Spectroscopy“seeing the unseeable”

Using electromagnetic radiation as a probe to obtain information about atoms and molecules that are too small to see.

Electromagnetic radiation is propagated at the speed of light through a vacuum as an oscillating wave.

electromagnetic relationships:

λυ = c λ ∝ 1/υ

E = hυ E ∝ υ

E = hc/λ E ∝ 1/λ

λ = wave length

υ = frequency

c = speed of light

E = kinetic energy

h = Planck’s constant

λ

c

some electromagnetic radiation ranges

Approx. freq. range Approx. wavelengths

Hz (cycle/sec) meters

Radio waves 104 - 1012 3x104 - 3x10-4

Infrared (heat) 1011 - 3.8x1014 3x10-3 - 8x10-7

Visible light 3.8x1014 - 7.5x1014 8x10-7 - 4x10-7

Ultraviolet 7.5x1014 - 3x1017 4x10-7 - 10-9

X rays 3x1017 - 3x1019 10-9 - 10-11

Gamma rays > 3x1019 < 10-11

Provides information about the vibrations of functional groups in a molecule

Infrared Spectroscopy

Therefore, the functional groups present in a molecule can be deduced from an IR

spectrum

Infrared radiation

λ = 2.5 to 17 μm

υ = 4000 to 600 cm-1

These frequencies match the frequencies of covalent bond stretching and bending vibrations. Infrared spectroscopy can be used to find out about covalent bonds in molecules.

IR is used to tell:

1. what type of bonds are present

2. some structural information

IR source è sample è prism è detector

graph of % transmission vs. frequency

=> IR spectrum

4000 3000 2000 1500 1000 500

v (cm-1)

100

%T

0

E2

E2

∆E = hνE2

E1

∆E = hν

Frequency

Intensity

νFrequency

Intensity

ν

Absorbance Emission

7

The functional group concept of organic chemistry

IR spectroscopy can identify functional groups!

Some Important and Characteristic Infrared Absorption Frequencies and Wavelengths for Some Common Stretching Motions

Atom Group Typical of Frequency (cm-1) Wavelength (µ) O-H (free) Alcohols (dilute) 3550-3650 cm-1 2.8 µ O-H (H bonded) Alcohols

(concentrated) Carboxylic acids

3200-3400 cm-1 3.0 µ

C C H Acetylene (CH) 3300 cm-1 3.0 µ C C H Benzene (CH),

Ethylene (CH) 3010-3100 cm-1 3.3 µ

C_____C_____H Ethane (CH) 2950-3000 cm-1 3.5 µ C C Acetylene 2100-2260 cm-1 4.5 µ C N Nitriles 2000-2300 cm-1 4.5 µ C O Carbonyl 1650-1750 cm-1 5.5- 6.0 µ C C Alkene 1620-1680 cm-1 6.0 µ C C Alkane 600-1500 cm-1 6.7-17 µ C O Alcohols,

Ethers 1000-1300 cm-1 10-7.7 µ

In general we will only be using the data in an IR spectrum for stretching vibrations which have energies higher than 1620 cm-1. Although the bands at lower energy are known and assigned, the region below 1620 cm-1 is very congested with single bond stretches of two heavy atoms (see C-C and C-O in table) and C-H bends and are beyond the scope of what we want to do.

IR spectra of ALKANES

C—H bond “saturated”

(sp3) 2850-2960 cm-1

+ 1350-1470 cm-1

-CH2- + 1430-1470

-CH3 + “ and 1375

-CH(CH3)2 + “ and 1370, 1385

-C(CH3)3 + “ and 1370(s), 1395 (m)

IR of ALKENES

=C—H bond, “unsaturated” vinyl

(sp2) 3020-3080 cm-1

+ 675-1000

RCH=CH2 + 910-920 & 990-1000

R2C=CH2 + 880-900

cis-RCH=CHR + 675-730 (v)

trans-RCH=CHR + 965-975

C=C bond 1640-1680 cm-1 (v)

IR spectra BENZENEs

=C—H bond, “unsaturated” “aryl”

(sp2) 3000-3100 cm-1

+ 690-840

mono-substituted + 690-710, 730-770

ortho-disubstituted + 735-770

meta-disubstituted + 690-710, 750-810(m)

para-disubstituted + 810-840(m)

C=C bond 1500, 1600 cm-1

IR spectra ALCOHOLS & ETHERS

C—O bond 1050-1275 (b) cm-1

1o ROH 1050

2o ROH 1100

3o ROH 1150

ethers 1060-1150

O—H bond 3200-3640 (b) «

nn--Hexane: CHHexane: CH33CHCH22CHCH22CHCH22CHCH22CHCH33

11--Hexene: CHHexene: CH22=CHCH=CHCH22CHCH22CHCH22CHCH33

1650 cm1650 cm--11

3100 cm3100 cm--11

??

IR: Masses, Atoms and Springs

F = force, restoring back to equilibrium position

k = characteristic stretching constant

x = displacement from the equilibrium position

A Model: Picture the atoms of a diatomic molecule as point masses connected by springs (bonds).

As a first approximation use Hooke’s Law

F = -kx

ν =1

2πk

mr

mr =m1m2

m1 + m2

IR Stretching Frequencies of two bonded atoms:

ν = frequencyk = spring strength (bond stiffness)mr = reduced mass (~ mass of largest atom)

What Does the Frequency, ν, Depend On?

Internuclearseparation

Heavy atomsand /or

weak bonds

Internuclearseparation

Light atomsand/or

strong bonds

X Y X Y

X Y X Y

X YX Y

PE

r re re

a b

c d

F = dPE/dr

Vibrations, potential energy and motion

IR Stretching Frequencies: What Do they Depend On?

Directly on the strength of the bonding between the two atoms (ν ~ k)

Inversely on the reduced mass of the two atoms (v ~ 1/m)

Expect: ν will increase with increasing bond strength (bond order) and decreasing mass

Examples of stretching frequencies and correlations with bond strengths (bond order)

CC--CC

C=CC=C

C C

Bond strength*

350350

600600

840840

Bond order

11

22

33

νν

2200 cm2200 cm--11

1600 cm1600 cm--11

1000 cm1000 cm--11

*In kJ/mol*In kJ/mol For same reduced mass!For same reduced mass!

Quantum mechanics: The frequency (ν) depends on the energy gap between vibrational levels

E = hν = hc/λ (cm-1)

The natural frequency (8.67 x 1013 s-1) is absorbed selectively

Only the natural frequency will be absorbed

The absorption intensity depends on how efficiently the energy of an electromagnetic wave of frequency ν can be transferred to the

atoms involved in the vibration

The greater the change in dipole moment during a vibration, the higher the intensity of absorption of a photon

What does the absorption intensity depend on?

Dipole Moment Must Change during for a vibration to be “IR active”!

In order to interact strongly with the EM radiation, the motion of the molecule must be such that the dipole

moment changes.

Which of the vibrations are “IR” active”?

What is the intensity of an IR signal of: O2 or N2 or H2?

Ans: In order to absorb infrared radiation, a molecular vibration must cause a change in the dipole moment of

the molecule

O2, N2 and H2 DO NOT ABSORB IR LIGHT!

The are not “Greenhouse” gases

Does O=C=O absorb IR light?

Ans: vibrations of O=C=O which cause a change in the dipole moment of the molecular absorb IR light

vibrations of O=C=O which do not cause a change in the dipole moment of the molecular DO NOT absorb IR light

No dipoleNo dipolegeneratedgenerated

Dipole Dipole generatedgenerated

Which should have a higher stretching frequency, CO, CO+, or CO-? Why?

Ans: The higher the bond order, the stronger the bond and the higher the frequency for the IR stretch.

Bond order: CO = 3, CO+ = 5/2, CO- = 5/2

CO will have the higher stretching frequency

CO+ and CO- will have similar, lower frequencies

CO = (σ)2(π)4 CO+ = (σ)2(π)3 CO- = (σ)2(π)4(π∗)1

region of infrared that is most useful lies between2.5-16 µm (4000-625 cm-1)

depends on transitions between vibrational energy states

stretching

bending

Infrared Spectroscopy: further review

Stretching Vibrations of a CH2 Group

Symmetric Antisymmetric

Bending Vibrations of a CH2 Group

In plane In plane

Bending Vibrations of a CH2 Group

Out of plane Out of plane

2000200035003500 30003000 25002500 1000100015001500 500500

Wave number, cmWave number, cm--11

Infrared Spectrum of Hexane

CHCH33CHCH22CHCH22CHCH22CHCH22CHCH33

CC——H stretchingH stretchingbendingbending bendingbending

bendingbending

2000200035003500 30003000 25002500 1000100015001500 500500

Wave number, cmWave number, cm--11

Infrared Spectrum of 1-Hexene

HH22C=CHCHC=CHCH22CHCH22CHCH22CHCH33

HH——CCC=CC=C——HH

C=CC=C

HH22C=CC=C

Structural unit Frequency, cm-1

Stretching vibrations (single bonds)

sp C—H 3310-3320

sp2 C—H 3000-3100

sp3 C—H 2850-2950

sp2 C—O 1200

sp3 C—O 1025-1200

Infrared Absorption Frequencies

Structural unit Frequency, cm-1

Stretching vibrations (multiple bonds)

Infrared Absorption Frequencies

C C 1620-1680

—C N

—C C— 2100-2200

2240-2280

Structural unit Frequency, cm-1

Stretching vibrations (carbonyl groups)

Aldehydes and ketones 1710-1750

Carboxylic acids 1700-1725

Acid anhydrides 1800-1850 and 1740-1790

Esters 1730-1750

Amides 1680-1700

Infrared Absorption Frequencies

C O

Structural unit Frequency, cm-1

Bending vibrations of alkenes

Infrared Absorption Frequencies

CH2RCH

CH2R2C

CHR'cis-RCH

CHR'trans-RCH

CHR'R2C

910-990

890

665-730

960-980

790-840

Structural unit Frequency, cm-1

Bending vibrations of derivatives of benzene

Monosubstituted 730-770 and 690-710

Ortho-disubstituted 735-770

Meta-disubstituted 750-810 and 680-730

Para-disubstituted 790-840

Infrared Absorption Frequencies

2000200035003500 30003000 25002500 1000100015001500 500500

Wave number, cmWave number, cm--11

Infrared Spectrum of tert-butylbenzene

HH——CC

ArAr——HH

MonsubstitutedMonsubstitutedbenzenebenzene

CC66HH55C(CHC(CH33))33

Structural unit Frequency, cm-1

Stretching vibrations (single bonds)

O—H (alcohols) 3200-3600

O—H (carboxylic acids) 3000-3100

N—H 3350-3500

Infrared Absorption Frequencies: functional groups

2000200035003500 30003000 25002500 1000100015001500 500500

Wave number, cmWave number, cm--11

Infrared Spectrum of 2-Hexanol

HH——CC

OO——HH

OHOH

CHCH33CHCH22CHCH22CHCH22CHCHCHCH33

2000200035003500 30003000 25002500 1000100015001500 500500

Wave number, cmWave number, cm--11

Infrared Spectrum of 2-Hexanone

HH——CC

C=OC=O

OO

CHCH33CHCH22CHCH22CHCH22CCHCCH33