint region

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20 =C-H C-H C C C=C O-H O-H( 氢键 ) C-C,C-N,C-OC C C C=O S-H P-H N-O N-N C-F C-X N-H C N C=N C-H,N-H,O-H

21 2.31 Carbon-Carbon Bonds Increasing bond order leads to higher frequencies: C-C 1200 cm -1 (fingerprint region) C=C cm -1 C C 2200 cm -1 (weak or absent if internal) Conjugation lowers the frequency: isolated C=C cm -1 conjugated C=C cm -1 aromatic C=C approx cm -1 C=C peaks are generally weak to moderate in intensity.

22 Bonds with more s character absorb at a higher frequency. sp 3 (alkane) C-H just below 3000 cm -1 (to the right) sp 2 (alkene or aromatic hydrocarbon) C-H just above 3000 cm -1 (to the left) sp (lk (alkyne)ch C-H at 3300 cm -1

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24 IR Spectrum of n-octane

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26 2.3.2 O-H and N-H Bonds Both O-H and N-H appear around 3300 cm -1, but they look different. Alcohol O-H broad with rounded tip. Secondary amine (R 2 NH) broad with one sharp spike Primary amine (RNH 2 ) broad with two sharp spikes. No signal for a tertiary amine (R 3 N)

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28 2.3.3 Carbonyls Carbonyl stretches are generally strong: Aldehyde ~1710 cm -1 Ketone ~1710 cm -1 Carboxylic acid ~1710 cm -1 Ester ~ cm -1 Amide ~ cm -1 Conjugation shifts all carbonyls to lower frequencies.

29 Aldehydes

30 Ketones

31 Carboxylic Acids

32 Esters C=O stretch at ~ cm -1 and C-O stretch at cm -1 strong (Note: other functional groups may have peaks in the cm -1 region too!) O O

33 Anhydride C=O

34 Amides C=O stretch at cm -1 N-H stretch (if present) around 3300 cm -1

35 Nitriles C N absorbs just above 2200 cm -1 (med strong) The alkyne C C signal is much weaker and is just below 2200 cm -1

36 2.3.4 Fingerprint region IR spectrum results from a combination of all possible stretching and/or bending vibrations of the individual bonds and the whole molecule. Simple stretching: ~ cm -1. Complex vibrations: cm -1, called the fingerprint region. Each molecule has a unique IR spectrum. The IR spectrum is a fingerprint i for the molecule. 1400~ 900 cm -1 :COCN C-O, C-N, C-F, FCPCS C-P, C-S, P-O, Si-O, OC=SS=OP=O C=S, S=O,

37 The Eight Important Region ~3000 cm -1 υ OH, υ NH ~3000cm -1 υ C=H ~2700cm υ CH ~2100cm -1 υ C C or υ C N ~1650cm -1 υ C=O ~1500cm -1 υ C=C C ~1300cm 1 υ C-H ~650cm 1 υ C-H

38 2.4 Factors influencing IR Absorption Outer factors Inner factors

39 2.4.1 Outer factors Concentration

40 242Innerfactors Masses of attached atoms. As masses increase, wavenumber decreases. C-H C-CC C-O C-ClCl C-Br C-I 3000 cm cm cm cm cm cm -1 Strength of chemical bond. As bond strength increases, wavenumber increases. C, C triple bond C=CC C-CC 2150 cm cm cm -1

41 Hybridization. Bonds are stronger in the order sp > sp 2 > sp 3. C-H H(sp) C-H H(sp 2 ) C-H H(sp 3 ) 3300 cm cm cm -1 Resonance. Conjugation lowers the energy to vibrate bond. isolated ketones α,β-unsaturated ketone α,β,γ-unsaturated ketone 715 cm cm cm-1 Different isomers. Electronic effect. Hydrogen bond Inductive effect Conjugated effect

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45 2.5 Application of IR Example 1: Interpret the following IR spectrum by assigning each of the major peaks. Identify what functional groups are present

46 Example 2: Which one of the following compounds is the most reasonable structure for the IR spectrum shown below? 1721

47 Example 3: Which of the following compounds is the most reasonable structure for the IR spectrum shown below? O NH 2 O OH OH O OH O OH

48 Exercise 1 H C CH 2

49 Exercise 2. A compound with formula C 8 H 7 N. H 3 C CN

50 Exercise 3 CH 3 CH 2 CH 2 CH 2 NH 2

51 Homework 1. Review this chapter 2. Do exercises 3. Check the website of bin/cre_index.cgi?lang=eng index cgi?lang eng 4. Preview NMR