Introduction to UV spectroscopy, Instrumentation, electronic excitation and terms used. Absorption and intensity shifts, factors affecting position and intensity of UV bands, applications.

1. ULTRAVIOLET
SPECTROSCOPY
PALLAVI KUMBHAR
MSC PART-1

2. CONTENTS:
 INTRODUCTION
 INSTRUMENTATION
 ELECTRONIC EXCITATION
 TERMS USED IN UV SPECTROSCOPY
 ABSORPTION AND INTENSITY SHIFTS
 FACTORS AFFECTING POSITION AND INTENSITY OF UV BANDS;
 APPLICATIONS OF UV SPECTROSCOPY
 REFERENCES

3. INTRODUCTION:
― Ultraviolet (UV) and Visible Spectroscopy deals with the recording of the absorption of
radiations in the ultraviolet and visible regions of the electromagnetic spectrum.
― The UV radiation region extends from 10 nm to 400 nm and the visible radiation region
extends from 400 nm to 800 nm.
1. Near UV Region: 200 nm to 400 nm
2. Far UV Region: below 200 nm
― Far UV spectroscopy is studied under vacuum condition.
― UV spectroscopy is based on the principle of Beer Lamberts law.
A = ɛ c l
Where, A = Absorbance
C = concentration of the solution
ɛ = molar absorptivity coefficient
l = path length

4. INSTRUMENTATION:
SPECTROPHOTOMETER CONSISTS OF;
Schematic diagram of a UV-Vis spectrophotometer
1. Radiation Source
2. Monochromator
3. Sample And Reference Cells
4. Detectors
5. Amplifier
6. Recorder

5. ELECTRONIC EXCITATION:
The absorption of electromagnetic radiation of wavelength 200-750 nm can cause excitation of
electrons from occupied bonding molecular orbital to unoccupied antibonding molecular orbital.
This excitation is called electronic excitation.

6. σ - σ* Transition
π - π* transition
n - σ* transition
n - π* transition
σ - π* transition
π - σ* transition
• In alkanes
• Eg; C-H bond from Methane
• Compounds containing multiple bonds like alkenes, alkynes, carbonyl,
nitriles, aromatic compounds, etc
• Saturated compounds containing atoms with lone pair of electrons
like O, N, S and halogens are capable of n - σ* transition.
• Compounds containing double bond involving hetero atoms (C=O, C≡N, N=O)
undergo such transitions
• These electronic transitions are forbidden transitions & are only theoretically
possible.
• The UV spectrum is of only a few broad of absorption.
}
The relative energies are in the following order;
σ - σ * > n - σ * > π- π* > n - π*
TYPES OF ELECTRONIC TRANSITIONS:

7. TERMS USED IN UV SPECTROSCOPY
1. CHROMOPHORE:
― The part of a molecule responsible for imparting colour, are called as chromophores.
― The functional groups containing multiple bonds capable of absorbing radiations above 200 nm due to
n - π* & π - π* transitions.
― For example; NO2, N=O, C=O, C=N, C≡N, C=C, C=S, etc
2. AUXOCROME:
― The functional groups attached to a chromophore which modifies the ability of the chromophore to
absorb light , altering the wavelength or intensity of absorption.
― The functional group with non-bonding electrons that does not absorb radiation in near UV region but
when attached to a chromophore alters the wavelength & intensity of absorption.
― For example;.
1. Benzene (λmax = 255 nm) 2. Phenol (λmax = 270 nm) 3. Aniline (λmax = 280 nm)

8. ABSORPTION AND INTENSITY SHIFTS
1. Bathochromic Shift (Red shift)
2. Hypsochromic Shift (Blue Shift)
3. Hyperchromic Effect
4. Hypochromic Effect

9. 1) BATHOCHROMIC SHIFT (RED SHIFT):
 The shift of an absorption maximum to a longer wavelength
due to the presence of an auxochrome or solvent effect is
called a bathochromic shift or red shift.
λmax = 255nm λmax = 265nm
2) HYPSOCHROMIC SHIFT (BLUE SHIFT):
 The shift of an absorption maximum to a shorter wavelength is
called hypsochromic or blue shift.
3) HYPERCHROMIC EFFECT:
 An effect which leads to an increase in absorption intensity
Emax is called hyperchromic effect.
Pyridine 2-Methylpyridine
λmax = 257nm λmax = 260nm
4) HYPOCHROMIC EFFECT:
 An effect which leads to a decrease in absorption intensity
Emax is called hypochromic effect.
ɛ = 19000 ɛ = 10250
Napthalene 2-Methylnapthalene

10. FACTORS
AFFECTING
POSITION AND
INTENSITY OF
UV BANDS
EFFECT OF
CONJUGATION
EFFECT OF
SOLVENT
EFFECT
OF PH
STERIC
EFFECT

11. EFFECT OF CONJUGATION
• In the presence of conjugated double bonds, the electronic energy levels of a chromophore move closer
together.
• The energy required to produce a transition from an occupied electronic energy level to an unoccupied
level decreases, and the wavelength of the light absorbed becomes longer.
• Extension of conjugation leads to both bathochromic and hyperchromic shifts.
Comparison of π - π* energy gap in series
of polyenes of increasing chain length
EFFECT OF CONJUGATION IN ELECTRONIC TRANSITIONS

12. EFFECT OF SOLVENT:
1. Polar solvents such as water and alcohol forms hydrogen
bond with polar molecules.
2. Polar solvents have lone pair of electrons which forms
solute-solvent complexes through hydrogen bonding.
3. It diminishes the hyperfine structure of UV spectra.
4. Pure non-polar solvents does not react with solute molecules
as it does not form hydrogen bonds. For eg; Iso-octane.
Absorbance
Wavelength (nm)
UV spectra of Phenol in Ethanol
and Iso-octane
In π- π* transition In n - π* transition

13. Bathochromic and
Hyperchromic effect
Hypsochromic and
Hypochromic effect
Hypsochromic and
Hypochromic effect
EFFECT OF pH
Phenol Phenoxide anion
(Acidic/Neutral medium) (Basic medium)
Aniline Protonated
(Basic/Neutral medium) (Acidic medium)

14. STERIC EFFECT
Trans - Stilbene Cis – Stilbene
λmax = 295 nm λmax = 280 nm
Ɛmax = 27000 Ɛmax = 13500
Trans – Stilbene:
― Coplanar π system is achieved easily
Cis – Stilbene:
― Bulky phenyl groups on one side of the double
bond
― Planar structure is distorted
― Lower λmax and Ɛmax
― Hypsochromic and hypochromic effect

15.  Detection of Impurities
 Structure elucidation of organic compounds
 UV absorption spectroscopy can be used for the quantitative determination of compounds that absorb UV
radiation.
 UV absorption spectroscopy is used in qualitative determination of compounds.
 Kinetics of reaction can also be studied using UV spectroscopy.
 Many drugs are either in the form of raw material or in the form of formulation. They can be assayed by
making a suitable solution of the drug in a solvent and measuring the absorbance at specific wavelength.
 Molecular weights of compounds can be measured spectrophotometrically.
 UV spectrophotometer may be used as a detector for HPLC.
APPLICATIONS OF ULTRAVIOLET AND VISIBLE
SPECTROSCOPY

16. REFERENCES
 Pavia, Lamp, Kriz and Vyvyan; Introduction to spectroscopy
 Principles of Instrumental Analysis by Skoog, Holler and Crouch
 https://microbenotes.com/uv-spectroscopy-principle-instrumentation-applications