various factor affecting vibrational frequency in IR,
• Coupled vibrations
• Fermi resonance
• Electronic effects
• Hydrogen bonding
and their examples
a substance can absorb any visible light or external radiation and then again emit it. this called fluorescence and the process of reduction in fluorescence intensity is called quenching. this presentation is all about quenching of fluorescence.
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metastable ions
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applications
This presentation include the detailed explanation of various parts of a UV-Visible spectrophotometer and two types of UV-Visible spectrophotometers-Single beam and Doube beam. It also include the comparison between single beam and double beam spectrophotometers.
a substance can absorb any visible light or external radiation and then again emit it. this called fluorescence and the process of reduction in fluorescence intensity is called quenching. this presentation is all about quenching of fluorescence.
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fragmentation process
general rules for fragmentation
general modes of fragmentation
metastable ions
isotopic peaks
applications
This presentation include the detailed explanation of various parts of a UV-Visible spectrophotometer and two types of UV-Visible spectrophotometers-Single beam and Doube beam. It also include the comparison between single beam and double beam spectrophotometers.
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This presentation discuss about he factors affecting IR vibrational frequencies. It discuss about various factors such as Bond Order, Fermi resonance, Inductive effect etc..
Introduction to Spectroscopy,
Introduction to UV, electronic transitions, terminology, chromophore, Auxochrome, Examples and Applications.
Introduction to IR, Fundamental vibrations, Types of Vibrations, Factors affecting the vibrational freaquencies, Group frequencies, examples and applications.
UV-Visible spectroscopy, a versatile analytical technique used to study the interaction of molecules with light within the ultraviolet and visible regions of the electromagnetic spectrum. It is a fundamental tool in chemistry, biochemistry, and related fields for analyzing the electronic structure of molecules, determining their concentrations, and studying their behavior.
1. Principle of UV-Visible Spectroscopy:
UV-Vis spectroscopy is based on the principle that molecules absorb specific wavelengths of light due to electronic transitions.
When molecules absorb light in the UV or visible range, they move from a ground state to an excited state.
2. Instrumentation:
UV-Vis spectrophotometer is the key instrument used for this technique.
Components include a light source, sample holder, monochromator, and a detector.
The sample is placed in a cuvette, and the spectrophotometer measures the absorbance of light passing through the sample.
3. Beer-Lambert Law:
The Beer-Lambert law relates the concentration of a solution, the path length (distance that light travels through the solution), and the absorbance of light by the solution.
A = ε * c * l, where A is absorbance, ε is the molar absorptivity (a constant for a specific compound and wavelength), c is the concentration, and l is the path length.
4. Absorbance Spectra:
UV-Vis spectroscopy generates absorbance spectra, which are plots of absorbance versus wavelength.
Peaks in the spectra indicate the wavelengths of light that are absorbed by the sample, providing information about the electronic structure of the molecules.
5. Applications:
Quantitative Analysis: UV-Vis is widely used for quantitative analysis of compounds by measuring the absorbance of a sample and comparing it to a standard curve.
Identification of Compounds: The unique absorbance spectra can be used to identify compounds.
Kinetics: UV-Vis can monitor reaction kinetics by following the change in absorbance over time.
Pharmaceutical Analysis: It is crucial in quality control for pharmaceuticals.
Environmental Analysis: UV-Vis is used in environmental monitoring, such as water quality analysis.
6. Advantages:
It's a rapid and simple technique.
It can be highly sensitive for many compounds.
It is non-destructive to the sample.
7. Limitations:
It does not provide structural information about the molecules.
It may not be suitable for analyzing complex mixtures.
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Various factor affecting vibrational frequency in IR spectroscopy.
1. various factor affecting vibrational frequency in IR
Coupledvibrations
Fermi resonance
Electronic effects
Hydrogenbonding
COUPLED VIBRATIONS
An isolated C-H bondhas only one stretching vibrationalfrequency
where as methylenegroup showstwo stretching vibrations, symmetrical
and asymmetrical.
Becauseofmechanicalcouplingor interaction betweenC-H stretching
vibrations in the CH2 group.
Assymetric vibrations occurat higherfrequenciesor wavenumbers than
symmetric stretching vibrations.
Theseare known as coupledvibrations becausethesevibrations occurat
different frequenciesthan that required for an isolated C-H stretching.
A strong vibrationalcoupling is presentin carboxylic acid anhydridesin
which symmetricaland asymmetrical stretching vibrations appearin the
region 1720 – 1825 cm-1.
2. The interaction is very effective probablybecauseofthe partial double
bondcharacterin the carbonyloxygenbondsdueto resonancewhichalso
keepsthe system planarfor effective coupling.
For interaction to occur, the vibrations must be of samesymmetry species.
There mustbe a commonatom betweenthe groups for strong coupling
betweenstretching vibrations.
For couplingof bendingvibrations , a common bondis necessary.
Interaction is greatestwhen coupledgroupsabsorb, individually, nearthe
same frequency.
Coupling is negligible whengroups are separatedby one or more carbon
atoms and the vibrations are mutually perpendicular.
FERMI RESONANCE
Resonance
A vibration of large amplitude producedby a relatively small vibration.
Coupling of two fundamentalvibration modesproducestwo new modes
of vibration ,with frequencieshigherandlower than that observedin
absenceofinteraction. Interaction canalso take place between
fundamentalvibrations andovertonesor combination tone vibrations and
suchinteractions are knownas FermiResonance.
In this , a moleculetransfers its energyfrom fundamentalvibrational level
to overtoneor combination tone leveland back.
Resonancepushesthetwo levels apart andmixes their character,
consequentlyeachlevelhas partly fundamentaland partly overtoneor
combinat
3. For eg. symmetricalstretching vibration of CO2 in Ramanspectrum shows
bandat 1337 cm-1.Thetwo bendingvibrations are equivalentand absorbat
the same frequencyof 667.3cm-1.
The first overtoneof this is 2 X 667.3= 1334.6cm-1..
Fermi resonance occurs
There is mixing of 1337cm-1and 1334.6cm-1to give two bandsat1285.5
cm-1 andat 1388.3cm-1 with intensity ratio 1 : 0.9 respectively.
Hydrogen bonding
It occursin any systemcontaininga proton donorgroup(X-H) and a
proton acceptor.if the s-orbital of the proton can effectively overlap the P
or π orbital of the acceptorgroup.
The strongerthe hydrogenbond, the longerthe O-H bond,the lowerthe
vibration frequencyandbroaderand more intensewill be the absorption
band.
4. The N-H stretching frequenciesof amines are also affected by hydrogen
bondingas that of the hydroxylgroup butfrequencyshifts for aminesare
lesserthan that for hydroxylcompounds.
Becausenitrogenis less electronegativethan oxygenso the hydrogen
bondingin amines is weakerthan that in hydroxylcompounds.
Intermolecular hydrogenbondsgivesrise to broadbands,while
intramolecular hydrogenbondsgivesharp and well defined bands.
The inter and intramolecularhydrogenbondingcan be distinguishedby
dilution.
Intramolecular hydrogenbondingremains unaffectedon dilution and as a
result the absorption bandalso remains unaffectedwhere as in
intermolecular, bondsare brokenon dilution and as a result there is a
decreasein the bondedO-H absorption.
The strength of hydrogenbondingis also affected by :
Ring strain
Moleculargeometry
Relative acidity andbasicity of the proton donorand acceptor
groups
Electroniceffect
Changesin the absorptionfrequenciesfor a particular group take placewhen the
substituentsin the neighbourhoodofthat particular group are changed.
It includes :
1. .Inductive effect
2. .Mesomeric effect
3. Field effect
5. INDUCTIVE EFFECT
The introduction of alkyl groupcauses+I effect which results in the
lengtheningor the weakeningof the bond
Hencethe force constantis lowered and wave numberof absorption
decreases.
Let us comparethe wave numbersof v (C=O) absorptionsfor the following
compounds:
Formaldehyde(HCHO) 1750cm-1.
Acetaldehyde(CH3CHO) 1745 cm-1.
Acetone(CH3COCH3) 1715cm-1.
Introduction of an electronegativeatom or group causes –Ieffect which
results in the bondorder to increase.
Hencethe force constantincreasesand the wave numberof absorptionrises.
Mesomeric effect :
It causeslengtheningorthe weakeningof a bond leadingin the lowering of
absorptionfrequency.
6. As nitrogen atom is less electronegativethan oxygenatom, the electron pair
on nitrogen atom in amide is more labile and participatesmore in
conjugation.
Due to this greaterdegreeof conjugation,the C=O absorption frequencyis
much less in amides as comparedto that in esters.
FIELD EFFECT
In ortho substitutedcompounds,thelone pair of electrons on two atoms
influence eachother throughspaceinteractions and changethe vibrational
frequenciesof both the groups.Thiseffect is called field effect