2. CONTENTS OF THIS TEMPLATE
● What is Spectroscopy?
● Principles of UV-Visible Spectroscopy
● Features of UV-Visible Spectroscopy
● Absorption Laws
● Instrumentation
● Electronic Transitions
● Absorption and Intensity Shifts
● Auxochrome and Chromophore concepts
● Woodward-fieser Rules
● Applications
4. Spectroscopy
● It is the branch of science that
deals with the study of interaction
of electromagnetic radiation with
matter.
● Electromagnetic radiation consist
of discrete packages of energy
which are called the photons.
● A photon consists of an oscillating
electric field and oscillating
magnetic field which are
perpendicular to each other.
Source: https://www.turito.com/blog/physics/electromagnetic-waves
5. Spectroscopy
● Spectrum is a graph of intensity of
absorbed or emitted radiation by
sample versus frequency (ν) or
wavelength (λ)
● A spectrometer is an instrument
used to measure the spectrum of
a compound.
Source: https://qr.ae/prR8qw
7. Principle
● It is the branch of science that
deals with the study of interaction
of electromagnetic radiation (in
the UV-Visible spectra region) with
matter.
● A record of the amount of light
absorbed by the sample as a
function of the wavelength of light
in mμ or nm units is called
absorption spectrum which
generally consists of absorption
bands.
Source: Elementary Organic Spectroscopy by Y R Sharma
9. Features
● The absorption spectrum of UV has
higher sensitivity and lower detection
limit. So it is commonly used in
quantitative analysis.
● Since ultraviolet region has shorter
electromagnetic radiation
wavelength, and energy is larger, it
can reflect the situation of the
transition of the mid valence
electron.
● The far ultraviolet region (below 200
nm) is not much studied due to
absorption by oxygen and nitrogen.
Must be carried out under vacuum.
Source: Elementary Organic Spectroscopy by Y R Sharma
11. Beer’s law
Beer’s law: When a beam of monochromatic radiation is passed through a solution of an
absorbing substance, the rate of decrease of intensity of radiation with thickness of the
absorbing solution is proportional to the intensity of incident radiation as well as the
concentration of the solution.
Mathematically, this law is stated as
-(dI/dx)= k’ Ic
Where -(dl/dx) = rate of decrease of intensity of radiation with thickness of the absorbing
medium,
dI = infinitesimally small decrease in the intensity of radiation on passing through infinitesimally
small thickness, dx of the medium,
c = concentration of the solution in moles litre,
k’= molar absorption coefficient and its value depends upon the nature of the absorbing
substance.
12. Lambert’s law
Lambert’s law: When a beam of monochromatic radiation passes through a homogeneous
absorbing medium, the rate of decrease of intensity of radiation with thickness of absorbing
medium is proportional to the intensity of the incident radiation.
Mathematically, the law is expressed as:
-(dI/dx) = kI
-(dl/dx) = rate of decrease of intensity of radiation with thickness of the absorbing medium,
dI = infinitesimally small decrease in the intensity of radiation on passing through infinitesimally
small thickness, dx of the medium,
I=intensity of radiation after passing through a thickness x, of the medium,
k=proportionality constant or absorption coefficient. It’s value depends upon the nature of the
absorbing medium.
13. Beer-Lambert’s law
● Beer-Lambert’s law: On combining the two laws, the Beer-Lambert Law can be
formulated as below :
log(I0/I)=𝛜.c.l = A
Where I0= Intensity of incident light.
I = Intensity of transmitted light.
c = Concentration of solution in moles litre–1
l = Path length of the sample (usually 1 cm).
𝛜 = Molar extinction coefficient (or molar absorptivity).
A = Absorbance.
● Limitation of Beer-Lambert’s law: This law is not obeyed:
a) When different forms of the absorbing molecules are in equilibrium as in keto-enol
tautomers.
b) When fluorescent compounds are present.
c) When solute and solvent form complexes through some sort of association.
17. Electronic Transitions
● Electronic transitions take place
when electrons in a molecule are
excited from one energy level to a
higher energy level, when they absorb
ultraviolet or visible light.
● According to the molecular orbital
theory, when a molecule is excited by
the absorption of energy (UV or
visible light), its electrons are
promoted from a bonding to an
antibonding orbital.
● Antibonding orbitals are the high
energy molecular orbitals. Source: Elementary Organic Spectroscopy by Y R Sharma
18. Electronic Transitions
1. 𝝈→𝝈* transitions: It is a high energy process since 𝝈
bonds are, in general, very strong. The organic
compounds in which all the valence shell electrons
are involved in the formation of sigma bonds do not
show absorption in the normal ultra-violet region. Eg;
methane, propane.etc
2. n→𝝈* transition: This type of transition takes place in
saturated compounds containing one hetero atom
with unshared pair of electrons (n electrons). Eg;
halides, alcohols, ethers, aldehydes, ketones etc.
3. 𝝅→𝝅* transitions: This type of transition occurs in the
unsaturated centres of the molecule i.e., in
compounds containing double or triple bonds and also
in aromatics. Eg; alkenes, alkynes, carbonyl
compounds. etc
Source: Elementary Organic Spectroscopy by Y R Sharma
19. Electronic Transitions
1. n→𝝅* transition; In this type of transition, an electron of unshared electron pair on hetero
atom gets excited to 𝝅* antibonding orbital. This type of transition requires least amount of
energy out of all the transitions and hence occurs at longer wavelengths. Eg; Saturated
aldehydes. Etc
Depending upon the symmetry and value of εmax, the transitions can be classed as :
(a) Allowed Transitions (b) Forbidden Transitions
● The transitions with values of εmax, more than 104 are usually called allowed transitions. They
generally arise due to 𝝅→𝝅* transitions.
● The forbidden transition is a result of the excitation of one electron from the lone pair
present on the heteroatom to an antibonding 𝝅* orbital. n→𝝅* transition near 300 mμ in
case of carbonyl compounds with εmax value between 10–100, is the result of forbidden
transition. The values of εmax for forbidden transition are generally below 104.
● The transition (allowed or forbidden) is related with the geometries of the lower and the
higher energy molecular orbitals and also on the symmetry of the molecule as a whole.
23. Chromophore
● It is defined as any isolated covalently bonded group that shows a characteristic
absorption in the ultraviolet or the visible region.
● Some of the important chromophores are ethylenic, acetylenic, carbonyls, acids, esters,
nitrile group etc.
● There are two types of chromophores-
1. Chromophores in which the group contains electrons and they undergo n⇀π*
transitions. Such chromophores are ethylenes, acetylenes etc
2. Chromophores which contain both π electrons and n (non-bonding) electrons. Such
chromophores undergo two types of transitions i.e., π⇀π* and n⇀π*. Examples of
this type are carbonyls, nitriles, azo compounds, nitro compounds etc
24. Chromophore
● Non-conjugated alkenes show an intense absorption below 200 nm & are therefore
inaccessible to UV spectrophotometer.
● Non-conjugated carbonyl group compound give a weak absorption band in the 200-300
nm region.
● Conjugation of C=C and carbonyl group shifts the λmax of both groups to longer
wavelength.
1,5 hexadiene
λmax = 178 nm
2,4 hexadiene
λmax = 227 nm
25. Auxochrome
● An auxochrome can be defined as any group-which does not itself act as a chromophore
but whose presence brings about a shift of the absorption band towards the red end of
the spectrum (longer wavelength).
● Some common auxochromic groups are —OH, —OR, —NH2, —NHR, —NR2, —SH etc.
● The effect of the auxochrome is due to its ability to extend the conjugation of a
chromophore by the sharing of non-bonding electrons. Thus, a new chromophore results
which has a different value of the absorption maximum as well as the extinction
coefficient.
30. Applications
● Quantitative Analysis
○ Molar concentration of the solute can be determined.
● Qualitative Analysis
○ Characterization of Aromatic compounds and Olefins.
● Detection of functional groups
○ Applied to detect the presence or absence of the chromophore.
○ If the spectrum is transparent above 200 mμ, it shows the absence of (i)
conjugation (ii) a carbonyl group (aldehydes and ketones) (iii) benzene or aromatic
compounds and also (iv) bromo or iodo atoms.
● Detection of impurities
● Determination of molecular weight using Beer’s law
31. Applications
● Determination of extent of conjugation
● Determination of preference over two Tautomeric forms
● Detection of Geometrical isomers
