Organic Chemistry

500 CHAPTER 13 Mass Spectrometry and Infrared Spectroscopy

2.52.6 2.7 2.8 2.9 3 3.5 4 4.5 5 5.5Wavelength ( 6 μm) 7 8 9 10 11 12 13 14 15 16

4000380036003400320030002800260024002200 2000 1800 1600 1400 1200 1000 800 600

Wavenumber (cm−^1 )

CH 3 CCH 2 CCH 3

CH 3

O OH

OH

% Transmittance

C O

0

100

Figure 13.13

The infrared spectrum of 4-hydroxy-4-methyl-2-pentanone.

Obtaining an Infrared Spectrum

The instrument used to obtain an infrared spectrumis called an IR spectrometer. An

infrared spectrumis obtained by passing infrared radiation through a sample of the

compound. A detector generates a plot of percent transmission of radiation versus the

wavenumber (or wavelength) of the radiation transmitted (Figure 13.13). At 100%

transmission, all the energy of the radiation passes through the molecule. Lower

values of percent transmission mean that some of the energy is being absorbed by the

compound. Each downward spike in the IR spectrum represents absorption of energy.

The spikes are called absorption bands. Most chemists report the location of absorp-

tion bands using wavenumbers.

A newer type of IR spectrometer, called a Fourier transform IR (FT-IR) spectrome-

ter, has several advantages. Its sensitivity is better because, instead of scanning

through the frequences, it measures all frequencies simultaneously. With a conven-

tional IR spectrometer, it can take 2 to 10 minutes to scan through all the frequencies.

In contrast, FT-IR spectra can be taken in 1 to 2 seconds. The information is digitized

and Fourier transformed by a computer to produce the FT-IR spectrum. The spectra

shown in this text are FT-IR spectra.

An IR spectrum can be taken of a gas, a solid, or a liquid sample. Gases are ex-

panded into an evacuated cell (a small container). Solids can be compressed with an-

hydrous KBr into a disc that is placed in the light beam. Solids can also be examined

as mulls. A mull is prepared by grinding a few milligrams of the solid in a mortar.

Then a drop or two of mineral oil is added and the grinding continued. In the case of

liquid samples, a spectrum can be obtained of the neat (undiluted) liquid by placing a

few drops of it between two optically polished plates of NaCl that are placed in the

light beam. Alternatively, a small container (called a cell) with optically polished NaCl

or AgCl windows is used to hold samples dissolved in solvents. Ionic substances with-

out covalent bonds are used for discs, plates, and cells because they don’t absorb IR ra-

diation. (Glass, quartz, and plastics have IR-absorbing covalent bonds.)

When solutions are used, they must be in solvents that have few absorption bands in

the region of interest. Commonly used solvents are and In a double-

beam spectrophotometer, the IR radiation is split into two beams—one that passes

through the sample cell and the other that passes through a cell containing only the

solvent. Any absorptions of the solvent are thus canceled out, so the absorption spec-

trum is that of the solute alone.

The Functional Group and Fingerprint Regions

Electromagnetic radiation with wavenumbers from 4000 to has just the right

energy to correspond to the stretching and bending vibrations of organic molecules.

Electromagnetic radiation in this energy range is known as infrared radiationbe-

cause it is just below the “red region”of visible light. (Infrais Latin for “below.”)

600 cm-^1

CH 2 Cl 2 CHCl 3.