Infrared Spectroscopy 紅 外 線 光 譜 儀

Transcription

1 Infrared Spectroscopy 紅 外 線 光 譜 儀

2 Introduction Spectroscopy is an analytical technique which helps determine structure. It destroys little or no sample (nondestructive method). The amount of light absorbed by the sample is measured as wavelength is varied. Chapter 12 2

3 Types of Spectroscopy Infrared (IR) spectroscopy measures the bond vibration frequencies in a molecule and is used to determine the functional group. Mass spectrometry (MS) fragments the molecule and measures the masses. Nuclear magnetic resonance (NMR) spectroscopy detects signals from hydrogen atoms and can be used to distinguish isomers. Ultraviolet (UV) spectroscopy uses electron transitions to determine bonding patterns. => Chapter 12 3

4 Electromagnetic Spectrum Examples: X rays, microwaves, radio waves, visible light, IR, and UV. Electromagnetic radiation has the characteristics of both waves and particles The wave nature of electromagnetic radiation is described by wavelength (l) or frequency (n) The relationship between wavelength (or frequency) and energy (E) is well defined Wavelength and frequency are inversely proportional (n= c/l) The higher the frequency, the greater the energy of the wave The shorter the wavelength, the greater the energy of the wave Chapter 12 4

5 The Spectrum and Molecular Effects => Chapter 12 5 =>

6 The IR Region Just below red in the visible region. Wavelengths usually mm. More common units are wavenumbers, or cm -1, the reciprocal of the wavelength in centimeters. Wavenumbers are proportional to frequency and energy. Chapter 12 => 6

7 Molecular Vibrations Covalent bonds vibrate at only certain allowable frequencies. => Chapter 12 7

8 Stretching Frequencies Frequency decreases with increasing atomic weight. Frequency increases with increasing bond energy. => Chapter 12 8

9 Chapter 12 9

10 Vibrational Modes Nonlinear molecule with n atoms usually has 3n - 6 fundamental vibrational modes. Chapter 12 10

11 Chapter 12 11

12 Fingerprint of Molecule No two molecules will give exactly the same IR spectrum (except enantiomers). Simple stretching: cm -1 has the most common vibrations, and we can use it to get information about specific functional groups in the molecule. Complex vibrations (bending): cm -1, called the fingerprint region and has the most complex vibrations. Chapter 12 12

13 IR-Active and Inactive A polar bond is usually IR-active. A nonpolar bond in a symmetrical molecule will absorb weakly or not at all. Chapter 12 13

14 Chapter 12 14

15 An Infrared Spectrometer Chapter =>

16 FT IR Spectrometer Has better sensitivity. Less energy is needed from source. Completes a scan in 1 to 2 seconds. Takes several scans and averages them. Has a laser beam that keeps the instrument accurately calibrated.

17 Carbon-Carbon (C-C) Bond Stretching Stronger bonds absorb at higher frequencies: C-C 1200 cm -1 C=C 1660 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 => Chapter 12 17

18 Chapter 12 18

19 Carbon-Hydrogen (C-H) Stretching Bonds with more s character absorb at a higher frequency. sp 3 C-H, just below 3000 cm -1 (to the right) sp 2 C-H, just above 3000 cm -1 (to the left) sp C-H, at 3300 cm -1 => Chapter 12 19

20 Examples Chapter 12 20

21 An Alkane IR Spectrum n >3000; d 1465, 1375 Chapter =>

22 An Alkene IR Spectrum Chapter =>

23 An Alkyne IR Spectrum Chapter =>

24 O-H and N-H Stretching Both of these occur 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) => Chapter 12 24

25 Chapter 12 25

26 Chapter 12 26

27 The O-H stretching absorption is very characteristic In very dilute solutions, hydrogen bonding is absent and there is a very sharp peak at cm -1 In more concentrated solutions, the hydroxyl groups hydrogen bond to each other and a very broad and large peak occurs at cm -1 Chapter 12 27

28 An Alcohol IR Spectrum Chapter =>

29 An Amine IR Spectrum Chapter =>

30 Carbonyl(n C=O) Stretching The C=O bond of simple ketones, aldehydes, and carboxylic acids absorb around n 1710 cm -1. => Chapter 12 30

31 Usually, it s the strongest IR signal. Carboxylic acids will have O-H also. Aldehydes have two C-H signals around 2700 and 2800 cm -1. Chapter 12 31

32 A Ketone IR Spectrum Chapter =>

33 An Aldehyde (n HC=O) IR Spectrum Chapter =>

34 O-H Stretch of a Carboxylic Acid This O-H absorbs broadly, cm -1, due to strong hydrogen bonding. Chapter =>

35 Variations in C=O Absorption Conjugation of C=O with C=C lowers the stretching frequency to ~1680 cm -1. The C=O group of an amide absorbs at an even lower frequency, cm -1. The C=O of an ester absorbs at a higher frequency, ~ cm -1. Carbonyl groups in small rings (5 C s or less) absorb at an even higher frequency. => Chapter 12 35

36 Chapter 12 36

37 習 題 Portions of the infrared spectra of three cyclic ketones and three exocyclic alkenes show the influence of ring strain on the C=O and C=C stretching frequency. Please indicate your combination and explain your reasons. H 2 C O CH 2 O CH 2 O Chapter 12 37

38 習 題 The C=O vibration frequencies are varied to different molecules which reveals the influence of conjugation and other factors. Show the order of the carbonyl absorption of the following molecules and explain your reasons for full credits. (I) (II) (III) O O O O O OH O CH 3 (IV) O (V) O O CH 3 O CH 3 Chapter 12 38

39 An Amide IR Spectrum Chapter =>

40 習 題 The absorption of carbonyl group in amide is usually in the range from 1680 to 1630 cm -1. However, the following compound has the C=O band appearing about 1700cm -1. Please explain this observation. N C=O:1700cm -1 O Chapter 12 40

41 Carbon - Nitrogen Stretching (C~N) C - N absorbs around 1200 cm -1. C = N absorbs around 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. => Chapter 12 41

42 A Nitrile IR Spectrum Chapter =>

43 Chapter 12 43

44 Summary of IR Absorptions Chapter => =>

45 TODAY S CHEMIST AT WORK Chapter 12 45

46 In addition to the physical symptoms, diseases cause changes in the chemical composition of the organs,tissues, or fluids they affect; these differences are the basis of everyday clinical chemical tests, tissue staining, and medical imaging techniques. IR spectroscopy not only probes the chemical composition of a sample but also determines the precise position and amplitude of IR absorption bands that reflect interactions among the matrix constituents. Because of its sensitivity to both molecular structure and molecular interactions, the spectrum is often referred to as a molecular fingerprint of the sample; the specificity of that fingerprint is the basis for biomedical applications. Chapter 12 46

47 Strengths and Limitations IR alone cannot determine a structure. Some signals may be ambiguous. The functional group is usually indicated. The absence of a signal is definite proof that the functional group is absent. Correspondence with a known sample s IR spectrum confirms the identity of the compound. => Chapter 12 47