Electronic Excitation by UV/Vis Spectroscopy :

Transcription

1 Electronic Excitation by UV/Vis Spectroscopy : X-ray: core electron excitation UV: valance electronic excitation IR: molecular vibrations Radio waves: Nuclear spin states (in a magnetic field)

2 The wavelength and amount of light that a compound absorbs depends on its molecular structure and the concentration of the compound used. The concentration dependence follows Beer s Law. A=εbc Where A is absorbance (no units, since A = log10 P0 / P ) ε is the molar absorbtivity with units of L mol-1 cm-1 b is the path length of the sample - that is, the path length of the cuvette in which the sample is contained (typically in cm). c is the concentration of the compound in solution, expressed in mol L-1

3 Molecules have quantized energy levels: ex. electronic energy levels. hv energy energy } Ε = hv Q: Where do these quantized energy levels come from? A: The electronic configurations of associated with bonding. Each electronic energy level (configuration) has associated with it the many vibrational energy levels we examined with IR.

4 C C σ σ hv σ σ σ σ H H C C H H H λ max = 135 nm (a high energy transition) Absorptions having λ max < 200 nm are difficult to observe because everything (including quartz glass and air) absorbs in this spectral region.

5 C C σ π σ π π hv π Ε= hv =hc/λ σ σ π π Example: ethylene absorbs at longer wavelengths: λ max = 165 nm ε= 10,000

6 C σ π σ π n hv n π π σ n π The n to pi* transition is at even lower wavelengths but is not as strong as pi to pi* transitions. It is said to be forbidden. Example: Acetone: n σ λ max = 188 nm ; ε= 1860 n π λ = 279 nm ; ε= 15 σ

7 λ C C σ > σ 135 nm C C π > π 165 nm H C C n > σ 183 nm weak π > π 150 nm n > σ 188 nm n > π 279 nm weak 180 nm C A 279 nm

8 Conjugated systems: C C LUM HM Preferred transition is between Highest ccupied Molecular rbital (HM) and Lowest Unoccupied Molecular rbital (LUM). Note: Additional conjugation (double bonds) lowers the HM-LUM energy gap: Example: 1,3 butadiene: λ max = 217 nm ; ε= 21,000 1,3,5-hexatriene λ max = 258 nm ; ε= 35,000

9 Similar structures have similar UV spectra: λ max = 238, 305 nm λ max = 240, 311 nm λ max = 173, 192 nm

10 Woodward-Fieser Rules for Dienes Homoannular Heteroannular Parent λ=253 nm λ=214 nm =217 (acyclic) Increments for: Double bond extending conjugation +30 Alkyl substituent or ring residue +5 Exocyclic double bond +5 Polar groupings: -C()CH3 +0 -R +6 -Cl, -Br +5 -NR SR +30 C C C C Homoannular heteroannular acyclic exocyclic For more than 4 conjugated double bonds: λ max = (# of alkyl groups) + n( n)

11 Parent: 214 (heteroannular) 3 alkyls +15 (3x5) +5 (exocyclic) TTAL 234 nm (Actual = 235 nm) Parent: 253 (homoannular) 3 alkyls +15 (3x5) +5 (exocyclic) TTAL 273 nm (Actual = 275 nm) H Parent: 202 (5-member ring ) +35 (alpha hydroxyl) +12 (beta alkyl - note part of ring) Total: 249

12 Lycopene: λ max = (8) + 11*( *11) = 476 nm λ max (Actual) = 474.

13 π π* for ethylene and butadiene =>

14 Ultraviolet Spectroscopy nm photons excite electrons from a π bonding orbital to a π* antibonding orbital. Conjugated dienes have M s that are closer in energy. A compound that has a longer chain of conjugated double bonds absorbs light at a longer wavelength. =>

15 btaining a UV Spectrum The spectrometer measures the intensity of a reference beam through solvent only (I r ) and the intensity of a beam through a solution of the sample (I s ). Absorbance is the log of the ratio Graph is absorbance vs. wavelength. =>

16 The UV Spectrum Usually shows broad peaks. Read λ max from the graph. Absorbance, A, follows Beer s Law: A = εcl where ε is the molar absorptivity, c is the sample concentration in moles per liter, and l is the length of the light path in centimeters.

17 UV Spectrum of Isoprene =>

18

19 Woodward-Fieser Rules =>

20 Conjugated Systems (Ch. 15, pp ), Recommended Problems from the Text: 15-1 through through through

21 EFEITS DE SLVENTE Aumento da polaridade do solvente: deslocamento do máximo da banda de absorção para o vermelho (isto é para comprimemntos de onda maiores): energias menores. Modelo: π* é mais estabilizado do que π pelo aumento da polaridade do solvente. Conseqüência: p* DE p * p DE p

22 Aumento da polaridade do solvente: deslocamento do máximo da banda de absorção π, π* para o vermelho (isto é para comprimentos de onda maiores): energias menores. deslocamento do máximo da banda de absorção n, π* para o azul (isto é para comprimentos de onda menores): energias maiores. p* p π* é mais estabilizado do que π n é mais estabilizado do que π* transição π π* transição n π* deslocamento batocrômico DE DE p* p l aumenta polaridade do solvente menor p * p * n deslocamento hipsocrômico DE polaridade do solvente DE n l diminui maior Modelo: como

23 H

24 H Base value: 202 nm

25 H Base value: α-hydroxy group: 202 nm 35 nm

26 H Base value: α-hydroxy group: β-alkyl group: 202 nm 35 nm 12 nm

27 H Base value: α-hydroxy group: β-alkyl group: 202 nm 35 nm 12 nm 249 nm

28 H Base value: α-hydroxy group: β-alkyl group: 202 nm Base value: 35 nm 12 nm 249 nm 215 nm

29 H Base value: α-hydroxy group: β-alkyl group: 202 nm Base value: 35 nm α-alkyl group: 12 nm 249 nm 215 nm 10 nm

30 H Base value: α-hydroxy group: β-alkyl group: 202 nm Base value: 215 nm 35 nm α-alkyl group: 10 nm 12 nmβ-alkyl group: nm

31 H Base value: α-hydroxy group: β-alkyl group: 202 nm Base value: 215 nm 35 nm α-alkyl group: 10 nm 12 nmβ-alkyl group: nm 237 nm

32 Me Me

33 Me Me Base value: 215 nm

34 Me Me Base value: α-alkyl group: 215 nm 10 nm

35 Me Me Base value: 215 nm α-alkyl group: 10 nm Two β-alkyl groups 24 nm

36 Me Me Base value: 215 nm α-alkyl group: 10 nm Two β-alkyl groups 24 nm Two exocyclic double bonds 10 nm Me Me Me Me

37 Me Me Base value: 215 nm α-alkyl group: 10 nm Two β-alkyl groups 24 nm Two exocyclic double bonds 10 nm 259 nm

38 Me H Me Base value: 215 nm α-alkyl group: 10 nm Two β-alkyl groups 24 nm Two exocyclic double bonds 10 nm 259 nm

39 Me H Me Base value: 215 nm α-alkyl group: 10 nm Two β-alkyl groups 24 nm Two exocyclic double bonds 10 nm 259 nm Base value: 215 nm

40 Me H Me Base value: 215 nm α-alkyl group: 10 nm Two β-alkyl groups 24 nm Two exocyclic double bonds 10 nm 259 nm Base value: β-hydroxy group: 215 nm 30 nm

41 Me H Me Base value: 215 nm α-alkyl group: 10 nm Two β-alkyl groups 24 nm Two exocyclic double bonds 10 nm Base value: 215 nm β-hydroxy group: 30 nm β-alkyl group: nm

42 Me H Me Base value: 215 nm α-alkyl group: 10 nm Two β-alkyl groups 24 nm Two exocyclic double bonds 10 nm Base value: 215 nm β-hydroxy group: 30 nm β-alkyl group: nm 259 nm

43

44 Base value: 215 nm

45 Base value: Double bond Extending conjug. 215 nm 30 nm

46 Base value: Double bond Extending conjug. β-alkyl group: 215 nm 30 nm 12 nm

47 Base value: Double bond Extending conjug. β-alkyl group: γ-alkyl group: 215 nm 30 nm 12 nm 18 nm

48 Base value: Double bond Extending conjug. β-alkyl group: γ-alkyl group: >γ-alkyl group: 215 nm 30 nm 12 nm 18 nm 18 nm

49 Base value: Double bond Extending conjug. β-alkyl group: γ-alkyl group: >γ-alkyl group: Two exocyclic double bonds 215 nm 30 nm 12 nm 18 nm 18 nm 10 nm

50 Base value: Double bond Extending conjug. β-alkyl group: γ-alkyl group: >γ-alkyl group: Two exocyclic double bonds 215 nm 30 nm 12 nm 18 nm 18 nm 10 nm 303 nm

51 Base value: Double bond Extending conjug. β-alkyl group: γ-alkyl group: >γ-alkyl group: Two exocyclic double bonds 215 nm 30 nm 12 nm 18 nm 18 nm 10 nm 303 nm

52 Base value: Double bond Extending conjug. β-alkyl group: γ-alkyl group: >γ-alkyl group: Two exocyclic double bonds 215 nm 30 nm 12 nm 18 nm 18 nm 10 nm Base value: 215 nm 303 nm

53 Base value: Double bond Extending conjug. β-alkyl group: γ-alkyl group: >γ-alkyl group: Two exocyclic double bonds 215 nm 30 nm 12 nm 18 nm 18 nm 10 nm Base value: α-alkyl group: 215 nm 10 n 303 nm

54 Base value: Double bond Extending conjug. β-alkyl group: γ-alkyl group: >γ-alkyl group: Two exocyclic double bonds 215 nm 30 nm 12 nm 18 nm 18 nm 10 nm 303 nm Base value: 215 nm α-alkyl group: 10 n 2 β-alkyl groups: 24 nm

55 Base value: Double bond Extending conjug. β-alkyl group: γ-alkyl group: >γ-alkyl group: Two exocyclic double bonds 215 nm 30 nm 12 nm 18 nm 18 nm 10 nm 303 nm Base value: 215 nm α-alkyl group: 10 n 2 β-alkyl groups: 24 nm 249 nm

56

57 Base value: 202 nm

58 Base value: α-alkyl group 202 nm 10 nm

59 Base value: 202 nm α-alkyl group 10 nm Two β-alkyl groups 24 nm

60 Base value: 202 nm α-alkyl group 10 nm Two β-alkyl groups 24 nm exocyclic double bond 5 nm

61 Base value: 202 nm α-alkyl group 10 nm Two β-alkyl groups 24 nm exocyclic double bond 5 nm 241 nm

62 CH Br Ac

63 CH Br Ac

64 CH Br Ac Base value: 215 nm

65 CH Br Ac Base value: α-alkoxy (Me) 215 nm 35 nm

66 CH Br Ac Base value: α-alkoxy (Me) β-alkoxy (Me) 215 nm 35 nm 30 nm

67 CH Br Ac Base value: α-alkoxy (Me) β-alkoxy (Me) Double bond Extending conj 215 nm 35 nm 30 nm 30 nm

68 CH Br Ac Base value: 215 nm α-alkoxy (Me) 35 nm β-alkoxy (Me) 30 nm Double bond Extending conj 30 nm >γ alkyl group 18 nm

69 CH Br Ac Base value: 215 nm α-alkoxy (Me) 35 nm β-alkoxy (Me) 30 nm Double bond extending conj 30 nm >γ alkyl group 18 nm Homodiene 39 nm

70 CH Br Ac Base value: 215 nm α-alkoxy (Me) 35 nm β-alkoxy (Me) 30 nm Double bond extending conj 30 nm >γ alkyl group 18 nm Homodiene 39 nm 367 nm

71 Br CCH 3 triene Table 7.5 Me N H

72 Br CCH 3 triene Table 7.5 Me N H Base value (heteroannular diene): 214 nm

73 Br CCH 3 triene Table 7.5 Me N H Base value (heteroannular diene): Double bond extending conjugation 214 nm 30 nm

74 Br CCH 3 triene Table 7.5 Me N H Base value (heteroannular diene): Double bond extending conjugation alk group 6 nm 214 nm 30 nm

75 Br CCH 3 triene Table 7.5 Me N H Base value (heteroannular diene): Double bond extending conjugation alk group N(Alk)2 6 nm 60 nm 214 nm 30 nm

76 Br CCH 3 triene Table 7.5 Me N H Base value (heteroannular diene): Double bond extending conjugation alk group 6 nm N(Alk)2 60 nm 5 alkyl residues 25 nm 214 nm 30 nm

77 Br CCH 3 triene Table 7.5 Me N H Base value (heteroannular diene): 214 nm Double bond extending conjugation 30 nm alk group 6 nm N(Alk)2 60 nm 5 alkyl residues 25 nm 4 exocyclic double bonds 20 nm Br CCH 3 Br CCH 3 Br CCH 3 Br CCH 3 Me N H Me N H Me N H Me N H

78 Br CCH 3 triene Table 7.5 Me N H Base value (heteroannular diene): 214 nm Double bond extending conjugation 30 nm alk group 6 nm N(Alk)2 60 nm 5 alkyl residues 25 nm 4 exocyclic double bonds 20 nm 355 nm

79 Ac N

80 Ac N Base value: 202 nm

81 Ac N Base value: Extended conjugation 202 nm 30 nm

82 Ac N Base value: Extended conjugation β-nr nm 30 nm 95 nm

83 Ac N Base value: Extended conjugation β-nr 2 δ-ac 202 nm 30 nm 95 nm 6 nm

84 Ac N Base value: 202 nm Extended conjugation 30 nm β-nr 2 95 nm δ-ac 6 nm 2 γ and higher alkyls 36 nm

85 Ac N Ac N Base value: 202 nm Extended conjugation 30 nm β-nr 2 95 nm δ-ac 6 nm 2 γ and higher alkyls 36 nm Two exocyclic olefins 10 nm Ac N

86 Ac N Base value: 202 nm Extended conjugation 30 nm β-nr 2 95 nm δ-ac 6 nm 2 γ and higher alkyls 36 nm Two exocyclic olefins 10 nm 379 nm