15-1

Infrared Spectroscopy

Unit 15

15-2

Electromagnetic Radiation

Electromagnetic radiation: light and other forms of radiant energy

Wavelength (): the distance between consecutive peaks on a wave

Frequency (): the number of full cycles of a wave that pass a given point in a second

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

15-3

Electromagnetic Radiation

Common units used to express wavelength

Angstrom (Å) 1 Å = 10-10 m

Relationto MeterUnit

1 mm = 10-3 m

1 nm = 10-9 m1 m = 10-6 m

Nanometer (nm)Micrometer (m)

Millimeter (mm)Meter (m) ----

15-4

Molecular Spectroscopy

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

Vibrational energy levels

Nuclear spin states

Electronic energy levels

Infrared

Radio fequency

Ultraviolet-visible

Absorption ofElectromagneticRadiation Results in Transition Between

Region of theElectromagneticSpectrum

Nuclear magneticresonance

Infrared

Ultraviolet-visible

Type of Spectroscopy

15-5

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

2.5 x 10-6 m

10-2 m•cm -1

2.5 x 10-5 m

15-6

Infrared Spectroscopy

IR spectrum of 3-methyl-2-butanone

15-7

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

15-8

Molecular Vibrations

For a molecule to absorb IR radiation• the bond undergoing vibration must be polar and• its vibration must cause a periodic 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

H3C

C C

CH3

H3C CH3

H3C-C C-CH3

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

15-9

Molecular Vibrations

For a nonlinear molecule containing n atoms, there are 3n - 6 allowed fundamental vibrations

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

15-10

Molecular vibrations

Fundamental stretching and bending vibrations for a methylene group

15-11

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• 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 the bonds’ strength; force constants for single, double, and triple bonds are approximately 5, 10, and 15 x 105 dynes/cm

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

K = 4.12

15-12

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

K = 4.12

15-13

Correlation Tables

Characteristic IR absorptions for the types of bonds and functional groups we deal with most often

C-HN-HO-H

C=OC=C

C-O 1000-1250 strong

weak to medium1600-16801630-1820 strong

2700-3300 weak to medium

weak to strong3200-3650medium3100-3550

IntensityStretchingFrequency (cm-1)Bond

15-14

Hydrocarbons

C-H

CH3

C=CC-H

C-HC C

C-C

C-HC=CC-H

Intensity

2850 - 3000 Medium

Stretching 3300 Medium to strongStretching 2100-2250 Weak

Weak to medium1600 - 1680StretchingWeak to medium3000 - 3100Stretching

Alkyne

Alkene

AlkaneVibration

Stretching

Hydro-carbon

Frequency(cm-1)

Bending 1450-1475 MediumBending 1375 and 1450 Weak to medium(Not useful for interpretation - too many bands

AreneStretching 3030 Weak to medium

Medium1450-1600StretchingBending 690-900 Strong

15-15

Alkanes

IR spectrum of decane

15-16

Alkenes

IR spectrum of cyclohexene

15-17

Alkynes

IR spectrum of 1-octyne

15-18

Aromatics

IR spectrum of toluene

15-19

Alcohols

• IR spectrum of 1-hexanol

O-H (free)

O-H (H bonded)

C-O

Bond IntensityFrequency, cm-1

Medium1000 - 1250

Medium, broad3200 - 3500

3600-3650 Weak

15-20

Ethers

IR spectrum of dibutyl ether

15-21

Ethers

IR spectrum of anisole

15-22

Amines

IR spectrum of 1-butanamine

15-23

IR of Molecules with C=O Groups

C=O

C=O

C-H

O HRCOHO

RCHO

RCR'O

C=O

Strong1700-1725StretchingCarboxylic acids

Aldehydes

VibrationCarbonyl GroupFrequency

(cm-1) Intensity

Stretching 1630-1820 StrongStretching 2720 Weak

Stretching 2500-3300 Strong (broad)

KetonesStretching 1630-1820 Strong

15-24

IR of Molecules with C=O Groups

C=O

sp3 C O

sp2 C O

C=O

C=O

N H

CNRC N

RCOCRO O

RCOR'O

RCNH2

O

C O Stretching 900-1300 Strong

Strong1735-1800StretchingCarboxylic esters

Stretching 1000-1100 Strong

Stretching 1200-1250 Strong

Acid anhydrides

Stretching 1740-1760 and1800-1850

Strong

Strong1630-1680StretchingAmides

Stretching 3200, 3400 Medium(1° amides have two N-H stretches)(2° amides have one N-H stretch)

NitrilesStretching 2200-2250 Medium

15-25

Aldehydes and Ketones

IR spectrum of menthone

15-26

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

1850 cm-11780 cm-11745 cm-11715 cm-1

O O O O

1690 cm-1 1700 cm-11717 cm-1

O OH

O

15-27

Carboxylic acids

IR spectrum of pentanoic acid

15-28

Esters

IR of ethyl butanoate