A scientist working with a

Fourier transform infrared

spectrometer. Inset: a model

of 3-methyl-2-butanone. For an

IR spectrum of this compound,

see Figure 12.2.

OUTLINE12.1 Electromagnetic

Radiation

12.2 Molecular Spectroscopy

12.3 Infrared Spectroscopy

12.4 Interpreting InfraredSpectra

12.5 Solving Infrared SpectralProblems

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Electromagnetic radiationLight and other forms of radiant

energy.

Wavelength IA)The distance between consecutive

peaks on a wave.

FrequencyThe number of full cycles of a

wave that pass a given point in asecond; it is given the symbol v(Greek nul and reported in hertz(Hz), which has the units S-I.

Hertz 1Hz)The unit in which frequency ismeasured: S-1 (read "per second").

Determination of molecular structure is one of the central themes of organicchemistry. For this purpose, chemists today rely almost exclusively oninstrumental methods, four of which we discuss in this text. We begin in

this chapter with infrared (IR) spectroscopy. Then, in Chapters 13 and 14, weintroduce nuclear magnetic resonance (NMR) spectroscopy and mass spectrom­etry (MS). A brief introduction to ultraviolet-visible spectroscopy is contained inChapter 20, as part of our discussion of conjugated systems.

12.1 Electromagnetic RadiationGamma rays, x-rays, ultraviolet light, visible light, infrared radiation, microwaves,and radio waves are all types of electromagnetic radiation. Because electromag­netic radiation behaves as a wave traveling at the speed of light, it can be describedin terms of its wavelength and its frequency.

Table 12.1 summarizes wavelengths (.A.), frequencies, and energies of variousregions of the electromagnetic spectrum. The wavelengths of visible light fall inthe range 400-700 nm. Infrared radiation (which can be felt as heat but not seen)falls in the range 2-15 /-Lm.

Frequency, the number of full cycles of a wave that pass a given point in a sec­ond, is given the symbol v (Greek nu) and is reported in hertz (Hz), which has theunits S-I. Wavelength and frequency are inversely proportional, and one can becalculated from the other using the following relationship:

Av = c

where c is the velocity of light, 3.00 X 108 m/s. For example, consider the infraredradiation of wavelength 1.5 X 10-5 m (15 /-Lm). The frequency of this radiation is2.0 X 1013 Hz.

v = 3 X 108

m = 2.0 X 1013 Hz8/1.5 X 10 5 m

456 Chapter 12 Infrared Spectroscopy

Figure 12.1

Absorption of energy in theform of electromagneticradiation causes an atom ormolecule in energy state E1

to change to a higher energystate E2•

E2 ----

Energy in theform ofelectromagnetic

i¥l radiation

~~

Atom or moleculein energy state E1

Absorptionof radiation

•

Atom or moleculein energy state E2

Infrared spectroscopy is useful to organic chemists not only for the determinationof molecular structure but also for many other applications. For example, infraredspectroscopy is used to identify certain illegal substances in forensic science, and atype of infrared frequency measurement is used as an important probe of atmos­pheric and mineral composition in space exploration programs such as the recentMars rover missions.

A. The Vibrational Infrared Spectrum

Table 12.3 Types of Energy Transitions Resulting from Absorption of Energyfrom Three Regions of the Electromagnetic Spectrum

Organic molecules are flexible. As we discussed in Chapter 2, atoms and groupsof atoms can rotate about single covalent bonds. In addition, covalent bonds canstretch and bend just as if their atoms were joined by flexible springs. Infraredspectroscopy, also called IR spectroscopy, probes stretching and bending vibra­tions of organic molecules.

The vibrational infrared region, which extends from 2.5 X 10-6 to 2.5 X 10-5

m in wavelength, is used for infrared spectroscopy. Radiation in this region is mostcommonly referred to by its frequency in wavenumbers, P, the number of waves percentimeter, with units cm-1 (read: reciprocal centimeters). The frequency in wave­numbers is the reciprocal of the wavelength in centimeters, or the frequency (v) inhertz divided by c, the speed of light.

_ 1 10-2 (m'cm-1) vp=---= =-

A (em) A (m) c

When expressed in frequencies, the vibrational region of the infrared spectrumextends from 4000 to 400 cm-1.

Infrared URI spectroscopyA spectroscopic technique inwhich a compound is irradiatedwith infrared radiation, absorp­tion ofwhich causes covalentbonds to change from a lowervibration state to a higher one.Infrared spectroscopy is particu­larly valuable for determining thekinds offunctional groups presentin a molecule.

Vibrational infrared regionThe portion of the infrared re­gion that extends from 4000 to400 em-I.

Wavenumbers jj

The frequency of electromag­netic radiation expressed as thenumber of waves per centimeter,with units cm- I (read: reciprocalcentimeters) .

v = 10-2 m' cm-1

= 4000 cm-1

2.5 X 1O-6 mv = 10-

1 m 'cm-1= 400 cm-1

2.5 X 1O-5 m

Region ofElectromagneticSpectrum

Radio frequency

Infrared

Ultraviolet­visible

Frequency(hertz)

3 X 107 - 9 X 108

1 X 1013 - 1 X 1014

2.5 X 1014 - 1.5 X 1015

Type ofSpectroscopy

Nuclear magneticresonance

Infrared

Ultraviolet­visible

Absorption ofElectromagneticRadiation Resultsin Transitions Between

Nuclear spin levels

Vibrational energylevels

Electronic energylevels

458 Chapter 12 Infrared Spectroscopy

B. Molecular Vibrations

Infrared activeAny molecular vibration that leadsto a substantial change in dipolemoment is observed in an IRspectrum.

Atoms joined by covalent bonds are not permanently fixed in one position butrather undergo continual vibrations relative to each other. The energies associatedwith these vibrations are quantized, which means that, within a molecule, only spe­cific vibrational energy levels are allowed. The energies associated with transitionsbetween vibrational energy levels in most covalent molecules correspond to fre­quencies in the infrared region, 4000-400 cm-1.

For a molecule to absorb this radiation, its vibration must cause a periodic changein the bond dipole moment. If two charges are connected by a spring, a change in dis­tance between the charges corresponds to a change in dipole moment. In general,the greater the bond dipole, the greater the change in dipole moment caused by avibration. Any vibration that leads to a substantial change in dipole moment is said tobe infrared active. The greater the change is, the more intense the absorption will be.Covalent bonds whose vibration does not result in a change in bond dipole moment,for example, as a result of symmetry in the molecule, are said to be infrared inactive.The carbon-earbon double and triple bonds in symmetrically substituted alkenes andalkynes, for example 2,3-dimethyl-2-butene and 2-butyne, do not absorb infrared ra­diation because vibration does not result in a substantial bond dipole change.

H3C\ lH3

C=C/ \

H3C CH3

2,3-Dimethyl-2-butene 2-Butyne

Raman spectroscopyA vibrational molecular spectros­copy that is complementary to

infrared (IR) spectroscopy in thatinfrared inactive vibrations areseen in Raman spectra.

For a nonlinear molecule containing n atoms, 3n - 6 allowed fundamental vibra­tions exist. For a molecule as simple as ethanol, CH3CH20H, there are 21 fundamen­tal vibrations, and for hexanoic acid, CH3(CH2)4COOH, there are 54. Thus, even forrelatively simple molecules, a large number of vibrational energy levels exist, and thepatterns of energy absorption for these and larger molecules are very complex.

The simplest vibrational motions in molecules giving rise to absorption of in­frared radiation are stretching and bending motions. Figure 12.3 illustrates thefundamental stretching and bending vibrations for a methylene group.

A different technique called Raman spectroscopy is complementary to infra­red spectroscopy in that infrared-inactive vibrations are seen in Raman spectra,while Raman-inactive vibrations are the ones that are infrared active. A more com­plete description of Raman spectroscopy is beyond the scope of this text, but it is avery useful technique for studying certain molecules.

C. Characteristic Absorption PatternsAnalysis of the modes of vibration for a molecule is very complex because all theatoms contribute to the vibrational modes. However, we can make useful general­izations about where absorptions due to particular vibrational modes will appear

Symmetric stretching

Wagging Twisting

Bending vibrations

H"\a H"\a1>( 1>(

H Ii;~ ~

Scissoring Rocking

-~~<; ~

HH HH)( )(~ §

/ I>(H/\ H\

Asymmetric stretching

Stretching vibrations

Figure 12.3

Fundamental stretching andbending vibrations for amethylene group.

460 Chapter 12 Infrared Spectroscopy

400

400

400

600

600

600

800

800

800

1000

1000

10001200

1200

1200

1400

1400

14001600

1600

1600

Wavenumber (em-I)

Wavenumber (em-I)

Wavenumber (em-I)

1800

1800

18002000

2000

2000

12.6 • Following are infrared spectra ofnonane and I-hexanol. Assign each compound itscorrect spectrum.

2400

2400

24002800

2800

28003200

3200

32003600

3600

3600

Micrometersl002f-,5 --'- ---l...4 ----' ----'6 .....7__---'-_---l...9_..J.IO 1J...l_----J1L...2_..J.13_...J1L...4_11...5 ...J2O'--_---,

90

80

~70

860~ 50

'~ 40

la~ 30

20

10

o4000

PROBLEMS

Micrometers100 2f-.5 --'- ---l...4 ----' ----'6 .....7__---'-_---l...9_..J.IO 1J...l_----J1L...2 _..J.13_...J1L...4_11...5 ...J2O'--_---,

90

Online homework for this chapter may be assigned in Organic OWL.• indicates problems assignable in Organic OWL.Red numbers indicate applied problems.

12.5 • Following are infrared spectra of methylenecyclopentane and 2,3-dimethyl-2-butene.Assign each compound its correct spectrum.

Micrometers100 2f-.5 --'- ---l...4 ----' ----'6 .....7__---'-_---l...9_..J.IO 1J...l_----J1L...2 _..J.13_...J1L...4_11...5 ...J2O'--_---,

90

80

~70

860~ 50

's",40

~ 30

20

10

o4000

80

~70

860~ 50

's",40

~ 30

20

10

o4000

474 Chapter 12 Infrared Spectroscopy • Assignable in OWL