#ULTRAVIOLET-VISIBLE SPECTROSCOPY#

2. The Electromagnetic Spectrum

3.  Atom- a set of discrete E levels
 Three types:
i) Electronic E levels (ΔEelectronic)
ii) Vibrational E levels (ΔEvibrational)
iii) Rotational E levels (ΔErotational)
Order of E levels:
ΔErotational < ΔEvibrational < ΔEelectronic

4.  Near UV region : 200-400 nm- for transition study
 Far UV region : below 200 nm
 Visible range : 400-800 nm
 Solvents- Ethyl alcohol/Hexane/Water- not absorb in UV
region
 Complementary to fluorescence spectroscopy

5. Instrumentation
Ultraviolet Spectrophotometer (Double beam)
Hydrogen Discharge Lamp for UV
Tungsten filament lamp for visible region
Prism
Quartz for UV, Glass for visible
Compound sample+ ethanol solvent
UV/Visible spectra :
Absorption Spectra

6. Light source
 Hydrogen discharge lamp, deuterium gas discharge lamp, Quartz lamp, mercury
arc
Sample cell
Quartz cuvettes- UV spectroscopy
Glass cuvettes - visible spectroscopy
Detector
Photoelectric cell, Photovoltaic cell, Photomultiplier tube
Convert light E into electrical E/current
Output current α intensity of light falling
Source

7. Lambert's law
 When monochromatic radiation is passed through a solution-
decrease in intensity of radiation with thickness of the solution is
directly proportional to the intensity of the incident radiation.
-dI α I
dx
-dI = K I
dx
-dI = K dx
I
On integration
ln I = -K. l
Io
log I = -K .l ; -K = E ; I0 = A
Io 2.303 2.303 I
A = E. l

8.  When monochromatic radiation is passed through a solution-
decrease in intensity of radiation with thickness of the
solution is directly proportional to the intensity of the incident
radiation as well as conc. of the solution
-dI α C I
dx
-dI = K’. C I
dx
-dI = K’ . C dx
I
On integration
ln I = -K’. C. l
Io
log I = -K’ . l
Io 2.303
A = E. C.l

9.  Linear relationship
 Absorption oa a light by solution is directly proportional to
the conc . of solution and path length.
A = ε * l * c
 ε = molar absorptivity coefficient
 l = path length
 C = conc . of solution
 Absorbannce
[A = log (I/I0)]
Percent transmittance
%T = (I/I0 * 100)

10. 1) Beer’s law: A α C
2) Absorption of λmax from UV-visible region- causes excitation
of bonding e-s in a covalent bond to higher E antibonding
molecular orbitals.
3) Stronger covalent bond- higher E require for excitation-
shorter λ in UV region
4) Weaker covalent bond- Lower E require for excitation- Longer
λ in visible region

11.  Molecules containing π-electrons or non-bonding electrons (ne-
) absorb the energy from UV/ visible light causes excitation of
e-s in a bond to higher E state.
 These transitions consist of excitation of an e- from π e-s/ n e-s
to antibonding orbital.
 Three types of e-s present in molecules- σ, n, π e-s
a) σ e-s–involved in saturated /sigma bond
b) n e-s – not participate in bonding- Lone pair of e-s –N, O, S
c) π e-s – involved in unsaturated compounds and aromatics

12. 1) σ σ * Transitions:
• Bonding s orbital e- is excited to the corresponding
antibonding orbital.
 E required - large.
 E.g. methane (C-H bonds, σ σ * transitions) -
absorbance maximum 125 nm.
2) n σ * Transitions:
 Saturated compounds containing atoms with lone pairs (non-
bonding es- O, N,S and halogens)
 Need less E than σ σ * transitions.
 Wavelength range 150 - 250 nm.
 Organic functional groups - small UV region. eg. Amines (-
NH2), alcohols (-OH)

13. 3) π π* transitions:
 π e-s in a bonding orbital is excited to antibonding orbital.
 Eg. Unsaturated compounds- Alkene and alkynes
Carbonyl, nitriles
a) E-band: double bond compound (Alkenes) – absorption large- 160-
175 nm region
b) K-band: Conjugated double bond like butadiene, mesityl oxide-
absorption large- 200-280 nm
c) B-bands: Aromatic and heteroaromatic molecules-
broad bands
 Near UV region 220-270 nm
 Absorbtivity between 1000 - 10,000


14. 4) n π* transitions:
 Unsaturated Compounds containing atoms with lone pairs
(non-bonding electrons O, N,S and halogens)
 Very weak bands - absorption coefficient 10-100
 R-bands 270-320 nm (λ)
 eg. Aldehydes and Ketones

16. Terms in UV-Visible Spectrometry
1. Chromophore
 Part of a molecule -responsible for its color; Absorption
above 200 nm
 Covalently unsaturated groups for electronic absorption in
UV-Visible region.
 Delocalised systems of es in a compound - gives its colour,
 e.g. C = C, C = O, C = N, C = S, N = O, NO2
NO2

17. 2. Auxochrome (Auxanein: to increase)
 A saturated group with nonbonded electrons (lone pair of es)
which when attached to a chromophore increase the
wavelength and intensity of absorption e.g.: Benzene,
Phenol, Aniline
 λmax 225 nm 269nm 270nm
 - CH3 -OH
 auxochrome auxochrome
CH3 OH

18. 3) Bathochromic shift (Red Shift)
 The shift of an absorption (λmax) to a longer wavelength
 It is due to the presence of an auxochrome (substitution)/
change of solvent
 p- nitrophenol Alkaline medium

λmax 255 nm 265 nm
NO2
NO2
OH
O-

19. 4) Hypsochromic shift (Blue Shift)
 The shift of an absorption (λmax) to a shorter wavelength
 This effect is due to
 Removal of conjugation (auxochrome)
 Changing the polarity of solvent
 e.g. : Aniline shows blue shift in acidic medium
Aniline Acidic medium
λmax 280 nm 265 nm

NH2 +NH3
H+

20. 5) Hyperchromic shift :
 When absorption intensity of a compound is increased
 Introduction of auxochrome- increase the conjugation-
increase the intensity
 e.g. :
Pyridine 2-methyl pyridine
λmax 275 nm 260 nm
ε 2750 3560
N N CH3

21. 6) Hypochromic shift :
 When absorption intensity of a compound is decreased
 Introduction of group- distort the geometry of the
molecule
 e.g. : Napthalene 2-methyl napthalene
ε 19,000 10,250
Absorption
(A)

22.  Molecular structure / environment - influence max
and  :
 Shift to longer   bathochromic (red) shift
 Shift to shorter   hypsochromic (blue) shift
 Increase in  hyperchromic effect
 Decrease in   hypochromic effect

23. 1. Qualitative analysis: characterizing aromatic
compounds and conjugated olefins
2. Quantitative analysis: determination of different
analytes, such as transition metal ions, highly
conjugated organic compounds, and biological
macromolecules
3. Detection of impurities: in organic compounds
4. Detection of isomers- cis or trans isomer
5. Determination of mol. wt - Beer’s law
6. Determination of structure of several vitamins
7. To study kinetics of chemical reaction.

25.  Electrode potential difference developed due to the formation of
electrical double layer at the interface of metal and the charge in
the solution.
 Depending on the nature of the metal electrode to lose /gain e-s,
the galvanic cell has two electrode potentials:
1) Oxidation potential- electrode is –ve charged - act as anode
2) Reduction potential- electrode is +ve charged - act as cathode