Modern Optical Spectroscopy

Shu-Ping Lin, Ph.D.

Institute of Biomedical Engineering E-mail: splin@dragon.nchu.edu.tw

Website: http://web.nchu.edu.tw/pweb/users/splin/

Introduction of Molecular Vibrations & IR Spectroscopy

Vibrations

What is a vibration in a molecule?

Any change in shape of the molecule- stretching of bonds, bending of bonds, or internal rotation around single bonds

Can a vibration change the dipole moment of a molecule?

Asymmetrical stretching/bending and internal rotation change the dipole moment of a molecule. Asymmetrical stretching/bending are IR active.

Symmetrical stretching/bending does not. Not IR active

Infrared (IR) electromagnetic radiation causes vibrations in molecules (wavelengths of 2500-15,000 nm or 2.5 – 15 mm)

What wavelength of electromagnetic radiation is involved in causing vibrations in molecules?

For a vibration at 4111 cm-1 (the stretch in H2), how many vibrations occur in a second?

120 x 1012 vibrations/sec or a vibration every 8 x 10-15 seconds!

120 trillion vibration per second!!!!

How Does the Mass Influence the Vibration?

H2 I2

MM =2 g/mole

MM =254 g/mole

The greater the mass - the lower the wavenumber

How Much Movement Occurs in the Vibration of a C-C Bond?

For a C-C bond with a bond length of 154 pm, the variation is about 10 pm.

For C-C-C bond angle a change of 4o is typical. This moves a carbon atom about 10 pm.

4o 10 pm

10 pm

154 pm

stretching vibration

bending vibration

A Little Physics of Electromagnetic Radiation

Energy (E) E = hn = hc/l where h is Planck’s constant, c is the speed of

light, n is frequency or the number of vibrations per second and l is the wavelength

Wavenumber (n’) n’ = 1/ l given in cm-1

Period (P) P = 1/n

the time between a vibration

= hcn’

Energy, frequency, and wavenumber are directly proportional to each other.

Definition of Infrared Absorption Bands

There are two regions in IR absorption spectra: the "functional group region" and the "fingerprint region". The functional group region spans from 4000 to 1300 cm-1. In this region the bands are characteristic of specific functional groups in a molecule. The fingerprint region spans from 1300 to 900 cm-1. In this portion of the spectrum the energy of the absorption bands varies depending on the structure of a molecule.

What Type of Vibrations Would Occur in Pentane?

Let’s examine the IR spectrum of pentane.

IR spectrum

Increasing wavenumber (energy, frequency)

Increasing absorption of IR radiation

Increasing wavelength

C-H stretching

C-C bending

C-H bending

IR Spectra of chloroform and deuterochloroform

Mode of vibration CHCl3

Calculated*

CHCl3

Measured

CDCl3

Measured

C-H stretching 3002 3020 2256

C-H bending 1120 1219 912

C-Cl stretching 701 773 737

C-Cl bending 418 671 652 * Spartan ’02 AM1 minimization

Shift of peak due to replacement of H with D (2x mass)

Incr

easi

ng a

bso

rbance

Some Results

Calculated values using computational software give lower wave numbers

Increasing mass of substituted atoms shifts wave numbers to lower values

(Excel spreadsheet)

Stretching energies > bending energies > internal rotation energies (occur at higher wavelengths)

Does the stretching energy have any relationship to the strength of the bond?

Wavenumber vs. Bond Energy

0

1000

2000

3000

4000

5000

200 300 400 500 600Bond energy (kJ/mole)

Wavenum

ber (

cm

-1 )

W = 6.3286BE + 401.38

r2 = 0.7979

Let’s Examine the Carbonyl Group on Three Compounds

formaldehyde phosgene acetone

How does the C=O stretching energy compare for these three molecules?

2053 cm-1 1951 cm-1 2063 cm-1

The carbonyl group has a range of 1700-3000 cm-1.

Functional group analysis in organic compounds

Unlike atomic spectroscopy where sharp energy transitions occur due to well quantized electron transitions, molecular spectroscopy tends to show bands.

Molecular vibrations are influenced by the surrounding groups!

Use of IR spectra

Identification of functional groups on a molecule – this is a very important tool in organic chemistry

Spectral matching can be done by computer software and library spectra

Since absorbance follows Beer’s Law, can do quantitative analysis

Correlation Chart -1

Correlation Chart -2

4000 3000 2000 1000

WAVENUMBER (cm-1)

Basic Functional Groups

C-H

O-H

ChC C=C

alkenes

aromatic

C=O C-O

C-H

O-H

bendin

g

stre

tchin

g

C-C

400

IR source sample prism detector

graph of % transmission vs. frequency

=> 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 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)

n-pentane

CH3CH2CH2CH2CH3

3000 cm-1

1470 &1375 cm-1

2850-2960 cm-1

sat’d C-H

cyclohexane

no 1375 cm-1

no –CH3

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)

1-decene

910-920 & 990-1000 RCH=CH2

C=C 1640-1680

unsat’d

C-H

3020-3080 cm-1

4-methyl-1-pentene

910-920 & 990-1000 RCH=CH2

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

ethylbenzene

690-710, 730-770

mono-

1500 & 1600

Benzene ring

3000-3100 cm-1

Unsat’d C-H

o-xylene

735-770

ortho

p-xylene

810-840(m)

para

m-xylene

meta

690-710, 750-810(m)

styrene

no sat’d C-H

910-920 & 990-1000

RCH=CH2 mono

1640

C=C

2-phenylpropene

mono 880-900

R2C=CH2

Sat’d C-H

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)

1-butanol

CH3CH2CH2CH2-OH

C-O 1o

3200-3640 (b) O-H

2-butanol

C-O 2o

O-H

tert-butyl alcohol

C-O 3o O-H

2-butanone

C=O

~1700 (s)

C9H12

C-H unsat’d & sat’d

1500 & 1600

benzene

mono

C9H12 – C6H5 = -C3H7

C8H6

C-H unsat’d

1500, 1600

benzene

mono

C8H6 – C6H5 = C2H

phenylacetylene

3300

C-H

C4H8

1640-1680

C=C

880-900

R2C=CH2

isobutylene CH3 CH3C=CH2

Unst’d

H2C C

HCH2

CH3

CH3CH3CH2CH2CH2CH3

H2C

H2C

CH2CH2CH2CH3

biphenyl allylbenzene 1,2-diphenylethane

o-xylene n-pentane n-butylbenzene

A

B

C

D

E

F

In a ―matching‖ problem, do not try to fully analyze each spectrum. Look for differences in the possible compounds that will show up in an infrared spectrum.

Homework

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