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1 CHEM 241 UNIT 5: PART B INFRA-RED RED SPECTROSCOPY 1

2 Spectroscopy of the Electromagnetic Spectrum Radiant energy is proportional to its frequency (cycles/s = Hz) as a wave (Amplitude is its height) Different types are classified by frequency or wavelength ranges 2

3 Absorption Spectra Organic compound exposed to electromagnetic radiation, can absorb energy of only certain wavelengths (unit of energy) Transmits, energy of other wavelengths. Changing wavelengths to determine which are absorbed and which are transmitted produces an absorption spectrum Energy absorbed is distributed internally in a distinct and reproducible way (See Figure 12-11) 3

4 Electromagnetic Radiation Electromagnetic radiation: light and other forms of radiant energy Wavelength (λ): the distance between consecutive identical points on a wave Frequency (ν): the number of full cycles of a wave that pass a point in a second Hertz (Hz): the unit in which radiation frequency is reported; s -1 (read per second ) 4

5 Planck s Equation. hc E = hν = λ E = energy of 1 photon h = Planck s constant, 6.62 x J-s ν = frequency, s -1 λ = wavelength, cm c = speed of light, 3.00 x cm/s 5

6 Infrared Spectroscopy of Organic Molecules IR region lower energy than visible light (below red produces heating as with a heat lamp) m to m region used by organic chemists for structural analysis IR energy in a spectrum is usually measured as wavenumber (cm -1 ), the inverse of wavelength and proportional to frequency Absorption of IR radiation in this region causes bonds to change from a lower vibrational energy level to a higher one Specific IR absorbed by organic molecule related to its structure 6

7 Infrared Spectroscopy The frequency of IR radiation is commonly expressed in wavenumbers Wavenumber (1/λ ) (ν): the number of waves per centimeter, cm -1 (read reciprocal centimeters) Expressed in wavenumbers, the vibrational IR extends from 4000 cm -1 to 400 cm -1 10,000 µm cm -1 = 4000 cm µm 10,000 µm cm -1 = 400 cm µm 7

8 Infrared Spectroscopy For the IR spectra recorded in this text, calibrations are 8

9 Molecular Vibrations Atoms joined by covalent bonds undergo continual vibrations relative to each other The energies associated with these vibrations are quantized; within a molecule, only specific vibrational energy levels are allowed The energies associated with transitions between vibrational energy levels for most covalent bonds are from 2 to 10 kcal/mol (8.4 to 42 kj/mol) 9

10 Molecular Vibrations For a molecule to absorb IR radiation, the bond undergoing vibration must be polar and its vibration must cause a periodic change in the bond moment (dipole moment) Covalent bonds which do not meet these criteria are said to be IR inactive the C-C double and triple bonds of symmetrically substituted alkenes and alkynes, for example, do not absorb IR radiation because they are not polar bonds 10

11 Infrared Energy Modes IR energy absorption corresponds to specific modes, corresponding to combinations of atomic movements, such as bending and stretching of bonds between groups of atoms called normal modes Energy is characteristic of the atoms in the group and their bonding Corresponds to vibrations and rotations 11

12 Vibrational Modes Energy is absorbed when the energy of the radiation is the same as the energy difference between two vibrational frequencies. 12

13 Interpreting Infrared Spectra Most functional groups absorb at about the same energy and intensity independent of the molecule they are in Characteristic higher energy IR absorptions in Table 12.1 can be used to confirm the existence of the presence of a functional group in a molecule IR spectrum has lower energy region characteristic of molecule as a whole ( fingerprint region) See samples in Figure

14 Characteristic Absorptions 14

15 Regions of the Infrared Spectrum cm -1 N-H, C-H, O-H (stretching) N-H, O- H 3000 C-H cm -1 C C and C N (stretching) cm -1 double bonds (stretching) C=O C=C cm -1 Below 1500 cm -1 fingerprint region 15

16 Differences in Infrared Absorptions Molecules vibrate and rotate in normal modes, which are combinations of motions (relates to force constants) Bond stretching dominates higher energy modes Light objects connected to heavy objects vibrate fastest: C-H, N-H, O-H For two heavy atoms, stronger bond requires more energy: C C, C N > C=C, C=O, C=N > C-C, C-O, C-N, C-halogen 16

17 Infrared Spectra of Hydrocarbons C-H, C-C, C=C, C C have characteristic peaks absence helps rule out C=C or C C 17

18 Infrared Spectra of Some Common Functional Groups Spectroscopic behavior of functional group is discussed in later chapters Brief summaries presented here 18

19 IR: Alcohols and Amines O H 3400 to 3650 cm 1 Usually broad and intense N H 3300 to 3500 cm 1 Sharper and less intense than an O H 19

20 IR: Aromatic Compounds Weak C H stretch at 3030 cm 1 Weak absorptions cm 1 range Medium-intensity absorptions 1450 to 1600 cm 1 See spectrum of phenylacetylene, Figure

21 IR: Carbonyl Compounds Strong, sharp C=O peak 1670 to 1780 cm 1 Exact absorption characteristic of type of carbonyl compound 1730 cm 1 in saturated aldehydes 1705 cm 1 in aldehydes next to double bond or aromatic ring 21

22 C=O in Ketones 1715 cm 1 in six-membered ring and acyclic ketones 1750 cm 1 in 5-membered ring ketones 1690 cm 1 in ketones next to a double bond or an aromatic ring 22

23 C=O in Esters 1735 cm 1 in saturated esters 1715 cm 1 in esters next to aromatic ring or a double bond 23

24 Interpreting IR Spectra The region between 1500 and 400 cm -1 is known as the fingerprint region. The region is very rich in structural information. We do not have to fully interpreted IR spectra to get useful information. Characteristic absorptions (see Table 12.1) 24

25 25

26 26

27 Hexane C-H rock C-H stretch C-H bend 27

28 1-Hexene Olefinic C-H C=C 28

29 1-Hexyne C-C triple bond alkyne C-H 29

30 1-Hexanol O-H OH 30

31 Benzene C=C ring aromatic C-H C=C ring C=C ring 31

32 Ethylbenzene C=C ring aromatic C-H C-H stretch C=C ring C=C ring 32

33 Acetic Acid O-H C-H stretch O C=O C-O-H O-H H 3 C C OH 33

34 IR examples More Examples 34

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