1. An Introduction toAn Introduction to
UV – IRUV – IR
SpectroscopySpectroscopy
By : Daruosh ShafieeBy : Daruosh Shafiee

2. Different areas ofDifferent areas of
Electromagnetic spectrumElectromagnetic spectrum
 Spectrum X: breaking the linkSpectrum X: breaking the link
 Spectrum UV: electron transferSpectrum UV: electron transfer
 Spectrum IR: vibrationsSpectrum IR: vibrations
 Spectrum MW: rotational movementsSpectrum MW: rotational movements
 Spectrum RF: nuclear spinSpectrum RF: nuclear spin

3. IR SpectrophotometryIR Spectrophotometry
Almost all covalently bonded compounds are absorbed inAlmost all covalently bonded compounds are absorbed in
thethe
IR region. IR region has a wavelength longer than theIR region. IR region has a wavelength longer than the
wavelength of visible light (VIS) and shorter thanwavelength of visible light (VIS) and shorter than
microwaves (MW) is. The most important part of themicrowaves (MW) is. The most important part of the
spectrum to study the behavior of chemicals in thespectrum to study the behavior of chemicals in the
range of 660 cm-1 4000, respectively.range of 660 cm-1 4000, respectively.
E = h.E = h.νν = h.C/= h.C/λλ
EE ∝∝ νν ∝∝ 1/1/λλ

4. In the process of absorbing IR frequenciesIn the process of absorbing IR frequencies
of molecular vibrations with frequencies match isof molecular vibrations with frequencies match is
absorbed and the absorbed energy is used toabsorbed and the absorbed energy is used to
increase the range of motion of molecularincrease the range of motion of molecular
vibrations.vibrations.
Point :Point :
Only bonds with a dipole moment (the asymmetry inOnly bonds with a dipole moment (the asymmetry in
the bond) can absorb IR radiation and symmetrical,the bond) can absorb IR radiation and symmetrical,
such as H-H bonds do not absorb IR radiation.such as H-H bonds do not absorb IR radiation.
IR AbsorptionIR Absorption

5. The application of IR SpectrumThe application of IR Spectrum
1- Each frequency (energy) has its own vibration and a1- Each frequency (energy) has its own vibration and a
bond in two molecules is in two different environmentsbond in two molecules is in two different environments
with different structures, so they never have same IRwith different structures, so they never have same IR
spectrum. The IR spectrum can be used like fingerprintsspectrum. The IR spectrum can be used like fingerprints
for molecular identification. By comparing the IR spectrafor molecular identification. By comparing the IR spectra
of the two materials can be figured out whether they areof the two materials can be figured out whether they are
one or not?one or not?
2- IR spectrum also gives us information about the2- IR spectrum also gives us information about the
structure of the molecule. For example, the existence ofstructure of the molecule. For example, the existence of
such functional groups such as Csuch functional groups such as C ≡≡C, CC, C≡≡N, C=O, O-H, N-N, C=O, O-H, N-
H and conversion of the index in certain areas of the IRH and conversion of the index in certain areas of the IR
spectrum that represents the will of the linkages in thespectrum that represents the will of the linkages in the
molecule.molecule.

6. The stronger bond be required higher frequencyThe stronger bond be required higher frequency
(energy) to its vibration:(energy) to its vibration:
CC≡≡C > C=C > C-CC > C=C > C-C
21502150cmcm-1-1
1650 cm1650 cm-1-1
1200 cm1200 cm-1-1
IR absorption of various bondsIR absorption of various bonds

7. As the mass of the atoms attached to the carbonAs the mass of the atoms attached to the carbon
increases the frequency (energy) to reduce vibrationincreases the frequency (energy) to reduce vibration
link:link:
C-H > C-C > C-O > C-Cl > C-BrC-H > C-C > C-O > C-Cl > C-Br
3000cm3000cm-1-1
1200cm1200cm-1-1
1100cm1100cm-1-1
800cm800cm-1-1
600cm600cm-1-1
Concerns the interpretation of peaks IR:Concerns the interpretation of peaks IR:
IR spectroscopy is a technique for detection ofIR spectroscopy is a technique for detection of
functional groups. Important functional groups thatfunctional groups. Important functional groups that
routinely combined for evaluation include:routinely combined for evaluation include:
C=OC=O O-HO-H N-HN-H C=CC=C CC≡≡CC CC≡≡NN‫و‬‫و‬......

8. IR InterpretationIR Interpretation
To interpret IR peaks the following steps should beTo interpret IR peaks the following steps should be
considered:considered:
1. Is the carbonyl group (C = O) exist?1. Is the carbonyl group (C = O) exist?
There is a strong absorption in theThere is a strong absorption in the 1700 - 18001700 - 1800 cm-1 incm-1 in
the form of a strong peakthe form of a strong peak..

9. 2. If the carbonyl group (C=O) exists, the2. If the carbonyl group (C=O) exists, the
following processes must be considered. Iffollowing processes must be considered. If
not go to step 3.not go to step 3.
--Acids: Are there groups O-H? (R-CO-Acids: Are there groups O-H? (R-CO-
OHOH((
There is a very strong and broad absorptionThere is a very strong and broad absorption
in the 2500 - 3400 cm-1in the 2500 - 3400 cm-1

10.  Amides: Does the group N-H there? (R-CO-Amides: Does the group N-H there? (R-CO-
NH2)NH2)
 There are moderate absorptions nearThere are moderate absorptions near 3500 cm-13500 cm-1

11.  Esters: Does the group C-O there? (R-CO-Esters: Does the group C-O there? (R-CO-
OR)OR)
 There are strong appeal in theThere are strong appeal in the 1000-1300 cm-11000-1300 cm-1

12.  Aldehydes: Is aldehyde C-H groups are there?Aldehydes: Is aldehyde C-H groups are there?
(RH-CO-H)(RH-CO-H)
Two weak absorption in the 2700 - 2800 cm-1Two weak absorption in the 2700 - 2800 cm-1
to C-H absorption ordinaryto C-H absorption ordinary

13. - Ketones: (R-CO-R)- Ketones: (R-CO-R)
If none of the above conditions is notIf none of the above conditions is not
available.available.

14. 3. If the carbonyl group (C=O) did not exist:3. If the carbonyl group (C=O) did not exist:
Alcohol (phenols):Alcohol (phenols):
Does the group O-H there? (R-OH)Does the group O-H there? (R-OH)
There is strong & broad absorption in the 3200-3500 cm-There is strong & broad absorption in the 3200-3500 cm-
11

15.  Amines: Does the group N-H there? (R-NH2)Amines: Does the group N-H there? (R-NH2)
There are moderate absorption near 3400cm-1There are moderate absorption near 3400cm-1

16.  Ethers: Does the group C-O there? (R-A-Ethers: Does the group C-O there? (R-A-
R)R)
There are strong attractions in area 1000-1300 cm-There are strong attractions in area 1000-1300 cm-
11

17. 4- Double bonds or aromatic: Is there Group4- Double bonds or aromatic: Is there Group
C=C?C=C?
- Poor absorption in the 1650 cm-1 is indicative of C=C- Poor absorption in the 1650 cm-1 is indicative of C=C
- Moderate or high conversions of 1400-1650 cm-1 and- Moderate or high conversions of 1400-1650 cm-1 and
usually crowded area and the absorbance at 650-usually crowded area and the absorbance at 650-
800 cm-1 represents the aromatic.800 cm-1 represents the aromatic.
..

18. 5. The triple bonds C5. The triple bonds C≡≡C or CC or C≡≡N ?N ?
- Medium and sharp absorption about 2250 cm-1- Medium and sharp absorption about 2250 cm-1
indicatesindicates CC≡≡NN
- Medium and sharp absorption about 2150 cm-1- Medium and sharp absorption about 2150 cm-1
indicatesindicates CC≡≡CC

19. 66- Hydrocarbons: (R-H)- Hydrocarbons: (R-H)
- There is none of the above attractions- There is none of the above attractions ..

32. UV SpectroscopyUV Spectroscopy
The frequency (Energy) of UV is in the range ofThe frequency (Energy) of UV is in the range of
electronic transitions. The atoms or moleculeselectronic transitions. The atoms or molecules
can absorb UV radiation and then the electroncan absorb UV radiation and then the electron
can stimulate from one basic electronic level tocan stimulate from one basic electronic level to
higher levels, and it is the base of UVhigher levels, and it is the base of UV
spectroscopy.spectroscopy.

33. The principle of AbsorptionThe principle of Absorption
SpectroscopySpectroscopy
Beer-Lambet LawBeer-Lambet Law::
Direct relation between Absorption versusDirect relation between Absorption versus
Concentration in Quantitative analysisConcentration in Quantitative analysis
Absorption coefficient (Absorption coefficient ( εε) has the range of 0 to 1,000,000) has the range of 0 to 1,000,000
<<10,00010,000))strong attraction) and <1,000 (weak absorptionstrong attraction) and <1,000 (weak absorption((

34. Beer-Lambert limitationsBeer-Lambert limitations
1. Instrumental deviations1. Instrumental deviations
1.1. beam of light (1.1. beam of light ( λλmax)max)
The absorption coefficient (The absorption coefficient ( εε) depends on the wavelength.) depends on the wavelength.
1.2. Stray radiations (side lights)1.2. Stray radiations (side lights)
Absorbance linearity with respect to the concentrationAbsorbance linearity with respect to the concentration
overshadowed.overshadowed.
2. Chemical Deviations2. Chemical Deviations
2.1. Beer-Lambert law holds true in low concentrations.2.1. Beer-Lambert law holds true in low concentrations.
2.2. Interaction between molecules increased in higher2.2. Interaction between molecules increased in higher
concentrations and density affect them. In fact, theconcentrations and density affect them. In fact, the
interaction of molecules to affect their ability tointeraction of molecules to affect their ability to
attract and cause the linearity calibration.attract and cause the linearity calibration.
2.3. In high concentrations cause many changes in the2.3. In high concentrations cause many changes in the
refractive index solution to the absorbed doserefractive index solution to the absorbed dose
linearity.linearity.
2.4. Deviation from the separation or reactive chemical2.4. Deviation from the separation or reactive chemical
species, absorbing unwantedspecies, absorbing unwanted

35. Overview of SpectrophotometerOverview of Spectrophotometer

36. Some terms in UV spectrumSome terms in UV spectrum
 ChromophoresChromophores
Specific functional groups (unsaturated) that areSpecific functional groups (unsaturated) that are
responsible for absorbing the molecules. Such asresponsible for absorbing the molecules. Such as
C=C, C=O, NO2 and ...C=C, C=O, NO2 and ...
 Oxochromes:Oxochromes:
Saturation groups (substituted) that if connected toSaturation groups (substituted) that if connected to
numerous chromophores can change the wavelengthnumerous chromophores can change the wavelength
and intensity of UV absorption. Such as NH2, OHand intensity of UV absorption. Such as NH2, OH
and ...and ...
 Bathochromic displacement (red displacement)Bathochromic displacement (red displacement)
The absorption spectrum towards longer wavelengthsThe absorption spectrum towards longer wavelengths
transfer or substitution of solventtransfer or substitution of solvent
 Hypsochromic or blue shiftHypsochromic or blue shift
Transfer to the absorption spectrum of wavelengthsTransfer to the absorption spectrum of wavelengths
shorter or substitution of solventshorter or substitution of solvent

37. Solvents should be "Solvents should be " Solvents for spectroscopySolvents for spectroscopy ""
because the impurities do not disturb the UVbecause the impurities do not disturb the UV
spectrum (Solvents no effect on UV). Purityspectrum (Solvents no effect on UV). Purity
suitable solvent and solute to be ineffective.suitable solvent and solute to be ineffective.
CompoundCompound λ maxmax εε maxmax
BenzeneBenzene 256256 200200
TolueneToluene 261261 300300
m, Xylenem, Xylene 263263 300300
1,3,5- Trimethyl1,3,5- Trimethyl
BenzeneBenzene
266266 300300
Hexa methyl BenzeneHexa methyl Benzene 272272 300300

38. In multi-core aromatic compounds towardIn multi-core aromatic compounds toward
longer wavelengths absorbed by increasing thelonger wavelengths absorbed by increasing the
number of rings (Bathochromic shift) is shiftednumber of rings (Bathochromic shift) is shifted
to the visible region.to the visible region.