Rezaul Karim

Environmental Science and Technology

Jessore Science and Technology University

Instrumental Technique for Environmental Analysis

Chapter 3 Ultra Violet Spectroscopy

Chapter content and Reference

The UV/Vis spectral region

The origin of the absorptions

Electronic transitions of organic compounds

Chromophore groups

Solvent effects: solvatochromism

Fieser Woodward rules

Instrumentation in the UV/Visible

UV/Vis spectrophotometers

Quantitative analysis: laws of molecular absorption

UV Spectra and the Structure of Organic Molecules e.g. Conjugated Diene Systems; Conjugated Ketone Systems; Substitution of Benzene Rings

Analytical Applications: qualitative and quatitative

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Reference

1. Rouessac and Annick, 2007, Chemical

Analysis-Modern Instrumentation

Methods and Techniques, 2nd Edition,

John Wiley & Sons Ltd, The Atrium,

Southern Gate, England

2. Robinson, 1995. Undergraduate

instrumental analysis, Marcel Dekker,

Inc. NY, USA.

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The UV/Vis spectral region

This region of the spectrum is

conventionally divided into three sub-

domains termed

near UV (185400 nm),

visible (400700 nm) and

very near infrared (700 1100 nm).

Most commercial spectrophotometers

cover the spectral range of 185 to 900 nm.

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The lower limit of the instrument depends upon the nature of the optical components used and of the presence, or not, along the optical pathway of air.

Some instruments, on condition that they are operating in a vacuum, can attain 150 nm with samples in the gaseous state. This is the domain of vacuum or far ultraviolet.

The long-wavelength limit is usually determined by the wavelength response of the detector in the spectrometer.

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The origin of the absorptions

The origin of absorption in this domain is the interaction of photons with ions or molecules of the sample.

When a molecule absorbs a photon from the UV/Vis region, the corresponding energy is captured by one (or several) of its outermost electrons.

As a consequence there occurs a modification of its electronic energy Eelec, a component of the total mechanical energy of the molecule along with its energy of rotation Erot and its energy of vibration Evib.

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A modification of Eelec will result in

alterations for both Erot and Evib resulting

in a vast collection of possible transitions

obtained in all three cases,.

since the polarities of the bonds within

the molecules will be disturbed their

spectra are given the generic name of

charge transfer spectra.

Etot =Erot + Evib +Eelec with Eelec >Evib>Erot

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The UV/Vis spectrum

The interaction of UV and visible radiation with matter can provide qualitative identification of molecules and polyatomic species, including ions and complexes.

Structural information about molecules and polyatomic species, e.g. organic molecules.

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A typical UV absorption spectrum

The UV/Vis spectrum

This qualitative information is usually obtained by observing the UV/VIS spectrum, the absorption of UV and visible radiation as a function of wavelength by molecules.

The spectrum may be plotted as wavelength vs. Absorbance/ transmittance, / molar absorptivity.

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A typical UV absorption spectrum

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Three different aspects of UV/Vis spectra.

Spectra of benzene

(a) in solution (a band spectrum);

(b) in the vapour state (a fine structure);

(c) an expansion of a section from the high resolution

(0.14nm total interval) line spectrum of iodine vapour.

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The spectrum of benzene vapour, obtained from a

drop of this compound, deposited in a silica glass

cuvette of 1 cm optical path, provides an excellent

test to evaluate the resolution of an UV-

spectrophotometer.

The recorded spectra of compounds in the

condensed phase, whether pure or in solution,

generally present absorption bands that are both

few and broad, while those spectra obtained from

samples in the gas state and maintained under a

weak pressure yield spectra of detailed

structure

Electronic transitions of

organic compounds Molecules are composed of atoms that are held together by sharing electrons to form chemical bonds.

Electrons in molecules move in molecular orbitals at discrete energy levels as defined by quantum theory.

When the energy of the electrons is at a minimum , the molecules are in the lowest energy state, or ground state.

The molecules can absorb radiation and move to a higher energy state, or excited state.

The process of moving electrons to higher energy states is called electronic excitation.

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Three distinct types of electrons

Sigma (1) bonds : single bonds, such as those between carbon and hydrogen in alkanes.

The amount of energy required to excite electrons in s bonds is usually more than UV photons of wavelengths 200 nm possess.

For this reason, alkanes and other saturated compounds (compounds with only single bonds) do not absorb UV radiation and are therefore frequently very useful as transparent solvents for the study of other molecules.

An example of such a nonabsorbing compound is hexane, C2H14 .

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