William H. Brown & Christopher S. Foote

Transcription

1 Requests for permission to make copies of any part of the work should be mailed to:permissions Department, Harcourt Brace & Company, 6277 Sea Harbor Drive, Orlando, Florida William H. Brown & Christopher S. Foote 14-1

2 Chapter

3 Infrared Spectroscopy u The IR region covers 7.8 x 10-7 m (just above the visible region) to 2.0 x 10-3 m (just below the microwave region) u Organic chemistry uses mainly the vibrational IR, which covers 2.5 x 10-6 m (2.5 m) to 2.5 x 10-5 m (25 m) u Absorption of IR radiation in this region causes bonds to change from a lower vibrational energy level to a higher one 14-3

4 Infrared Spectroscopy u The frequency of IR radiation is commonly expressed in wavenumbers ( ) - u Wavenumber: the number of waves per centimeter, cm -1 (read reciprocal centimeters) u Expressed in wavenumbers, the vibrational IR extends from 4000 cm -1 to 400 cm -1 10,000 m cm -1 = 4000 cm m 10,000 m cm -1 = 400 cm m 14-4

5 Infrared Spectroscopy u For the IR spectra recorded in this text, calibrations are Transmittance (%) Micrometers Wavenumber (cm -1 )

6 Molecular Vibrations u Atoms joined by covalent bonds undergo continual vibrations relative to each other u The energies associated with these vibrations are quantized; within a molecule, only specific vibrational energy levels are allowed u 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) 14-6

7 Molecular Vibrations u 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 u 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 14-7

8 Molecular Vibrations u For a nonlinear molecule containing n atoms, there are 3n -6 allowed fundamental vibrations u For even a relatively small molecule, a large number of vibrational energy levels exist and patterns of IR absorption can be very complex u The simplest vibrational motions are bending and stretching 14-8

9 Molecular Vibrations u Consider two covalently bonded atoms as two vibrating masses connected by a spring u As the bond vibrates, its energy continually changes from kinetic to potential and vice versa u The total energy (KE + PE) is proportional to the frequency of vibration 14-9

10 Molecular Vibrations u For a simple harmonic oscillator, the frequency of a stretching vibration is given by an equation derived from Hooke s law for a vibrating spring 1 2 c NK N = Avogadro s number c = velocity of light K = a force constant, which is a measure of the bonds strength m = the reduced mass 14-10

11 Molecular Vibrations u From this equation, we see that the position of absorption of a stretching vibration depends on the strength of the vibrating bond and the masses of the atoms connected by the bond u The stronger the bond and the lighter the atoms connected by the bond, the higher the wavenumber of the vibration u The intensity of absorption depends primarily on the polarity of the vibrating bond 14-11

12 Correlation Tables u Table 14.1 Characteristic IR absorptions for the types of bonds and functional groups we deal with most often Bond O-H N-H C-H C=O C=C C-O Frequency (cm -1 ) Intensity strong and broad medium medium to strong strong weak strong 14-12

13 Hydrocarbons-Table 14.2 Hydrocarbon Alkane C-H CH 2 CH 3 Alkene C-H C=C Alkyne C-H C C Vibration stretching stretching Frequency (cm -1 ) Intensity stretching strong bending 1450 medium bending 1375 and 1450 weak to medium weak to medium weak to medium stretching 3300 medium to strong stretching weak 14-13

14 Alcohols & Ethers u Table 14.3 Bond Frequency, cm -1 Intensity O-H (free) O-H (hydrogen bonded) C-O weak medium, broad medium 14-14

15 UV/Vis Spectroscopy u Most UV/visible spectrophotometers cover from 200 to 400 nm (the near ultraviolet) and 400 nm (violet light) to 700 nm (red light) Region of Spectrum ultraviolet visible Wavelength (nm) Energy (kcal/mol)

16 UV/Vis Spectroscopy u UV-Vis spectral data are plotted as absorbance (A) versus wavelength (nm) Absorbance (A) Wavelength (nm) 14-16

17 UV/Vis Spectroscopy u Absorbance: a quantitative measure of the extent to which a compound absorbs ultraviolet-visible radiation of a particular wavelength Absorbance (A) = log I o I I 0 is the intensity of the incident radiation on the sample I is the intensity of the radiation transmitted through the sample 14-17

18 Beer-Lambert law u Beer-Lambert law: the relationship between absorbance, concentration, and length of the sample tube Beer-Lambert Law: A = c l A = absorbance c = concentration (mol liter -1 ) l = length of the sample tube (cm) = molar absorptivity (liter mol -1 cm -1 ). Experimental values of range from 0 to

19 Electronic Transitions u Absorption of UV-vis radiation results in transition of electrons from a lower energy occupied MO to a higher energy unoccupied MO u For example, to * transitions in conjugated systems such as CH 2 =CH-CH=CH 2 1,3-Butadiene O CH 2 =CH-C-CH 3 3-Buten-2-one O CH Benzaldehyde 14-19

20 Electronic Transitions u Electronic transitions are usually accompanied by changes in vibrational or rotational energy levels u The energy levels for these vibrational and rotational excitations are quite closely spaced and their difference is considerably smaller than the energy differences between electronic excitations 14-20

21 Electronic Transitions u Transitions between vibrational and rotational energy levels are superposed on the electronic excitations u The result is a large number of UV-Vis absorption peaks so closely spaced that the spectrophotometer cannot resolve them u For this reason, UV-Vis absorption peaks usually are much broader than IR peaks 14-21

22 Electronic Transitions u Table 14.5 Wavelengths and energies required for to * transitions of ethylene and three conjugated polyenes Name ethylene 1,3-butadiene Structural Formula CH 2 =CH 2 CH 2 =CHCH=CH 2 max (nm) Energy (kcal/ mol) (3E)-1,3,5- hexatriene CH 2 =CHCH=CHCH=CH (3E, 5E)-1,3,5,7- octatetraene CH 2 =CH(CH=CH) 2 CH=CH

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