Organic Chemistry

502 CHAPTER 13 Mass Spectrometry and Infrared Spectroscopy

in Chapter 14 give you the opportunity to identify compounds, using information from

two or more instrumental methods.

Because it takes more energy to stretch a bond than to bend it, absorption bands

for stretching vibrations are found in the functional group region

whereas absorption bands for bending vibrations are typically found in the fingerprint

region Stretching vibrations, therefore, are the most useful vibra-

tions in determining what kinds of bonds a molecule has. The IR stretching

frequenciesassociated with different types of bonds are shown in Table 13.4 and will

be discussed in Sections 13.10 and 13.11.

11400 – 600 cm-^12.

14000 – 1400 cm-^12 ,

It takes more energy to stretch
a bond than to bend it.

Table 13.4 Important IR Stretching Frequencies

Type of bond Wavenumber (cm−^1 ) Intensity

C N 2260 – 2220

C C 2260 – 2100

C C 1680 – 1600

~1600 and ~1500– 1430

C O 1780 – 1650

C O 1250 – 1050

C N 1230 – 1020

O H 3650 – 3200

(alcohol)

O H 3300 – 2500

N H 3500 – 3300

(carboxylic acid)

C H 3300 – 2700

medium

medium to weak

medium

C N 1650 – 1550 medium

strong to weak

strong

strong

medium

strong, broad

strong, very broad

medium, broad

medium

The greater the change in dipole
moment, the more intense the
absorption.

13.9 The Intensity of Absorption Bands

The intensity of an absorption band depends on the size of the change in dipole mo-

ment associated with the vibration: The greater the change in dipole moment, the more

intense the absorption. Recall that the dipole moment of a bond is equal to the magni-

tude of the charge on one of the bonded atoms, multiplied by the distance between the

two charges (Section 1.3). When the bond stretches, the increasing distance between

the atoms increases the dipole moment. The stretching vibration of an bond will

be associated with a greater change in dipole moment than that of an bond

because the bond is more polar. Consequently, the stretching vibration of the

bond will be more intense. Likewise, the stretching vibration of an bond

is more intense than that of a bond because the bond is more polar.

O H >>N H C H

relative bond polarities

relative intensities of IR absorption

increasing polarity

increasing intensity

C¬H N¬H

O¬H N¬H

O¬H

N¬H

O¬H