Ultraviolet spectroscopy (organic)

Introduction to UV spectroscopy, Instrumentation, electronic excitation and terms used. Absorption and intensity shifts, factors affecting position and intensity of UV bands, applications.

1. ULTRAVIOLET SPECTROSCOPY PALLAVI KUMBHAR MSC PART-1

2. CONTENTS: INTRODUCTION INSTRUMENTATION ELECTRONIC EXCITATION TERMS USED IN UV SPECTROSCOPY ABSORPTION AND INTENSITY SHIFTS FACTORS AFFECTING POSITION AND INTENSITY OF UV BANDS; APPLICATIONS OF UV SPECTROSCOPY REFERENCES

3. INTRODUCTION: Ultraviolet (UV) and Visible Spectroscopy deals with the recording of the absorption of radiations in the ultraviolet and visible regions of the electromagnetic spectrum. The UV radiation region extends from 10 nm to 400 nm and the visible radiation region extends from 400 nm to 800 nm. Near UV Region: 200 nm to 400 nm Far UV Region: below 200 nm Far UV spectroscopy is studied under vacuum condition. UV spectroscopy is based on the principle of Beer Lamberts law. A = ɛ c l Where, A = Absorbance C = concentration of the solution ɛ = molar absorptivity coeffic ient l = path length

4. INSTRUMENTATION: SPECTROPHOTOMETER CONSISTS OF; Schematic diagram of a UV-Vis spectrophotometer

5. ELECTRONIC EXCITATION: The absorption of electromagnetic radiation of wavelength 200-750 nm can cause excitation of electrons from occupied bonding molecular orbital to unoccupied antibonding molecular orbital. This excitation is called electronic excitation .

6. In alkanes Eg ; C-H bond from Methane Compounds containing multiple bonds like alkenes, alkynes, carbonyl, nitriles, aromatic compounds, etc Saturated compounds containing atoms with lone pair of electrons like O, N, S and halogens are capable of n - σ* transition. Compounds containing double bond involving hetero atoms (C=O, C≡N, N=O) undergo such transitions These electronic transitions are forbidden transitions & are only theoretically possible. The UV spectrum is of only a few broad of absorption. } The relative energies are in the following order; σ - σ * > n - σ * > π- π* > n - π* TYPES OF ELECTRONIC TRANSITIONS:

7. TERMS USED IN UV SPECTROSCOPY CHROMOPHORE: The part of a molecule responsible for imparting colour, are called as chromophores. The functional groups containing multiple bonds capable of absorbing radiations above 200 nm due to n - π* & π - π* transitions. For example; NO 2 , N=O, C=O, C=N, C≡N, C=C, C=S, etc 2. AUXOCROME: The functional groups attached to a chromophore which modifies the ability of the chromophore to absorb light , altering the wavelength or intensity of absorption. The functional group with non-bonding electrons that does not absorb radiation in near UV region but when attached to a chromophore alters the wavelength & intensity of absorption. For example;. 1. Benzene ( λ max = 255 nm) 2. Phenol ( λ max = 270 nm) 3. Aniline ( λ max = 280 nm)

8. ABSORPTION AND INTENSITY SHIFTS

9. BATHOCHROMIC SHIFT (RED SHIFT): The shift of an absorption maximum to a longer wavelength due to the presence of an auxochrome or solvent effect is called a bathochromic shift or red shift. λ max = 255nm λ max = 265nm HYPSOCHROMIC SHIFT (BLUE SHIFT): The shift of an absorption maximum to a shorter wavelength is called hypsochromic or blue shift. HYPERCHROMIC EFFECT : An effect which leads to an increase in absorption intensity Emax is called hyperchromic effect. Pyridine 2-Methylpyridine λ max = 257nm λ max = 260nm 4) HYPOCHROMIC EFFECT: An effect which leads to a decrease in absorption intensity Emax is called hypochromic effect. ɛ = 1900 0 ɛ = 10250 Napthalene 2-Methylnapthalene

10. EFFECT OF CONJUGATION In the presence of conjugated double bonds, the electronic energy levels of a chromophore move closer together. T he energy required to produce a transition from an occupied electronic energy level to an unoccupied level decreases, and the wavelength of the light absorbed becomes longer. Extension of conjugation leads to both bathochromic and hyperchromic shifts. Comparison of π - π* energy gap in series of polyenes of increasing chain length EFFECT OF CONJUGATION IN ELECTRONIC TRANSITIONS

11. EFFECT OF SOLVENT: Polar solvents such as water and alcohol forms hydrogen bond with polar molecules. Polar solvents have lone pair of electrons which forms solute-solvent complexes through hydrogen bonding. It diminishes the hyperfine structure of UV spectra. Pure non-polar solvents does not react with solute molecules as it does not form hydrogen bonds. For eg ; Iso-octane. Absorbance Wavelength (nm) UV spectra of Phenol in Ethanol and Iso-octane In π- π* transition In n - π* transition

12. Bathochromic and Hyperchromic effect Hypsochromic and Hypochromic effect Hypsochromic and Hypochromic effect EFFECT OF pH Phenol Phenoxide anion (Acidic/Neutral medium) (Basic medium) Aniline Protonated (Basic/Neutral medium) (Acidic medium)

13. STERIC EFFECT Trans - Stilbene Cis – Stilbene λ max = 295 nm λ max = 280 nm Ɛmax = 27 00 Ɛmax = 1 3 500 T rans – Stilbene: Coplanar π system is achieved easily Cis – Stilbene: Bulky phenyl groups on one side of the double bond Planar structure is distorted Lower λ max and Ɛmax Hypsochromic and hypochromic effect

14. Detection of Impurities Structure elucidation of organic compounds UV absorption spectroscopy can be used for the quantitative determination of compounds that absorb UV radiation. UV absorption spectroscopy is used in qualitative determination of compounds. Kinetics of reaction can also be studied using UV spectroscopy. Many drugs are either in the form of raw material or in the form of formulation. They can be assayed by making a suitable solution of the drug in a solvent and measuring the absorbance at specific wavelength.  Molecular weights of compounds can be measured spectrophotometrically. UV spectrophotometer may be used as a detector for HPLC. APPLICATIONS OF ULTRAVIOLET AND VISIBLE SPECTROSCOPY

15. REFERENCES Pavia, Lamp, Kriz and Vyvyan; Introduction to spectroscopy Principles of Instrumental Analysis by Skoog, Holler and Crouch https://microbenotes.com/uv-spectroscopy-principle-instrumentation-applications