ULTRAVIOLET SPECTROSCOPY or ELECTRONIC SPECTROSCOPY

Transcription

1 ULTRAVILET SPECTRSCPY or ELECTRNIC SPECTRSCPY S. SANKARARAMAN Department of Chemistry Indian Institute of Technology Madras Chennai , INDIA

2 Absorption of electromagnetic radiation in the region nm Transition from lower electronic level to higher electronic level Typically kj/mol energy difference between electronic energy levels

3 Absorption of electromagnetic radiation in the UV-Vis region I o is incident intensity of the radiation and I is transmitted intensity The ratio I/I o is known as transmittance (T) log (I o /I) is known as absorbance (A) UV-Vis absorption spectroscopy

4 Beer Lambert law a quantitative correlation Absorbance = log(i 0 /I) = εc l ε = extinction coefficient / molar absorptivity c= concentration of substance in moll -1 l = path length in cm

5 excited electronic level S ground electronic level S 0 Electronic transitions with vibronic transition superimposed.

6 Frank Condon Principle: The time scale of electronic excitation (10-15 s) is much faster than the time taken for nuclear motion (10-13 S). Therefore electronic transitions occur without much change in the interatomic distances. That is why electronic transitions are represented as vertical transitions on the energy level diagrams

7 Electronic energy levels and transitions in organic molecules

8 Selection rules for electronic transitions π to π* transition is symmetry allowed -High intensity with εmore than 1000 n to π* transition is symmetry forbidden low intensity with εless than 100 Transition among electronic states of same spin multiplicity are allowed If there is change of spin multiplicity then it is forbidden. Therefore singlet to singlet transition or triplet to triplet transition are allowed But singlet to triplet or triplet to singlet are forbidden

9 C C C C C C C C

10 C N n C N n C N

11 C Electronic transitions in a carbonyl group C n C C C

12 Chromophore Example Excitation λ max, nm λ max Solvent C=C Ethene π π* ,000 hexane C C 1-Hexyne π π* ,000 hexane C= Ethanal n π* π π* ,000 hexane hexane N= Nitromethane n π* π π* ,000 ethanol ethanol C-X X=Br X=I Methyl bromide Methyl Iodide n σ* n σ* hexane hexane

13 Chromophore: Absorption of UV radiation results from excitation of electron from the ground to excited state. The groups or structural units responsible for this electronic excitation is known as a chromophore. For example π-electrons of benzene is a chromophore. Aromatic ring (π π* transition) C=C (π π* transition) C= (π π* and n π* transitions) N=N (π π* and n π* transitions) are examples of chromophores.

14 Auxochrome: Substituents that increase the intensity of absorption usually accompanied by shift of wavelength are called auxochromes. For example halogen, methoxy, hydroxy, amino substituents on an aromatic ring are auxochromes. Substituents and solvents can have the following effects on the absorption (a) Bathochromic shift (red shift) - shift of absorption maximum to longer wavelengths (b) Hypsochromic shift (blue shift) shift of absorption maximum to shorter wavelengths (c) Hyperchromic effect increase in intensity (d) Hypochromic effect decrease in intensity

15 Solvent cutoffs Acetonitrile 190 nm N-hexane 201 nm Cyclohexane 195 nm Water 190 nm Methanol 205 nm 1,4-dioxane 215 nm

16 Solvent effect on π π and n-π* transitions of carbonyl compounds Me non-polar solvent polar protic solvent Me Me Solvent λ max ε λ max ε n 1 3 n n 4 Hexane EtH H < 4 < n - bathochromic shift - hypsochromic shift

17 π 6 Effect of conjugation on π π* transition Among ethylene, 1,3-butadiene and 1,3,5-hexatriene π 4 π 5 π antibonding π 4 π 3 bonding π 3 π 2 π π 2 π 1 π 1

18 Effect of conjugation on absorption maximum and intensity LUM levels (nm) HM levels (yellow) range red (violet) π π* transition

19 Beta carotene λ max = 452 nm, ε = 1.5 x 10 5 Vitamin A (retinol) λ max = 325 nm, ε = 5.1 x 10 3 Lycopene λ max = 474 nm, ε = 1.9 x 10 5

20 Comparison of enone with that of C=C And C= (effect of conjugation) Me Me Me 230 and 327 nm Me <220 nm Me

21 Effect of conjugation on absorption maximum and intensity (nm) Me 217 Me 270 Me 312 Me 343 Me Me Me π π* transition

22 Conjugated vs cross conjugated enones (yellow) (red) 242 nm (24200) 281 nm (400) 434 nm (20) n- * forbidden 390 nm (3020) 610 nm (20) n- * forbidden

23 homoannular diene (cisoid) heteroannular diene (transoid) max 253 nm max 214 nm homoannular diene (cisoid) heteroannular diene (transoid) max 273 nm max 234 nm Woodward and Fieser devised an empirical rule to correlate structural variation of dienes with their absorption maximum

24 Woodward-Fieser empirical rule for dienes Structural variations Homoannular (cisoid diene) Heteroannular (transoid diene) Parent value (base value) 253 nm 214 nm Increment for (value to be added to base value) Extended conjugation Alkyl substituents or ring residues 5 5 Exocyclic double bonds 5 5 Polar groups: CCH R 6 6 -Cl or -Br 5 5 -NR

25 CH 3 C Transoid diene = 214 nm Exocyclic (1) = 5 Ring residues (3 x 5) = 15 -R group (1) = 6 Expected λ max = 240 nm bserved λ max = 241 nm cisoid diene = 253 nm Extended double bond (2) = 60 Exocyclic (3) = 15 Ring residues (5 x 5) = 25 -CCH 3 group (1) = 0 Expected λ max = 353 nm bserved λ max = 355 nm Note: three exocyclic bonds

26 THANK YU