Fundamentals of UV-Visible Spectroscopy Explained

1. UV-VISIBLE SPECTROSCOPYDr. R. P. ChavanHead, Department of Chemistry

2. CONTENTs • Introduction to spectroscopy • Types of spectrscopy • Electro magnetic waves • Principle of UV Light Absorption • Factors affecting the position & intensity of UV • Application of UV spectroscopy • Reference

3. Spectroscopy Using electromagnetic radiation as a probe to obtain information about atoms and molecules that are too small to see. Electromagnetic radiation is propagated at the speed of light through a vacuum as an oscillating wave.

4. Spectroscopy Types: –Ultraviolet Spectroscopy (UV) – Based on Electronic excitation of molecule •Use–Conjugated Molecules; Carbonyl Group, Nitro Group –Infrared Spectroscopy (IR) – Involves Vibrational Energy States •Use– Functional Groups; Structure of compound –Nuclear Magnetic Resonance (NMR) – InvolvesNuclearSpin States •Use– The number, type, and relative position of protons(Hydrogen nuclei) and Carbon-13 nuclei –Mass Spectrometry (MS) – High-Energy Electron Bombardment •Use– Molecular Weight, Presence of Nitrogen, Halogens

5. Electromagnetic Waves • Waves of energy emitted from any accelerating charges • Any object that is above absolute zero emits electromagnetic waves • The entire range of possibilities is called the “Electromagnetic Spectrum”

6. Principle of UV Light Absorption Ultraviolet light: wavelengths between 190 and 400 nm Visible light: wavelengths between 400 and 800 nm Ultraviolet/visible spectroscopy involves the absorption of ultraviolet light by a molecule causing the promotion of an electron from a ground electronic state to an excited electronic state.

7. Beer-Lambert’s law In Beer-Lambert’s law the fraction of incident radiation absorbed is proportional to the number of absorbing molecules in its path. log I₀/I = εcl Where ; I₀=Intensity of incident light C=concentration of solute molecule l=path length of the sample ε=molar extension coefficient of substance whose light is investigated log I₀/I = Absorbance log I/I₀ =Transmittance

8. Types of transition σ* Π* n π σ σ-σ* π-π* n- σ * n-π* >170 Kcal <170Kcal <150Kcal <105 Kcal <165nm >165nm >185nm >270nm

9. An electron in bonding σ orbital is excited to the corresponding antibonding orbitals. The energy required is large For example : methane σ to σ∗

10. n to σ∗ • Saturated compounds containing atoms with lone pairs are capable of n to σ∗ transitions • These transition usually need less energy than σ to σ∗ transitions. • The number of organic functional group with n to σ∗ peaks in uv region is small.

11. n to π∗ & πto π∗ • The most absorption spectroscopy of organic compound is based on transitions of n or π electrons to the π∗ excited state. • These transitions need an unsaturated group in the molecule to provide the π electrons. • πto π∗ transition occurs with compound containing double bond or triple bond whereas n to π∗ transition occurs with compound containing double bond including hetero atom

12. Chromophores Molecules having unsaturated bonds or free nonbonding electrons that can absorb radiation of relatively low energy and which imparts colour to an organic compound are called chromophores. Examples include alkenes, alkynes, ketones, aldehydes, phenyl and other aromatic species, etc.

13. Auxochrome Auxochrome is a functional group that does not absorb in UV region but has the effect of shifting chromophore peaks to longer wavelength as well As increasing their intensity.

15. Factors affecting the position & intensity of UV pH effect In certain compounds generally acids and bases pH changes affects primary and secondary bands. As pH decreases wavelength also decreases.

16. Solvents: • Polar solvents such as water, alcohols, esters, and ketones tend to obliterate spectral fine structure arising from vibrational effects; spectra that approach those of the gas phase are more likely to be observed in nonpolar solvents such as hydrocarbons. • Generally polar solvents shifts n to π∗ transition towards a shorter wavelength.

17. Effect of conjugation • An increase in extent of conjugation in doubly bonded system brings about a bathochromic shift. • eg; Benzene-254 nm Anthracene-350nm

18. APPLICATION OF UV-VISIBLE SPECTROSCOPY It is used in determination of molecular weight of molecules. It is used in determination of impurities present in the sample. The unknown concentration of the solution can be determine using this spectroscopy. It is used in characterisation of aromatic compound and detection of conjugation. Useful in finding out dissociation constants of acids & bases.