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Vibrational Spectroscopy

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Eric Painting

on 25 March 2014

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Transcript of Vibrational Spectroscopy

Infrared Spectroscopy
First generation IR spectrometers used a prism system to diffract light; however, gave inconsistent results (2-20 cm-1 resolution)

Second generation brought introduction of the
monochrometer, which offers a much larger scan
range (0.2 cm-1 resolution)

Third generation is the Fourier Transfer, which utilizes an interferometer (0.001 cm-1 resolution)

In theory, IR spectroscopy measures the
vibrational energy of a bond with a changing
molecular dipole

A molecule is exposed to multiple wavelengths of infrared light,
and the light is absorbed sending the molecule
into a different vibration state.

The energy measured is the relaxation of a molecule
from an excited state back to its ground state
Since energy relates to frequency, it is possible to calculate the energy of a bond once the vibrational frequency is determined, by Hooke's law.
Through the harmonic oscillator quantum model, the energy of vibrational states can be estimated from the frequency, E=vh
Harmonic Oscillator
Anharmonic Oscillator
In reality, a molecule is modeled best by an anharmonic oscillator
Very effective tool for chemical identification
for all phases.

Every chemical has a unique/characteristic spectrum

It is used extensively for:
- Mineral identification
- Speciation and concentration of chemicals in a mixture
- To determine compound characteristics, like bond angles or dipole orientation
IR Spectrum
with Functional Groups
Vibrational Motion
Vibrational Spectroscopy
Periodic movement of atoms within a molecule

Number of modes
Linear : 3N-5
Nonlinear : 3N-6

Types of Vibrational Motion
Energy of Motion
Simple harmonic motion
Dependent on bond strength and mass of atoms
Relation to frequency
Raman Spectroscopy
Sir C. V. Raman
1923 - Adolf Smekal predicted inelastic scattering of light

1928 - Raman observed scattering effect in sunlight*

1930 - Raman awarded Nobel Prize in Physics
Why did he win?*

1930-'34 - George Placzek developed systematic pioneering
theory, and the mercury arc became the principle light source

1960's - The use of lasers rekindled Raman spectroscopy, which
had formerly lost use due to commerical IR spectrophotometers.
Nobel Prize
All it took to win the prize:
Narrow band photographic filter
(Created monochromatic light)
Crossed filter
(Blocks the light created above)
Light affects a molecule and interacts with its bonds and electron cloud - The Raman effect is seen.

Raman effect occurs when a molecule is excited by a photon to a higher energy state, and then relaxes to a different energy state, emitting a photon.
The difference in energy between the original state and the new state "shift" the emitted photon's frequency away from the excitation wavelength.
What is required?
To exhibit the Raman effect, a change in the molecular polarization potential must be seen.
This is a change in the electron cloud.

Amount of change ∞ Raman spectra intensity

Raman vs. Infrared
1. Due to scattering of light by the vibrating molecules.

2. Vibration is Raman active if it causes change in polarizability.

3. Molecule need not possess a permanent dipole moment.

4. Water can be used as a solvent.

5. Sample prep. is not elaborate.
Can be in almost any state.

6. Gives indication of covalent character in molecule.

7. Cost of instrumentation is very high.
The result of absorption of light by vibrating molecules.

Vibration is IR active if there is change in the dipole moment.

Vibration concerned should have change in dipole moment.

Water cannot be used (intense abs.)

Sample prep is elaborate. Gaseous samples can rarely be used.

Gives indication of ionic character in molecule.

Comparatively inexpensive.
Fingerprint region analysis

Real time monitoring of anaesthetic and respiratory gas mixtures during surgery.
Utilizes Raman gas analyzers.

Raman scattering through the use of an anisotropic crystal can show information in the orientation of the crystal.

Spatially offset Raman spectroscopy (SORS) - Discover counterfeit drugs without opening their packaging. It's a non-invasive tool.

Where else could non-invasive tools be effective?
Lesion Identification
Biopsies of breast lesions discovered by mammography have a high cost to the patient and the healthcare system.

Non-invasive SORS and Raman transmission have been shown to penetrate depths of 8.7, and 16 mm in chicken breast tissue, respectively.

This technique can reveal the chemical breakdown of the lesion, giving information on the state of the disease.

Should the lesion be found to be benign, no harm has been done.
Types of Raman Spectroscopy
Surface-enhanced Raman spec (RS)

Resonance Raman spec.

Surface-enhanced resonance RS

Angle-resolved Raman spec.

Hyper Raman

Spontaneous Raman spec.

Optical tweezers Raman spec.

Stimulated Raman spec.
Spatially offset Raman spec.

Coherent anti-Stokes RS

Raman optical activity

Transmission Raman

Inverse Raman spec.

Tip-enhanced Raman spec.

Surface plasmon polariton enhanced Raman scattering.
State whether of the following vibrations are IR or Raman active
Molecular Nitrogen
Carbon Monoxide
Carbon Dioxide (Symmetrical stretching & asymmetrical stre.)
State the number of vibrational modes for the following molecules
1800 - Infrared radiation first discovered by Sir William Herschel
Discovered by diffracting sunlight through a prism and
noticing a raise in temperature, when holding a thermometer
past the red light

1905 - William Weber Colbentz published book indicating
character bands for certain functional groups

1937 - First IR spectrometer made by E. Lehrer

1950's - First commercial IR spectrometer
saturated C-H
saturated C-H
terminal alkyne
aromatic hydrocarbon
Determine whether the compound is an alkane, an alkene, an alkyne, or an aromatic hydrocarbon.
Then assign major peaks as marked.
More than one unsaturated group may be present.
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