Infrared Spectroscopy 紅 外 線 光 譜 儀
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
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
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
The Spectrum and Molecular Effects => Chapter 12 5 =>
The IR Region Just below red in the visible region. Wavelengths usually 2.5-25 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
Molecular Vibrations Covalent bonds vibrate at only certain allowable frequencies. => Chapter 12 7
Stretching Frequencies Frequency decreases with increasing atomic weight. Frequency increases with increasing bond energy. => Chapter 12 8
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Vibrational Modes Nonlinear molecule with n atoms usually has 3n - 6 fundamental vibrational modes. Chapter 12 10
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Fingerprint of Molecule No two molecules will give exactly the same IR spectrum (except enantiomers). Simple stretching: 1600-3500 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): 600-1400 cm -1, called the fingerprint region and has the most complex vibrations. Chapter 12 12
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
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An Infrared Spectrometer Chapter 12 15 =>
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.
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 1640-1680 cm -1 conjugated C=C 1620-1640 cm -1 aromatic C=C approx. 1600 cm -1 => Chapter 12 17
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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
Examples Chapter 12 20
An Alkane IR Spectrum n >3000; d 1465, 1375 Chapter 12 21 =>
An Alkene IR Spectrum Chapter 12 22 =>
An Alkyne IR Spectrum Chapter 12 23 =>
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
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The O-H stretching absorption is very characteristic In very dilute solutions, hydrogen bonding is absent and there is a very sharp peak at 3590-3650 cm -1 In more concentrated solutions, the hydroxyl groups hydrogen bond to each other and a very broad and large peak occurs at 3200-3550 cm -1 Chapter 12 27
An Alcohol IR Spectrum Chapter 12 28 =>
An Amine IR Spectrum Chapter 12 29 =>
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
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
A Ketone IR Spectrum Chapter 12 32 =>
An Aldehyde (n HC=O) IR Spectrum Chapter 12 33 =>
O-H Stretch of a Carboxylic Acid This O-H absorbs broadly, 2500-3500 cm -1, due to strong hydrogen bonding. Chapter 12 34 =>
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, 1640-1680 cm -1. The C=O of an ester absorbs at a higher frequency, ~1730-1740 cm -1. Carbonyl groups in small rings (5 C s or less) absorb at an even higher frequency. => Chapter 12 35
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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
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
An Amide IR Spectrum Chapter 12 39 =>
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
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
A Nitrile IR Spectrum Chapter 12 42 =>
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Summary of IR Absorptions Chapter 12 44 => =>
TODAY S CHEMIST AT WORK Chapter 12 45
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
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