SPECTROSCOPY
Atoms and molecules interact with electromagnetic radiation (EMR) in a wide variety of ways.
Atoms and molecules may absorb and/or emit EMR.
Absorption of EMR stimulates different types of motion in atoms and/or molecules.
The patterns of absorption or emission are called ‘spectra’.
The field of spectroscopy is concerned with the interpretation of spectra in terms of atomic and molecular structure (and environment).
Monday, November 2, 2009
INFRARED SPECTROSCOPY
INFRARED SPECTROSCOPY
Infrared radiation stimulates molecular vibrations.
Infrared spectra are traditionally displayed as %T (percent transmittance) versus wave number.
Useful in identifying presence or absence of functional groups.
¨IR range- 10cm- - 12800cm-
¨Sub regions
¨Near IR – 12500 – 4000cm-
¨Middle IR – 4000 – 667cm-
¨Far IR – 667 – 50 cm-
Finger print region – 900 – 1400cm- (identify the compound)
Water is a
-bent- non linear – triatomic molecule
Formula used for finding vibrations in non-linear molecule – (3N-6)
Carbon-di-oxide is a
– Linear – triatomic molecule
Formula used for finding vibrations in non-linear molecule - (3N-5)
Infrared radiation stimulates molecular vibrations.
Infrared spectra are traditionally displayed as %T (percent transmittance) versus wave number.
Useful in identifying presence or absence of functional groups.
¨IR range- 10cm- - 12800cm-
¨Sub regions
¨Near IR – 12500 – 4000cm-
¨Middle IR – 4000 – 667cm-
¨Far IR – 667 – 50 cm-
Finger print region – 900 – 1400cm- (identify the compound)
Water is a
-bent- non linear – triatomic molecule
Formula used for finding vibrations in non-linear molecule – (3N-6)
Carbon-di-oxide is a
– Linear – triatomic molecule
Formula used for finding vibrations in non-linear molecule - (3N-5)
INFRARED ABSORPTION BANDS
INFRARED ABSORPTION BANDS
POSITION -REDUCED MASS- BOND STRENGTH (STIFFNESS)
LIGHT ATOMS HIGH FREQUENCY
STRONG BONDS HIGH FREQUENCY
STRENGTH -CHANGE IN ‘POLARITY’
STRONGLY POLAR BONDS GIVE INTENSE BANDS
WIDTH-HYDROGEN BONDING
STRONG HYDROGEN BONDING GIVES BROAD BANDS
POSITION -REDUCED MASS- BOND STRENGTH (STIFFNESS)
LIGHT ATOMS HIGH FREQUENCY
STRONG BONDS HIGH FREQUENCY
STRENGTH -CHANGE IN ‘POLARITY’
STRONGLY POLAR BONDS GIVE INTENSE BANDS
WIDTH-HYDROGEN BONDING
STRONG HYDROGEN BONDING GIVES BROAD BANDS
INFRARED (VIBRATIONAL) SPECTROSCOPY
POLARITY
Only vibrations that cause a change in ‘polarity’ give rise to bands in IR spectra.
Which of the vibrations for CO2 are infrared active?
Symmetric stretch
Asymmetric stretch
Bending (doubly degenerate)
Two types of stretching vibrations
Symmetric stretch
Asymmetric stretch
Four types of Bending Vibrations
¨Scissoring
¨Rocking
¨Twisting
¨Wagging
Only vibrations that cause a change in ‘polarity’ give rise to bands in IR spectra.
Which of the vibrations for CO2 are infrared active?
Symmetric stretch
Asymmetric stretch
Bending (doubly degenerate)
Two types of stretching vibrations
Symmetric stretch
Asymmetric stretch
Four types of Bending Vibrations
¨Scissoring
¨Rocking
¨Twisting
¨Wagging
Sunday, November 1, 2009
IR INSTRUMENTATION
Source- Incandescent material
Other sources – Nernst glower , Globar , Carbon arc, Heated Nichrome
Monochromator – Quartz – prism
Detectors – Photo conductivity cell, Thermocouple, Pyroelectric detectors
APPLICATIONS OF IR SPECTROSCOPY
—Identification of a substance (penicillin and peptides)
—Study of geometrical isomerism
—Characteristic of organic substances
—Study of photosynthesis and reparative in plants
—Assessment of purity of a sample
—Ideal and rapid method
—Study of geometrical isomerism
—Characteristic of organic substances
—Study of photosynthesis and reparative in plants
—Assessment of purity of a sample
—Ideal and rapid method
PRINCIPLES OF NMR.
PRINCIPLES OF NMR
i) The nuclei of all elements carry a charge.
ii) When the spins of the protons and neutrons comprising these nuclei are not paired, the overall spin of the charged nucleus generates a magnetic dipole along the spin axis, and the intrinsic magnitude of this dipole is a fundamental nuclear property called the nuclear magnetic moment, ยต.
i) The nuclei of all elements carry a charge.
ii) When the spins of the protons and neutrons comprising these nuclei are not paired, the overall spin of the charged nucleus generates a magnetic dipole along the spin axis, and the intrinsic magnitude of this dipole is a fundamental nuclear property called the nuclear magnetic moment, ยต.
NMR Signals, Tetramethyl silane
NMR Signals
•The number of signals shows how many different kinds of protons are present.
•The location of the signals shows how shielded or deshielded the proton is.
•The intensity of the signal shows the number of protons of that type.
•Signal splitting shows the number of protons on adjacent atoms.
TETRAMETHYL SILANE
TMS is used as the internal standard
Solvents used are CDCl3,DMSO,CD3OD
•The number of signals shows how many different kinds of protons are present.
•The location of the signals shows how shielded or deshielded the proton is.
•The intensity of the signal shows the number of protons of that type.
•Signal splitting shows the number of protons on adjacent atoms.
TETRAMETHYL SILANE
TMS is used as the internal standard
Solvents used are CDCl3,DMSO,CD3OD
Carbon-13
Carbon-13
•12C has no magnetic spin.
•13C has a magnetic spin, but is only 1% of the carbon in a sample.
•Signals are weak, getting lost in noise.
•Hundreds of spectra are taken, averaged.
•12C has no magnetic spin.
•13C has a magnetic spin, but is only 1% of the carbon in a sample.
•Signals are weak, getting lost in noise.
•Hundreds of spectra are taken, averaged.
UV-Visible Spectroscopy
Why should we learn this stuff? After all, nobody solves structures with UV any longer!
* Many organic molecules have chromophores that absorb UV
* UV absorbance is about 1000 x easier to detect per mole than NMR
* Still used in following reactions where the chromophore changes. Useful because timescale is so fast, and sensitivity so high. Kinetics, esp. in biochemistry, enzymology.
* Most quantitative Analytical chemistry in organic chemistry is conducted using HPLC with UV detectors
* One wavelength may not be the best for all compound in a mixture.
* Affects quantitative interpretation of HPLC peak heights.
* Many organic molecules have chromophores that absorb UV
* UV absorbance is about 1000 x easier to detect per mole than NMR
* Still used in following reactions where the chromophore changes. Useful because timescale is so fast, and sensitivity so high. Kinetics, esp. in biochemistry, enzymology.
* Most quantitative Analytical chemistry in organic chemistry is conducted using HPLC with UV detectors
* One wavelength may not be the best for all compound in a mixture.
* Affects quantitative interpretation of HPLC peak heights.
Ultraviolet (UV) Spectroscopy
Ultraviolet (UV) Spectroscopy – The Output
The output from a UV scanning spectrometer is not the most informative looking piece of data!! It looks like a series of broad humps of varying height.
Absorbance has no units – it is actually the logarithm of the ratio of light intensity incident on the sample divided by the light intensity leaving the sample.
There are two particular strengths of UV
(i) it is very sensitive
(ii) it is very useful in determining the quantity of a known compound in a solution of unknown concentration. It is not so useful in determining structure although it has been used in this way in the past.
The concentration of a sample is related to the absorbance according to the Beer Lambert Law.
Beer Lambert Law A = e.c.l
A = absorbance
c = concentration in moles l-1
l = pathlength in
e = molar absorptivity (also known as extinction coefficient) which has units of moles-1 L cm -1.
The output from a UV scanning spectrometer is not the most informative looking piece of data!! It looks like a series of broad humps of varying height.
Absorbance has no units – it is actually the logarithm of the ratio of light intensity incident on the sample divided by the light intensity leaving the sample.
There are two particular strengths of UV
(i) it is very sensitive
(ii) it is very useful in determining the quantity of a known compound in a solution of unknown concentration. It is not so useful in determining structure although it has been used in this way in the past.
The concentration of a sample is related to the absorbance according to the Beer Lambert Law.
Beer Lambert Law A = e.c.l
A = absorbance
c = concentration in moles l-1
l = pathlength in
e = molar absorptivity (also known as extinction coefficient) which has units of moles-1 L cm -1.
INSTRUMENTATION OF UV-VISIBLE SPECTROSCOPY
INSTRUMENTATION
Radiation source
Wavelength selectors
Prisms – visible – glass prisms
ultraviolet – silica, quartz prisms
Sample holder – cells or cuvette
visible – glass
ultraviolet – fused silica cells
Solvent – ethyl alcohol
Radiation source
Wavelength selectors
Prisms – visible – glass prisms
ultraviolet – silica, quartz prisms
Sample holder – cells or cuvette
visible – glass
ultraviolet – fused silica cells
Solvent – ethyl alcohol
MASS SPECTROSCOPY
ESSENTIAL REQUIREMENTS OF MASS SPECTROSCOPY
Production of ions in gas phase = Cations (removing e-) and Anions (Adding e-)
* Acceleration to specific velocity
* Projection into suitable mass analyzer
* Detection of ions
Efficiency of cation depends - 1. Different Ionisation energy
2. Electron affinity
Production of ions in gas phase = Cations (removing e-) and Anions (Adding e-)
* Acceleration to specific velocity
* Projection into suitable mass analyzer
* Detection of ions
Efficiency of cation depends - 1. Different Ionisation energy
2. Electron affinity
Uses of Mass spectroscopy
Uses of Mass spectroscopy
— 1. Ionizing any material (in gas phase)
— 2. Investigation of any biological compounds
— 1. Ionizing any material (in gas phase)
— 2. Investigation of any biological compounds
ELECTRON IMPACT IONISATION
ELECTRON IMPACT IONISATION
1. Commonly used and simple source
2. Used for metabolic studies, drug studies
CHEMICAL IMPACT IONISATION
CHEMICAL IMPACT IONISATION
Determine molecular mass
Production of high intensity molecular or pseudo molecular ions.
Generation of ions gives species like CH3+.
Suitable reagent gas (methane) used
Uses: study of drugs
Determine molecular mass
Production of high intensity molecular or pseudo molecular ions.
Generation of ions gives species like CH3+.
Suitable reagent gas (methane) used
Uses: study of drugs
Isotope Ratio Mass Spectrometry
IRMS
Used in biological field
Used for metabolic studies
Used for the analysis of Human Breath
EI source, magnetic sector, mass analyser & Faraday cup detector
m/Z = B2R2/2V
FAST ATOM BOMBARDMENT
FAST ATOM BOMBARDMENT
•FAS
•Introducing ionising beam into materials
•Liquid matrices used
•Solid surfaces damaged by high intensity beam
•Collision occur between fast moving ions & slow moving ions
•Use- Fragmentation of peptides.
•FAS
•Introducing ionising beam into materials
•Liquid matrices used
•Solid surfaces damaged by high intensity beam
•Collision occur between fast moving ions & slow moving ions
•Use- Fragmentation of peptides.
PYROLOSIS MASS SPECTROMETRY
PYROLOSIS MASS SPECTROMETRY
—Materials subjected to high T
—Ejection of volatile substances
—EI source, mass analyzer
—Certain T allowed
—Studying the effects of different growth conditions, composition of cell walls.
—Materials subjected to high T
—Ejection of volatile substances
—EI source, mass analyzer
—Certain T allowed
—Studying the effects of different growth conditions, composition of cell walls.
ANALYSERS
ANALYSERS USED IN MASS SPECTROMETERS
ž1. MAGNETIC SECTOR ANALYSER
ž2. ELECTRIC SECTOR ANALYSER
ž3. QUADRUPOLE MASS FILTERS
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