The document discusses ultraviolet-visible spectroscopy. It describes how atoms have discrete energy levels and the electromagnetic spectrum is used to cause electronic, vibrational, and rotational transitions in molecules. UV-Vis spectroscopy can be used to qualitatively and quantitatively analyze compounds and determine properties like molecular structure. Key aspects covered include chromophores, auxochromes, Beer's law, and how molecular structure influences absorption maximum wavelength and absorptivity.

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