Molecular vibrations and ir spectroscopy
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Molecular Vibrations and 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 http://academic.pgcc.edu/~ssinex/VibMol/form.html.

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Molecular Vibrations and IR Spectroscopy

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Molecular vibrations and ir spectroscopy

Molecular VibrationsandIR Spectroscopy


Molecular vibrations and 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

http://academic.pgcc.edu/~ssinex/VibMol/form.html

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


Molecular vibrations and ir spectroscopy

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

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

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

120 trillion vibration per second!!!!

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

WOW !!!!!


Molecular vibrations and ir spectroscopy

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


Molecular vibrations and ir spectroscopy

154 pm

10 pm

10 pm

4o

How much movement occurs in the

vibration of a C-C bond?

stretching vibration

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

bending vibration

For C-C-C bond angle a change of 4o is typical.

This moves a carbon atom about 10 pm.


Vibrational distortions for ccl 2 fcclf 2

Vibrational Distortions for CCl2FCClF2

http://www2.ess.ucla.edu/~schauble/MoleculeHTML/CCl2FCClF2_html/CCl2FCClF2_page.html


A little physics of electromagnetic radiation

A little physics of electromagnetic radiation

= hcn’

  • 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 secondand l is the wavelength

  • Wavenumber (n’) n’ = 1/ l

    • given in cm-1

  • Period (P)P = 1/n

    • the time between a vibration

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


What type of vibrations would occur in pentane

What type of vibrations would occur in pentane?

Let’s examine the IR spectrum of pentane.


Molecular vibrations and ir spectroscopy

IR spectrum

C-H

stretching

C-H

bending

Increasing absorption of IR radiation

C-C

bending

Increasing wavelength

Increasing wavenumber (energy, frequency)


Molecular vibrations and ir spectroscopy

IR Spectra of chloroform and deuterochloroform

Increasing absorbance

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

* Spartan ’02 AM1 minimization


Some results

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)


Molecular vibrations and ir spectroscopy

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

W = 6.3286BE + 401.38

r2 = 0.7979


Let s examine the carbonyl group on three compounds

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

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!


Molecular vibrations and ir spectroscopy

Basic Functional Groups

C-H

bending

O-H

C=O

C-O

alkenes

O-H

stretching

aromatic

C-H

ChC

C=C

C-C

4000

3000

2000

1000

400

WAVENUMBER (cm-1)


Molecular vibrations and ir spectroscopy

Animations Showing Vibrations and Accompanying Spectra

http://wwwchem.uwimona.edu.jm/spectra/iranim/index.html

http://www.chem.umass.edu/~nermmw/Spectra/irspectra/index.htm


Use of ir spectra

Use of IR spectra

  • Identification of functional groupson 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


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