CHAPTER 12INFRARED SPECTROSCOPY

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Electromagnetic Radiation

• Electromagnetic radiation: Light and other forms of radiant energy.

• Wavelength (l): The distance between consecutive peaks on a wave.

• Frequency (n): The number of full cycles of a wave that pass a given point in a second and is reported in hertz, which has the units s-1.

• Hertz (Hz): The unit in which radiation frequency is reported: s-1 (read “per second”).

l

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Electromagnetic Radiation

• Table 12.2 Common units used to express wavelength(l)

l

Angstrom (Å) 1 Å = 10-10 m

Relationto MeterUnit

1 mm = 10-3 m

1 nm = 10-9 m

1 m = 10-6 mNanometer (nm)Micrometer (m)

Millimeter (mm)

Meter (m) ----

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Molecular Spectroscopy

• Molecular spectroscopy The study of which frequencies of radiation are absorbed or emitted by a particular substance and the correlation of these frequencies with details of molecular structure

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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 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)-

n = = 400 cm-1= 4000 cm-1n = 10-2 m•cm-1

2.5 x 10-6 m

10-2 m•cm-1

2.5 x 10-5 m

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Infrared Spectroscopy

• Figure 12.2 Infrared spectrum of 3-methyl-2-butanone.

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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 correspond to frequencies in the infrared region, 4000 to 400 cm-1.

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Molecular Vibrations

• For a molecule to absorb IR radiation

• The bond undergoing vibration must be polar and

its vibration must cause a periodic and substantial change in the bond 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, are IR inactive because they are not polar bonds.

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Molecular Vibrations

• For a nonlinear molecule containing n atoms, 3n - 6allowed fundamental vibrations exist.

• 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.

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Molecular vibrations

• Figure 12.3 Fundamental stretching and bending vibrations for a methylene group.

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Molecular Vibrations

• Consider two covalently bonded atoms as two vibrating masses connected by a spring.

• The total energy is proportional to the frequency of

vibration; E = hn where h is Planck’s constant.

• The frequency of a stretching vibration is given by an equation derived from Hooke’s law for a vibrating spring.

K = a force constant, which is a measure of bond strength; force constants for single, double, and triple bonds are approximately 5, 10, and 15 x 105 dynes/cm.

= reduced mass of the two atoms, (m1m2)/(m1 + m2), where m is the mass of the atoms in amu.

Kn = 4.12

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Molecular Vibrations

• From this equation, we see that the position of 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.

Kn = 4.12

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Correlation Tables

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Correlation Tables

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Alkanes

• Figure 12.4 Infrared spectrum of decane.

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Alkenes

• Figure 12.5 Infrared spectrum of cyclohexene.

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Alkynes

• Figure 12.6 Infrared spectrum of 1-octyne.

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Aromatics

• Figure 12.7 Infrared spectrum of toluene.

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Alcohols

• The free O-H is not intrinsically weak. It’s intensity depends on concentration and the hydrogen bonding character of the solvent.

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Ethers

• Figure 12.9 Infrared spectrum of dibutyl ether.

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Ethers

• Figure 12.10 Infrared spectrum of anisole.

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Amines

• Figure 12.11 Infrared spectrum of 1-butanamine, a 1° amine.

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IR of Molecules with C=O Groups

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IR of Molecules with C=O Groups

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Aldehydes and Ketones

• Figure 12.12 Infrared spectrum of menthone.

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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 frequency.

• Conjugation shifts the C=O absorption to lower frequency.

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Carboxylic acids

• Figure 12.13 Infrared spectrum of pentanoic acid.

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Amides

• Figure 12.14 Infrared spectrum of N-methylpropanamide (a secondary amide)

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Esters

• Figure 12.15 Infrared spectrum of ethyl butanoate.

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Problem 12.6

• On the following screen are two IR spectra.

• One spectra is of nonane, the other is of 1-hexanol. Assign each compound to its correct spectrum.

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Problem 12.6