Organic Chemistry

Section 13.10 The Position of Absorption Bands 503

The intensity of an absorption band also depends on the number of bonds responsi-
ble for the absorption. For example, the absorption band for the stretch will be
more intense for a compound such as octyl iodide, which has bonds, than
for methyl iodide, which has only three bonds. The concentration of the sample
used to obtain an IR spectrum also affects the intensity of the absorption bands. Con-
centrated samples have greater numbers of absorbing molecules and, therefore, more
intense absorption bands. In the chemical literature, you will find intensities referred
to as strong (s), medium (m), weak (w), broad, and sharp.

PROBLEM 16

Which would be expected to be more intense, the stretching vibration of a bond or

the stretching vibration of a bond?

13.10 The Position of Absorption Bands

Hooke’s Law
The amount of energy required to stretch a bond depends on the strengthof the bond
and the massesof the bonded atoms. The stronger the bond, the greater the energy
required to stretch it, because a stronger bond corresponds to a tighter spring. The
frequency of the vibration is inversely related to the mass of the atoms attached to the
spring, so heavier atoms vibrate at lower frequencies.
The approximate wavenumber of an absorption can be calculated from the
following equation derived from Hooke’s law, which describes the motion of a vi-
brating spring:

The equation relates the wavenumber of the stretching vibration to the force con-
stant of the bond (f) and the masses of the atoms (in grams) joined by the bond
and The force constant is a measure of the strength of the bond. The equation
shows that stronger bondsand lighter atomsgive rise to higher frequencies.

m 2 ).

(m 1

1 n

'

2

n

'

=

1

2 pc

c

f 1 m 1 +m 22

m 1 m 2

d

1 > 2

c=the speed of light

C“C

C“O

C¬H

17 C¬H

C¬H

THE ORIGINATOR OF

HOOKE’S LAW

Robert Hooke (1635–1703) was born on the Isle

of Wight off the southern coast of England. A brilliant scientist,

he contributed to almost every field of science. He was the first

to suggest that light had wave-like properties. He discovered

that Gamma Arietis is a double star, and he also discovered

Jupiter’s Great Red Spot. In a lecture published posthumously,

he suggested that earthquakes are caused by the cooling and

contracting of the Earth. He examined cork under a microscope

and coined the term “cell”to describe what he saw. He wrote

about evolutionary development based on his studies of micro-

scopic fossils, and his studies of insects were highly regarded as

well. Hooke also invented the balance spring for watches and

the universal joint currently used in cars.

Robert Hooke’s drawing of a

“blue fly” appeared in

Micrographia, the first book on

microscopy, published by

Hooke in 1665.

Lighter atoms show absorption bands at

larger wavenumbers.

^700 cm^1

C¬Cl

^1100 cm^1

C¬O

^2200 cm^1

C¬D

^3000 cm^1

C¬H

Stronger bonds show absorption bands

at larger wavenumbers.

^1100 cm^1

C¬N

^1600 cm^1

C“N

^2200 cm^1

C‚N

Effect of Bond Order
Bond order affects bond strength, so bond order affects the position of absorption
bands. A bond is stronger than a bond, so a bond stretches at a
higher frequency 1 ' 2100 cm-^12 than does a C“Cbond 1 ' 1650 cm-^12 .C¬C

C‚C C“C C‚C