Spectroscopic Methods

PART 1

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Spectroscopic Techniques for Sequence Characterization

3Useful Web site for fundamentals: www.organicworldwide.net

Useful Spectroscopic Techniques

High Resolution NMR of Polymer Solutions (Samples are dissolved)

Mass Spectrometry (Samples are vaporized)

Highly Useful Spectroscopic Techniques

FT-IR spectroscopy

Raman Spectroscopy

High Resolution Solid State NMR

UV and Visible Spectroscopy (insufficient resolution)

Spectroscopic Techniques Which are Sometimes Useful

Selection of Spectroscopic Technique

• Each technique is based upon a unique phenomenon:• Infrared spectroscopy; vibrational energy absorption• Raman spectroscopy: inelastic scattering from vibrational levels • NMR: nuclear energy absorption while the sample is

located in a magnetic field• Mass spectrometry: ionization

• One technique may be better suited than another for a particular problem

• It is important to know the limitations of each technique i.e., sample preparation, etc.

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IR SPECTROSCOPY

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Interaction of light with matter

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Characteristics of waves

Amplitude

Maximum height of the oscillating stuff

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Absorption and emission of light from matter

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The Beer_Lambert Law

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Types of energies in a molecules

E (Molecules or Atoms)= Transition +Electronic + Vibration + Rotation

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Quantized Energy levels

Uv-Visb frequencies

(200-400 nm)

IR frequencies (2.5 -15 m,

400 – 4000 (cm-1)

Microwave frequencies (1 – 10-3 m)

Electronic and Vibration energy levels

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Molecular Spectroscopy

Energy possessed by molecules is quantised.

When a molecule interacts with radiation there can be changes in electronic, vibrational or rotational energy.

These changes depend on the frequency of the radiation.

Analysis of the energy needed to change from one energy level to another forms basis of molecular spectroscopy.

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Infrared Spectroscopy

Substances exposed to radiation from frequency range 1014 Hz to 1013 Hz (wavelengths 2.5μm -15μm)

Causing vibrational energy changes in the molecule

These absorb infrared radiation of specific frequencies.

Point is to identify functional groups in the molecule

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Vibrations in Molecules and Bond Deformations

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Bond deformation SIMPLE diatomic molecules can only vibrate

one way, by stretching.

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H Br

For these molecules there is only one vibrational infrared absorption.

Bond deformation More complex molecules have more possible

deformations

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O C O

symmetric stretch

Bond deformation

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O C O

O C O

asymmetric stretch

Bond deformation

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O C O

Bond deformation

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O C O

Bond deformation

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O C O

Bond deformation

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O C O

Bond deformation

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O C O

Bond deformation

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O C O

Bond deformation

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O C O

Bond deformation

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O C O

bending

Wavenumber (cm-1)

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c = λ f

from this equation we can get the reciprocal of the wavelength (1/λ)

this is a direct measure of the frequency

the reciprocal is described as the wavenumber

it is the wavenumber, measured in cm-1 that is recorded on an

infrared spectrum

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wavenumber (1/λ) / cm-1

wavelength (λ) / μm

frequency (v) / Hz

1000200030004000

102.5

2.5 x 10131.0 x 1014

Simple version Sample placed in ir spectrometer Subjected to ir radiation Molecule absorbs energy Molecule bonds starts to undergo different types of vibration (stretching, bending etc.)

This produces different signals that the detector records as ‘peaks’ on the spectrum.

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In an IR Spectroscopic chart

Frequencies are different for each molecule

Energy required for vibration depends on strength of bond

Weaker bonds requiring less energy.

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Important …When an ir spectrum is obtained we do not try to explain the whole thing, simply look for one or two signals that are characteristic of different bonds. 56

O-H bondstretch

C-H bondstretch

C-O bondstretch

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OCC

H

H

H HH

H

O-H bondstretch

3670 cm-1

C-O bondstretch

1050 cm-1

C-H bondstretch

3010 -2850 cm-1

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Interpreting the spectra!

Usually match a particular bond to a particular absorption region.

The precise position of the peak depends on the bond environment, so only wavenumber regions can be quoted.

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absorption intensity The strongest (more intense) absorptions

occur when a large change in bond polarity associated with the vibration.

e.g. C=O bonds will give more intense absorptions than C=C bonds.

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Some typical absorptions

Below 1500cm-1 the IR spectrum can be quite complex

This region is characteristic of a particular molecule

Hence known as ‘fingerprint region’

Absorption range / cm-1

Bonds responsible Examples

4000-2500 Single bonds to H

O-H, C-H, N-H

2500-2000 Triple bonds C≡C, C≡N2000-1500 Double bonds C=C, C=OBelow 1500 various C-O, C-X

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