1、Organic Chemistry,William H. Brown Christopher S. Foote Brent L. Iverson,Infrared Spectroscopy,Chapter 12,Electromagnetic Radiation,Electromagnetic radiation: light and other forms of radiant energy Wavelength (): the distance between consecutive peaks on a wave Frequency (): the number of full cycl

2、es of a wave that pass a given point in a second Hertz (Hz): the unit in which radiation frequency is reported; s-1 (read “per second”),Electromagnetic Radiation,Common units used to express wavelength,Molecular Spectroscopy,Molecular spectroscopy: the study of which frequencies of electromagnetic r

3、adiation are absorbed or emitted by a particular substance and the correlation of these frequencies with details of molecular structure we study three types of molecular spectroscopy,Infrared Spectroscopy,The vibrational IR extends from 2.5 x 10-6 m (2.5 m) to 2.5 x 10-5 m (25 m) the frequency of IR

4、 radiation is commonly expressed in wavenumbers wavenumber : the number of waves per centimeter, with units cm-1 (read reciprocal centimeters) expressed in wavenumbers, the vibrational IR extends from 4000 cm-1 to 400 cm -1,n =,Infrared Spectroscopy,IR spectrum of 3-methyl-2-butanone,Molecular Vibra

5、tions,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 corr

6、espond to frequencies in the infrared region, 4000 to 400 cm-1,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 dipole moment Covalent bonds which do not meet these criteria are said to b

7、e IR inactive the C-C double and triple bonds of symmetrically substituted alkenes and alkynes, for example, are IR inactive because they are not polar bonds,C,C,2,3-Dimethyl-2-butene,2-Butyne,Molecular Vibrations,For a nonlinear molecule containing n atoms, there are 3n - 6 allowed fundamental vibr

8、ations For even a relatively small molecule, a large number of vibrational energy levels exist and patterns of IR absorption can be very complex The simplest vibrational motions are bending and stretching,Molecular vibrations,Fundamental stretching and bending vibrations for a methylene group,Molecu

9、lar Vibrations,Consider two covalently bonded atoms as two vibrating masses connected by a spring the total energy is proportional to the frequency of vibration the frequency of a stretching vibration is given by an equation derived from Hookes law for a vibrating spring K = a force constant, which

10、is a measure of the bonds strength; force constants for single, double, and triple bonds are approximately 5, 10, and 15 x 105 dynes/cm m = reduced mass of the two atoms, (m1m2)/(m1 + m2), where m is the mass of the atoms in grams,Molecular Vibrations,From this equation, we see that the position of

11、a stretching vibration is proportional to the strength of the vibrating bond is inversely proportional the masses of the atoms connected by the bond The intensity of absorption depends primarily on the polarity of the vibrating bond,Correlation Tables,Table 12.4 Characteristic IR absorptions for the

12、 types of bonds and functional groups we deal with most often,Hydrocarbons-Table 12.5,Alkanes,IR spectrum of decane (Fig 12.4),Alkenes,IR spectrum of cyclohexene (Fig 12.5),Alkynes,IR spectrum of 1-octyne (Fig 12.6),Aromatics,IR spectrum of toluene (Fig 12.7),Alcohols,IR spectrum of 1-hexanol (Fig 1

13、2.8),Ethers,IR spectrum of dibutyl ether (Fig 12.9),Ethers,IR spectrum of anisole (Fig 12.10),Amines,IR spectrum of 1-butanamine (Fig 12.11),IR of Molecules with C=O Groups,IR of Molecules with C=O Groups,Aldehydes and Ketones,IR spectrum of menthone (Fig 12.12),Carbonyl groups,The position of C=O stretching vibration is sensitive to its molecular environment as ring size decreases and angle strain increases, absorption shifts to a higher frequen