INTRODUCTION TO UV-VISIBLE SPECTROSCOPY

UV-visible spectroscopy is the classical and the most reliable technique for qualitative and quantitative analysis of organic compounds. It involves detection of light absorbed by the sample and correlates it with concentration of the solute.

1. UV-VISIBLE SPECTROMETRY AN INTRODUCTION BY: MOHAMMED JUNAID KHAN PhD Scholar IOP, Pt. RSU RAIPUR

2. INTRODUCTION The UV-VIS spectrometry is one of the oldest instrumental techniques of analysis and is the basis for a number of ideal methods for the determination of micro and semi-micro quantities of analytes in a sample . UV-VIS spectrum results from the interaction of electromagnetic radiation in the UV-VIS region with molecules, ions or complexes. It forms the basis of analysis of different substances such as, inorganic, organic and biochemicals . These determinations find applications in research, industry, clinical laboratories and in the chemical analysis of environmental samples. 9/17/2012 UV-VISIBLE SPECTROSCOPY 2

3. ORIGIN OF SPECTRUM The absorption of radiation in the UV-VIS region of the spectrum is dependent on the electronic structure of the absorbing species like, atoms, molecules, ions or complexes . A given electronic energy level has a number of vibrational energy levels in it and each of the vibrational energy level has a number of rotational energy levels in it. When a photon of a given wavelength interacts with the molecule it may cause a transition amongst the electronic energy levels if its energy matches with the difference in the energies of these levels. 9/17/2012 UV-VISIBLE SPECTROSCOPY 3

4. The absorption of radiation in the UV-VIS region of the spectrum causes the transitions amongst the electronic energy levels. ORIGIN OF SPECTRUM 9/17/2012 UV-VISIBLE SPECTROSCOPY 4

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7. The human eye sees the complementary color to that which is absorbed 9/17/2012 UV-VISIBLE SPECTROSCOPY 7

8. CHARECTERISTICS OF SPECTRUM In order to obtain a UV-VIS spectrum the sample is ideally irradiated with the electromagnetic radiation varied over a range of wavelength. A monochromatic radiation i.e., a radiation of a single wavelength is employed at a time. This process is called scanning. The amount of the radiation absorbed at each wavelength is measured and plotted against the wavelength to obtain the spectrum. Thus , a typical UV spectrum is a plot of wavelength or frequency versus the intensity of absorption. 9/17/2012 UV-VISIBLE SPECTROSCOPY 8

9. CHARECTERISTICS OF SPECTRUM The UV spectra of substances are characterised by two major parameters, namely , the position of the maximum of the absorption band called λmax , and the intensity of the bands. The λmax refers to the wavelength of the most absorbed radiation and is a measure of the difference in the electronic energy levels involved in the transition. The intensity on the other hand is indicative of the probability of the transition i.e., whether the transition is allowed or not . It is also is a measure of the concentration of the absorbing species. 9/17/2012 UV-VISIBLE SPECTROSCOPY 9

10. ABSORBING SPECIES The electronic transitions do not occur in all the species when a radiation in UV region is passed through the sample . Generally three types of species show the mentioned transitions, these are organic, inorganic and complexes formed by change transfer. 9/17/2012 UV-VISIBLE SPECTROSCOPY 10

11. ORGANIC ABSORPTION The wavelength of the radiation absorbed by an organic molecule is determined by the difference in energy between the ground state and the various excited electronic states of the molecule . In organic molecules that the constituent atoms are bonded through σ and π bonds. In addition, these have nonbonding electrons on the atoms like , N,O,S and halogens etc. There are a number of transitions possible involving the bonding and the nonbonding electrons. 9/17/2012 UV-VISIBLE SPECTROSCOPY 11

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13. ORGANIC ABSORPTION As a rule, the transitions occur from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) in a molecule . Of the six possible transitions indicated in the figure , only the two of the lowest energy ones (n → π ∗ and π → π ∗ ) can be achieved by the energies available in the 200 to 800 nm region. π → π ∗ transitions : These transitions are observed in molecules containing π electrons and occur at wavelengths approaching near ultraviolet regions . The hydrocarbons containing double and triple bonds are typical examples. For example, ethylene absorbs at 170 nm. The conjugation of unsaturation further increases λ max; but-1 , 3-diene absorbs at 217 nm. 9/17/2012 UV-VISIBLE SPECTROSCOPY 13

14. ORGANIC ABSORPTION The effect of conjugation is quite important in aromatic molecules. For example single ring aromatics absorb in the vicinity of 250 nm, Naphthalene in the vicinity of 300 nm and Anthracene in the vicinity of 360 nm. n →π ∗ transitions : These transitions are observed in molecules containing lone pairs or nonbonding electrons. In such transitions one of the nonbonding electrons may be excited into an empty π ∗ orbital . The energies required for these transitions are lower than that for π → π ∗ transitions and result in the absorption in the ultraviolet and visible region. The presence of atoms or groups containing n = electrons, can cause remarkable changes in the spectrum. Thus , nitrogen, sulphur and halogens tend to move absorption to higher wavelengths. 9/17/2012 UV-VISIBLE SPECTROSCOPY 14

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16. The π → π ∗ transitions are generally intense while the n → π ∗ transitions are weak. For example, acetone, exhibits a high intensity π → π ∗ transition at 195 nm and a low intensity n → π ∗ transition at 274 nm in its absorption spectrum. 9/17/2012 UV-VISIBLE SPECTROSCOPY 16 ORGANIC ABSORPTION

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18. CHROMOPHORES Only those molecules are likely to absorb light in the 200 to 800 nm region which contain π-electrons and may also have atoms with non-bonding electrons. Such groups that absorb light in the UV-VIS region are referred to as chromophores . Chromophore in Greek means, “ colour bearing”. 9/17/2012 UV-VISIBLE SPECTROSCOPY 18

19. Absorption characteristics of some common chromophoric groups 9/17/2012 UV-VISIBLE SPECTROSCOPY 19

20. 9/17/2012 UV-VISIBLE SPECTROSCOPY 20 T H A N K Y O U