introduction principle laws electronic transitions absorption spectrum shifts

1. By:
Dipanshu sharma
M-PHARMACY
(pharmaceutics)

2. 
TYPES:
1) Absorption Spectroscopy : the study of
absorbed radiation by molecule , in form of spectra
Eg: UV , IR
2) Emission Spectroscopy: the radiation
emitted by molecule can also be studied to reveal
the structure of molecule
Eg : flourimetry , flame photometry
SPECTROSCOPY
“The study of interaction of electromagnetic radiation
with molecule /atom”

3. 
 Wavelength range of UV radiation starts at blue end
of visible light (about 4000Å) and ends at 2000Å
UV REGION
INTRODUCTION TO UV
SPECTROSCOPY
BETWEEN 2000Å -4000Å
(Near ultravoilet region )
BELOW 2000Å
( Far or Vaccum ultravoilet
region )
Alternate title for this technique is electronic
spectroscopy since it involve promotion of electrons
from ground state to the higher energy state

4. 

5. 
Lambert’s law
Beer’s law
Beer – lambert’s law
ABSORPTION
LAWS:

6. “When a beam of monochromatic light is passed through a homogeneous
absorbing medium , the rate of decrease of intensity of radiation with
thickness of absorbing medium is propotinal to the intensity of the
incident light (radiation) “
dI/dt =KI
I = intensity of incident light of wavelength λ
t= thickness of medium
On integrating the eqaution ang putting I= 𝐼 𝑜
we Get ,
𝐼 𝑜/ 𝐼𝑡= kt
𝐼𝑡 = 𝐼 𝑜 𝑒−𝑘𝑡
𝐼 𝑜=intensity of incident light
𝐼𝑡=intensity of transmitted light
k=constant which depends upon absorbing medium & λ
Converting equation to natural log
𝐼𝑡 = 𝐼 𝑜10−0.4343𝑘𝑡
= 𝐼 𝑜10−𝐾𝑡
LAMBERT’S LAW

7. 
 “intensity of beam of monochromatic light decreases
exponentially with increase in concentration of
absorbing substance airthmetically “
𝐼𝑡 = 𝐼 𝑜 𝑒−𝑘𝑐
Taking natural log
𝐼𝑡 = 𝐼 𝑜10−0.4343𝑘𝑐 = 𝐼 𝑜10−𝐾𝑐
BEER’S LAW:

8. 
 On combining the two laws , the beer –Lambert law
can be formulated as :
 logI₀/I = €.c.t = A = log1/T = -log T
 € = molar exctintion coefficient
 C = concentration of solution
 A = absorbance
 T = transmittance
 i.e, their exist a relation between absorbance and
transmittance
BEER – LAMBERT’S
LAW:

9. 
UV spectroscopy measures the response of a
sample to a ultraviolet range of EMW
Molecules have either n , or σ electrons .
These electrons absorb UV radiation &
undergoes transitions from ground state to
excited state
PRINCIPLE:

10.  When the promotion of electron from bonding to
anti bonding orbital
the wavelength of radiation change from
minimum to maximum in the given range , and
absorbance is recorded . Then a plot of energy absorbed
Vs wavelength is called absorption spectrum.
Features of spectrum :
 λmax (wavelength at which max absorption )
 €max (intensity of max absorption )
ABSORPTION SPECTRUM:

11. 

12. 
 *
 *
 n *
 n *
 *
 *
DIFFERENT TYPES OF
TRANSITIONS

13. 
 Energy required for this transition is large i.e, occur
at far UVregion (126 – 135nm )
 Example : methane λmax 121.9nm , saturated
hydrocarbons
*

14. 
 This transition can in principle occur in molecule
having electron system (unsaturated
hydrocarbons)
 Require smaller energy i.e, occur at longer
wavelength
 Example: alkenes , alkynes, carbonyl compound ,
cyanides , azo compounds
 Alkenes absorb in the region 170 to 205 nm
*

15. 
 Saturated compounds containing atoms with lone
pair of electrons like O , N , S and halogens are
capable of these transitions
 These transition usually require less energy than
*
 Example : water absorb at 167nm
n *

16. 
 Compounds containing double or triple bond
involving hetero atoms ( C=O, C≡N, N=O)
undergoes such transitions
 These transitions require minimum energy and show
absorption at longer wavelength around 300nm
n *

17. 
 These are forbidden transitions & are only
theoretically possible
 Thus, these transition show absorption in region
above 200nm which is not possible in
UVspectrophotometer
*
&
*

18. 
 UV radiation may be absorbed by organic
compounds that contain N , O , S, halogen atom or
unsaturated hydrocarbons ( such as olefins ) .
Functional group that contain these groups and
absorb radiation in UV region are called
chromophores
CHROMPHORES

19. 
 The functional group attached to a chromophore
which modifies the ability of the chromphore to
absorb light , altering the wavelength or intensity of
absorption
 Example : Benzene λmax = 255nm
Phenol λmax = 270nm (-OH)
Aniline λmax = 280nm (-N𝐻2)
AUXOCHROME

20. 

21. 
ABSORPTION AND
INTENSITY SHIFTS

22. 

23. 
 When absorption maxima shift toward longer
wavelength due to presence of an auxocrome or by
the change of solvent
 Example : n * transition for carbonyl
compound experience shift when polarity of solvent
decreases
groups like (–OH -OCH3) causes absorption at longer
wavelength
BATHOCHROMIC
SHIFT(RED SHIFT)

24. 
 When absorption maxima shifts toward shorter
wavelength
 Occurs due to removal of conjugation and also by
change in polarity of solvent
 Example :Aniline show shift in acidic medium , loses
conjugation
HYPSOCHROMIC
SHIFT (BLUE SHIFT)
Aniline
λmax =280nm
In acidic
medium
λmax =265nm
Hypsochromic shift

25. 
 In this absorption intensity of absorption maxima
increases i.e, €max increases
 Introduction of auxochrome usually increases
intensity of absorption
 Example:
HYPERCHROMIC
SHIFT
Pyridine
λmax =257nm
2- methyl
pyridine
λmax =260nm
HYPERCHROMIC SHIFT

26. 
 In this absorption intensity of absorption maxima
decreases i.e, €max decreases
 Introduction of group which distort the geometry of
the molecule causes hypochromic shift
 Example:
HYPOCHROMIC
SHIFT
Biphenyl
λmax =250nm
2-methyl biphenyl
λmax =237nm
HYPOCHROMIC SHIFT
Due to the distortion caused by the methyl group in 2-
methyl biphenyl

27. 