Understanding Electronic Transitions in Organic Molecules Using UV-Vis Spectroscopy

1. p. 235

2. p. 235 Electronic transitions IR transitions

3. In organic molecules, electrons are in s, p or n orbitals so transitions are n→p*p→p*n→s* p. 236 usually, in conjugated molecules n→p* < p→p* < n→s* 3

4. p. 236 n →p* in carbonyl, nitro,... X: + C=C or C=O or N=O p→p* in alkenes, aromatics, carbonyl compounds n →s* in O: (ethers), N: (amines), S: (thiols/thioethers), Hal: (halo-organics) } uv s→p* carbonyl, .... } high energy, far uv s→s* alkanes

5. p. 237 A UV spectrum of Benzoic acid e 10,000 p→p* 1000 100 because many vibrational transitions, absorptions broad

6. p. 237 Beer’s Law: A = log(I0/I) = ecl Ais linearly related to c so a calibration plot of A vs. c can be used to determine unknown c from measured A • A = absorbance • I0 = incident light intensity • I = transmitted light intensity • = molar extinction coefficient (or molar absorptivity) c = concentration in mol L-1 l = pathlength in cm • is in units of L mol-1 cm-1 constant at a given wavelength for any compound i.e. it is a molecular property

7. p. 237 p→p* are allowed (orbitals in same plane) e > 100, commonly 103 to 105 n →p* are forbidden (orbitals orthogonal) e < 50 l = 230 nm Benzoic acid, p→p*e = 10,000 acetone, n →p*e = 18 l = 274 nm

8. p. 238 Simple chromophores absorb in the far uv p→p* C=C175nme = 1000 C=O180nme = 1000 n →s* C-O: 180nme = 1000 except n →p* C=O280 nme = 10-30 } forbidden (weak intensity) N=O 270 nme = 5-10

9. p. 238 BUT conjugation of a chromophore produces a RED (long l) or BATHOCHROMIC shift andaHYPERCHROMIC shift

10. p. 238 conjugation:lMAX increases ~ 35 nm per C=C

11. p. 239 C=C C=C—C=C C=C

12. p. 239 C=C C=C—C=C C=C—C=C—C=C p1* Higher energy transitions p1* p2* p* p2* p3* LUMO 0 p3 HOMO 0.45 p2 0.62 p 1.00 p2 1.25 p1 1.62 p1 1.80 Longest l transition is HOMO→LUMO transition

13. p. 239 Longest l transition is HOMO→LUMO transition C=C 175 nm 15000 C=C—C=C 217nm 21000 C=C—C=C—C=C 258nm 35000 b-carotene (C=C)11 465nm 125,000 }UV }VIS

14. p. 240 ALL transitions are bathochromically shifted on conjugation p→p* n →p* 190nm 1000 280nm 18 213nm 7000 320nm 27 245nm 9800 lost (in under benzene transitions)

15. p. 240 BENZENE p1* p2* p2 p1 184nm 68,000 204nm 8,800 254nm 250

16. p. 240 Substituents change the HOMO-LUMO gap, and therefore change lMAX Ph-H 204 254 Ph-OH 210 270 Ph-O- 235 287 Ph-NH2 230 280 Ph-NH3+ 203 254 .. } pH dependant .. .. } pH dependant } Auxochromes: have conjugated lone pairs – smaller effect

17. p. 241 V I B G Y O R Colors of light: 400nm Violet 500nm Green 600nm Orange 670nm Red Na D line so a green laser is emitting light ~ 500 nm and a red laser at about 650 nm

18. p. 241 However, if you shine WHITE light on an object and it appears green, it is because it is reflecting green and absorbing other wavelengths approximately l of light objects are absorbing 400nm objects are absorbing 450nm objects are absorbing 500nm objects are absorbing 550nm objects are absorbing 600nm objects are absorbing 670nm

19. p. 241 Carrots absorb at 452 nm so appear orange Tomatoes absorb at 474nm so see red Chlorophyll absorbs at 670nm so see green Plums absorb 550nm see violet Intense colours, LARGE e >1000 Pale colours, SMALL e >50

20. p. 242 INORGANIC COMPOUNDS Main group compounds usually absorb in far UV so appear WHITE NaCl CaCO3 MgSO4 Al2O3 salt chalk Epsom salts Alumina Transition Metal Compounds CuSO4KMnO4K2Cr2O7HgI2Cr2O3 Intense colors involve CHARGE-TRANSFER bands Pale colors involve d → d transitions

21. p. 243 KMnO4K2Cr2O7HgI2Cr2O3Benzene-Cr(CO)3 CHARGE-TRANSFER COMPLEXES e > 1000 L → M ligand to metal: : electron from ligand orbital transferred to empty metal orbital {formally L+-M-} CT* CT transition ALLOWED Ligand orbital [Donor] Metal orbital [acceptor] New CT bonding orbital

22. p. 244 Breathalyser Test CH3CH2OH + K2Cr2O7→ CH3CHO + 2Cr3+ e ~15 e > 1000 Measure change in A (absorbance) of dichromate solution not affected by weak Cr3+ absorption Then change is proportional to conc of alcohol in breath Some roadside screening devices have a column of dichromate on silica and measure how much of column changes

23. p. 244 Summary UV-Vis Flowchart

24. ASSIGNMENT 10