Spectroscopic Methods PART 1 1
Dec 25, 2015
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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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)
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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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.
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
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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