Infrared Spectroscopy (IR) Basics & Instruments

To Do’s •  Read Chapter 11. Skip Raman. •  Complete the end-of-chapter problems, 11-2 and 11-5 Answer Keys are available in CHB204H

Molecular Vibration •  Atoms in a molecule are not fixed in space. They vibrate. •  To a first approximation, it is like 2 balls, A and B, attached to each other by a spring. •  If it was really a spring, any energy state would be possible. But, for a molecule, the vibrational energy is quantized. 1st excited vibrational energy state A

B

Molecular vibration

E

ΔEvib

ground vibrational energy state

Electromagnetic Radiation and IR absorption

When we put energy into the system, a transition from ground state the the excited state occurs when; ΔEvib = hν ν=

c λ

Wave Numbers •  Typical IR absorptions occur at frequencies (ν) approximately 1013 s-1, an inconvenient large number. •  A more convenient unit, ν (the wave number in cm-1), is used in IR. ν= Also, ν =

ν c 1 λ

c = speed of light (3 x 1010 cms-1) λ = wavelength

Classic IR Instrument Iref

Isample

%T =

Isample Iref

x 100

l : light intensity

Or, use absorbance (A) Iref A = - log ( ) Isample

A Typical IR Spectrum

%T

Wavenumber (cm-1), reciprocal centimeter

ΔEvib is related to bond strength, mass of atoms etc

Structual analysis

FT-IR Instrument

Michaelson Interferometer

•  IR frequecy, ~1014 Hz, is too fast for the rapid changes in power to be directly measured as a function of time. Can not measure the FID signal directly. •  Michaelson interferometer creates an interference pattern at a frequency that is a factor of 1010 times slower.

Michaelson Interferometer I0

d

When x = 0, I = 0.5 I0 x = λ/4, I = 0

d+x I 0.5 I0

I I(x) = 0.5 cos (2πx/λ)

x − λ/2 − λ/4

λ/4

λ/2

See YouTube video: http://www.youtube.com/watch?v=j-u3IEgcTiQ

IR Light with Different Frequencies I 0.5 I0

x − λ/2 − λ/4

λ/2

λ/4 I

0.5 I0

x − λ/2 − λ/4

I

λ/4

λ/2 0.5 I0

x λ/4 λ/2

− λ/2 − λ/4

0

0 x Signals are canceled out, except for x=0.

Interferogram and Fourier Transform

baseline

With sample

FT

Sample Preparation •  Sample holder must be transparent to IR- salts •  Liquids –  Salt Plates (large NaCl crystals) –  Neat, 1 drop –  Samples dissolved in volatile solvents- 0.1-10%

•  Solids –  KBr pellets –  Mulling (dispersions), i.e. nujol (paraffin oil) mull

•  Quantitative analysis-sealed cell with NaCl/NaBr/ KBr windows

IR Samples (a) “Neat” samples

(b) KBr disks

Common IR Solvents

ATR-IR •  Attenuated total reflectance •  Penetrates several micrometers

ZnSe, germanium or diamond

Stretching Bands Hooke’s law in classical mechanics MA

A

B

MB

f spring constant reduced mass µ =

(1) f C-C



MA x M B MA + M B

stronger bond larger f

2π ν=

C≡C

f



and ν =

µ

1



2πc

ν c

f µ

c : speed of light f : proportional to bond strength (2) µ

ν C=C

ν=

1

C-H C-C C-O C-N

ν small µ

large ν

large µ

small ν

Phenylpropiolaldehyde

-C≡C-

C=O

Cyclohexanol (dilute solution)

O-H C-O

Mass Effects on CH Bend Frequency for X(CH3)4

Bendings

CHCl3 and CDCl3

Cl H

C

Cl

Cl Cl

H

C

Cl Cl

C-H stretch C-H bending Both stretching and bending bands are affected by mass.

Typical IR Bands

Peak Intensity Oδ C δ

More efficient interaction

vibration causes a large change in dipole

•  In order to interact with electromagnetic radiation, the bond has to change the dipole moment upon vibration

C-H C-O C=O

strong

C-C

weak

Pentynes

C≡C 2150 cm-1

Mechanical Coupling •  Two identical, separate groups vibrate with same frequencies, independently from each other. •  When part of the same molecule, they cannot vibrate independently. •  Displacement of atoms in one group causes effect in the other group. •  In-phase (symm.) and out-of-phase (unsymm.) combinations of the starting vibrations are observed.

CH2 Stretching Vibrations

Hexane

CH2 and CH3, symmetric and anti-symmetric

http://www.quimica3d.com/EN/IR/hexane.php

Hexane

CH3 bending

http://www.quimica3d.com/EN/IR/hexane.php

Isopropyl and tert-butyl Groups

http://www.quimica3d.com/EN/IR/23dimethylbutane.php

Carboxylic Acid Anhydrides

Overtone and Combination Bands

0 -> 1 : Fundamental bands 0 -> 2 etc : Overtone bands (ν0-2 ~ 2 x ν0-1) Combination bands are observed when two or more fundamental vibrations are excited simultaneously.

- Both overtone and combination bands are generally weak.

Aromatics

overtones

CH bendings

Fermi Resonance •  Mechanical coupling between a fundamental band and an overtone band. •  Some can be diagnostic.

Benzoyl Chloride

overtone

CH out-of-plane bending 865 cm-1

Secondary Amides

NH deformation ~ 1550 cm-1 overtone

Aldehydes

CH deformation ~ 1400 cm-1 overtone