By: Dipanshu sharmaM-PHARMACY (pharmaceutics)

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”

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 ultravoiletregion )

Alternate title for this technique is electronicspectroscopy since it involve promotion of electronsfrom ground state to the higher energy state

Lambert’s law

Beer’s law

Beer – lambert’s law

ABSORPTION LAWS:

“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 =KII = intensity of incident light of wavelength λt= thickness of medium

On integrating the eqaution ang putting I= 𝐼𝑜we Get ,

𝐼𝑜/ 𝐼𝑡= kt𝐼𝑡 = 𝐼𝑜𝑒

−𝑘𝑡

𝐼𝑜=intensity of incident light𝐼𝑡=intensity of transmitted lightk=constant which depends upon absorbing medium & λ

Converting equation to natural log𝐼𝑡 = 𝐼𝑜10

−0.4343𝑘𝑡 = 𝐼𝑜10−𝐾𝑡

LAMBERT’S LAW

“intensity of beam of monochromatic light decreases

exponentially with increase in concentration of absorbing substance airthmetically “

𝐼𝑡 = 𝐼𝑜𝑒−𝑘𝑐

Taking natural log

𝐼𝑡 = 𝐼𝑜10−0.4343𝑘𝑐 = 𝐼𝑜10

−𝐾𝑐

BEER’S LAW:

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:

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:

When the promotion of electron from bonding to

anti bonding orbital

the wavelength of radiation change fromminimum to maximum in the given range , andabsorbance is recorded . Then a plot of energy absorbedVs wavelength is called absorption spectrum.

Features of spectrum :

λmax (wavelength at which max absorption )

€max (intensity of max absorption )

ABSORPTION SPECTRUM:

*

*

n *

n *

*

*

DIFFERENT TYPES OF TRANSITIONS

Energy required for this transition is large i.e, occur at far UVregion (126 – 135nm )

Example : methane λmax 121.9nm , saturated hydrocarbons

*

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

*

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 *

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 *

These are forbidden transitions & are only

theoretically possible

Thus, these transition show absorption in region above 200nm which is not possible in UVspectrophotometer

*&

*

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

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

ABSORPTION AND

INTENSITY SHIFTS

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)

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

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

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

Biphenylλmax =250nm

2-methyl biphenylλmax =237nm

HYPOCHROMIC SHIFT

Due to the distortion caused by the methyl group in 2-methyl biphenyl