Infrared spectroscopy

Probing molecular structure based on bond vibrations

Unlike UV-visible spectroscopy, which examines transitions between electronic states (promotion of electrons from one orbital to another), we are interested in absorption of light to induce bond vibration.

This is also quantized hν = ΔEexcited-ground

IR terminology As with UV-vis we are interest in:

1)  What is the energy of a transition (E=hυ)?

•instead of l, usually express as wavenumber ( )

2)  How likely is the transition to occur (ε)– this is measured by how intense the peak is, usually expressed as %tranmittance (low %t=intense)

υ ~

υ ~ = 1/λ = υ/c

Balls and springs

Hooke’s Law:

21

21

mmmm+

⋅=µ

µπυ

Kc21

=

reduced mass

What Hooke’s Law tells us…

1. The larger the force constant (i.e. the stronger the bond), the higher the

frequency of vibration and the higher the frequency of light absorbed to cause this

vibration.

All else being equal:

νsingle bonds < νdouble bonds < νtriple bonds

2. The smaller the masses of the balls (atoms), the higher the frequency:

νC-H > νC-D >> νC-O

HH

H

HH

H

DD

D

DD

D

C-H 3036 cm-1

C-C 1479 cm-1

C-H rocking 1000-1300 cm-1

C-H 2381 cm-1

C-C 1330 cm-1

C-H oop

Overtones and Combination bands

HH

H

HH

H

combination bands

vibrating strings

fundamental υ

2 • υ

3 • υ

observe bands that are integer values of fundamental υ (overtones)

Combination bands from two different vibrations coupled together = υ1+ υ2

Overtone and combination bands lower intensity than fundamental frequencies. Occur at slightly lower than integral frequencies

Dipole moments

O

Only vibrations that give rise to changes in dipole will be “IR active” (allowed transitions).

+q -q d

Dipole moment (µ) = q • d

+q -q d

no C=C band

C=O band (~1700 cm-1)

C=C band CH3

CH3

large Δµ

More dipoles….

N

High symmetry lowers ε (lower A)

Vibrational modes

νas

A B

Linear triatomic

IR-active only if A≠B

Bent triatomic

xx

degenerate

νs

IR-active: 2350 cm-1 in CO2

νs stretch

νas stretch

Scissoring or bending

scissoring

νs stretch νas stretch

Methane

twisting

wagging rocking

(C20H42)

Alkanes

CH stretch CH3

CH3

weak CC (pseudosymmetric as chains get longer)

CH2 rock

CH2 rock

~720 cm-1

branching increases CC bands

no CH2 rock?

(C20H42)

Alkanes CH2 rocking

CH stretch CH3

CH3

weak CC (pseudosymmetric as chains get longer)

CH2 rock

CH2 rock

~720 cm-1

CH2 rock: concerted motion increases in intensity as linear chain length increases without branching

H

H

H

HH

H H

H

Cycloalkanes

No CH2 rocking

CH3 groups only present in side chains