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X-ray: core electron excitation UV: valance electronic excitation IR: molecular vibrations Radio waves: Nuclear spin states (in a magnetic field) Electronic Excitation by UV/Vis Spectroscopy : 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=εεεεbc Where A is absorbance (no units, since A = log10 P0 / P ) εεεε 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 Molecules have quantized energy levels: ex. electronic energy levels. e n e r g y hv e n e r g y } ∆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. C C σ σ∗ hv σ σ∗ σ σ∗ C C H HH H HH λmax = 135 nm (a high energy transition) Absorptions having λmax < 200 nm are difficult to observe because everything (including quartz glass and air) absorbs in this spectral region. C C σ σ∗ hv pi pi∗ σ σ∗ pi pi∗ pi pi∗ Example: ethylene absorbs at longer wavelengths: λmax = 165 nm ε= 10,000 ∆Ε= hv =hc/λ σ σ∗ hv pi pi∗ n σ σ∗ pi pi∗ n C O pi∗n 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∗ λmax = 188 nm ; ε= 1860 n−pi∗ λmax = 279 nm ; ε= 15 C C C C C O C O H σ −> σ∗ 135 nm pi −> pi∗ 165 nm n −> σ∗ 183 nm weak pi −> pi∗ 150 nm n −> σ∗ 188 nm n −> pi∗ 279 nm weak λ A 180 nm 279 nm C O C C HOMO LUMO Conjugated systems: Preferred transition is between Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO). Note: Additional conjugation (double bonds) lowers the HOMO-LUMO energy gap: Example: 1,3 butadiene: λmax = 217 nm ; ε= 21,000 1,3,5-hexatriene λmax = 258 nm ; ε= 35,000 O O O Similar structures have similar UV spectra: λmax = 238, 305 nm λmax = 240, 311 nm λmax = 173, 192 nm 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: -OC(O)CH3 +0 -OR +6 -Cl, -Br +5 -NR2 +60 -SR +30 C C C C Homoannular heteroannular acyclic exocyclic For more than 4 conjugated double bonds: λmax = 114 + 5(# of alkyl groups) + n(48.0-1.7n) O Parent: 214 (heteroannular) 3 alkyls +15 (3x5) +5 (exocyclic) TOTAL 234 nm (Actual = 235 nm) Parent: 253 (homoannular) 3 alkyls +15 (3x5) +5 (exocyclic) TOTAL 273 nm (Actual = 275 nm) O HO Parent: 202 (5-member ring ketone) +35 (alpha hydroxyl) +12 (beta alkyl - note part of ring) Total: 249 Lycopene: λmax = 114 + 5(8) + 11*(48.0-1.7*11) = 476 nm λmax(Actual) = 474. => pi → pi* for ethylene and butadiene Ultraviolet Spectroscopy • 200-400 nm photons excite electrons from a pi bonding orbital to a pi* antibonding orbital. • Conjugated dienes have MO’s that are closer in energy. • A compound that has a longer chain of conjugated double bonds absorbs light at a longer wavelength. => Obtaining a UV Spectrum • The spectrometer measures the intensity of a reference beam through solvent only (Ir) and the intensity of a beam through a solution of the sample (Is). • Absorbance is the log of the ratio • Graph is absorbance vs. wavelength. => The UV Spectrum • Usually shows broad peaks. • Read λmax 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. UV Spectrum of Isoprene => Woodward-Fieser Rules => Conjugated Systems (Ch. 15, pp 638-678), Recommended Problems from the Text: 15-1 through 15-18 15-21 15-23 through 15-27 15-29 through 15-33 15-37 EFEITOS DE SOLVENTE 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: pi* é mais estabilizado do que pi pelo aumento da polaridade do solvente. Conseqüência: DE DE p p p * p* Aumento da polaridade do solvente: deslocamento do máximo da banda de absorção pi, pi* para o vermelho (isto é para comprimentos de onda maiores): energias menores. deslocamento do máximo da banda de absorção n, pi* para o azul (isto é para comprimentos de onda menores): energias maiores. Modelo: como pi* é mais estabilizado do que pi n é mais estabilizado do que pi* transição pi → pi* transição n → pi* DE DE p p p * p* menor DE DE p* p* maior n n l aumenta l diminui deslocamento batocrômico deslocamento hipsocrômico polaridade do solvente polaridade do solvente O OH UV-Vis problems α,β-unsaturated ketone Table 7.12 Base value: 202 nm O OH UV-Vis problems α,β-unsaturated ketone Table 7.12 Base value: 202 nm α-hydroxy group: 35 nm O OH UV-Vis problems α,β-unsaturated ketone Table 7.12 Base value: 202 nm α-hydroxy group: 35 nm β-alkyl group: 12 nm O OH UV-Vis problems α,β-unsaturated ketone Table 7.12 Base value: 202 nm α-hydroxy group: 35 nm β-alkyl group: 12 nm O OH UV-Vis problems α,β-unsaturated ketone Table 7.12 249 nm O Base value: 202 nm α-hydroxy group: 35 nm β-alkyl group: 12 nm O OH UV-Vis problems α,β-unsaturated ketone Table 7.12 249 nm Base value: 215 nm O Base value: 202 nm α-hydroxy group: 35 nm β-alkyl group: 12 nm O OH UV-Vis problems α,β-unsaturated ketone Table 7.12 249 nm Base value: 215 nm α-alkyl group: 10 nm O Base value: 202 nm α-hydroxy group: 35 nm β-alkyl group: 12 nm O OH UV-Vis problems α,β-unsaturated ketone Table 7.12 249 nm Base value: 215 nm α-alkyl group: 10 nm β-alkyl group: 12 O Base value: 202 nm α-hydroxy group: 35 nm β-alkyl group: 12 nm O OH UV-Vis problems α,β-unsaturated ketone Table 7.12 249 nm Base value: 215 nm α-alkyl group: 10 nm β-alkyl group: 12 237 nm UV-Vis problems Me O Me α,β-unsaturated ketone Table 7.12 UV-Vis problems Me O Me α,β-unsaturated ketone Table 7.12 Base value: 215 nm UV-Vis problems Me O Me α,β-unsaturated ketone Table 7.12 Base value: 215 nm α-alkyl group: 10 nm UV-Vis problems Me O Me α,β-unsaturated ketone Table 7.12 Base value: 215 nm α-alkyl group: 10 nm Two β-alkyl groups 24 nm UV-Vis problems Me O Me α,β-unsaturated ketone Table 7.12 Base value: 215 nm α-alkyl group: 10 nm Two β-alkyl groups 24 nm Two exocyclic double bonds 10 nm Me O Me Me O Me UV-Vis problems Me O Me α,β-unsaturated ketone Table 7.12 Base value: 215 nm α-alkyl group: 10 nm Two β-alkyl groups 24 nm Two exocyclic double bonds 10 nm 259 nm UV-Vis problems Me O Me α,β-unsaturated ketone Table 7.12 Base value: 215 nm α-alkyl group: 10 nm Two β-alkyl groups 24 nm Two exocyclic double bonds 10 nm 259 nm OH O UV-Vis problems Me O Me α,β-unsaturated ketone Table 7.12 Base value: 215 nm α-alkyl group: 10 nm Two β-alkyl groups 24 nm Two exocyclic double bonds 10 nm 259 nm OH O Base value: 215 nm UV-Vis problems Me O Me α,β-unsaturated ketone Table 7.12 Base value: 215 nm α-alkyl group: 10 nm Two β-alkyl groups 24 nm Two exocyclic double bonds 10 nm 259 nm OH O Base value: 215 nm β-hydroxy group: 30 nm UV-Vis problems Me O Me α,β-unsaturated ketone Table 7.12 Base value: 215 nm α-alkyl group: 10 nm Two β-alkyl groups 24 nm Two exocyclic double bonds 10 nm 259 nm OH O Base value: 215 nm β-hydroxygroup: 30 nm β-alkyl group: 12 UV-Vis problems Me O Me α,β-unsaturated ketone Table 7.12 Base value: 215 nm α-alkyl group: 10 nm Two β-alkyl groups 24 nm Two exocyclic double bonds 10 nm 259 nm OH O Base value: 215 nm β-hydroxy group: 30 nm β-alkyl group: 12 257 nm O UV-Vis problems O UV-Vis problems α,β-unsaturated ketone Table 7.12 Base value: 215 nm O UV-Vis problems α,β-unsaturated ketone Table 7.12 Base value: 215 nm Double bond Extending conjug. 30 nm O UV-Vis problems α,β-unsaturated ketone Table 7.12 Base value: 215 nm Double bond Extending conjug. 30 nm β-alkyl group: 12 nm O UV-Vis problems α,β-unsaturated ketone Table 7.12 Base value: 215 nm Double bond Extending conjug. 30 nm β-alkyl group: 12 nm γ-alkyl group: 18 nm O UV-Vis problems α,β-unsaturated ketone Table 7.12 Base value: 215 nm Double bond Extending conjug. 30 nm β-alkyl group: 12 nm γ-alkyl group: 18 nm >γ-alkyl group: 18 nm O UV-Vis problems α,β-unsaturated ketone Table 7.12 Base value: 215 nm Double bond Extending conjug. 30 nm β-alkyl group: 12 nm γ-alkyl group: 18 nm >γ-alkyl group: 18 nm Two exocyclic double bonds 10 nm O O O UV-Vis problems α,β-unsaturated ketone Table 7.12 Base value: 215 nm Double bond Extending conjug. 30 nm β-alkyl group: 12 nm γ-alkyl group: 18 nm >γ-alkyl group: 18 nm Two exocyclic double bonds 10 nm 303 nm O UV-Vis problems α,β-unsaturated ketone Table 7.12 Base value: 215 nm Double bond Extending conjug. 30 nm β-alkyl group: 12 nm γ-alkyl group: 18 nm >γ-alkyl group: 18 nm Two exocyclic double bonds 10 nm 303 nm O O UV-Vis problems α,β-unsaturated ketone Table 7.12 Base value: 215 nm Double bond Extending conjug. 30 nm β-alkyl group: 12 nm γ-alkyl group: 18 nm >γ-alkyl group: 18 nm Two exocyclic double bonds 10 nm 303 nm O Base value: 215 nm O UV-Vis problems α,β-unsaturated ketone Table 7.12 Base value: 215 nm Double bond Extending conjug. 30 nm β-alkyl group: 12 nm γ-alkyl group: 18 nm >γ-alkyl group: 18 nm Two exocyclic double bonds 10 nm 303 nm O Base value: 215 nm α-alkyl group: 10 n O UV-Vis problems α,β-unsaturated ketone Table 7.12 Base value: 215 nm Double bond Extending conjug. 30 nm β-alkyl group: 12 nm γ-alkyl group: 18 nm >γ-alkyl group: 18 nm Two exocyclic double bonds 10 nm 303 nm O Base value: 215 nm α-alkyl group: 10 n 2 β-alkyl groups: 24 nm O UV-Vis problems α,β-unsaturated ketone Table 7.12 Base value: 215 nm Double bond Extending conjug. 30 nm β-alkyl group: 12 nm γ-alkyl group: 18 nm >γ-alkyl group: 18 nm Two exocyclic double bonds 10 nm 303 nm O Base value: 215 nm α-alkyl group: 10 n 2 β-alkyl groups: 24 nm 249 nm O UV-Vis problems α,β-unsaturated ketone Table 7.12 O UV-Vis problems α,β-unsaturated ketone Table 7.12 Base value: 202 nm O UV-Vis problems α,β-unsaturated ketone Table 7.12 Base value: 202 nm α-alkyl group 10 nm O UV-Vis problems α,β-unsaturated ketone Table 7.12 Base value: 202 nm α-alkyl group 10 nm Two β-alkyl groups 24 nm O UV-Vis problems α,β-unsaturated ketone Table 7.12 Base value: 202 nm α-alkyl group 10 nm Two β-alkyl groups 24 nm exocyclic double bond 5 nm O O UV-Vis problems α,β-unsaturated ketone Table 7.12 Base value: 202 nm α-alkyl group 10 nm Two β-alkyl groups 24 nm exocyclic double bond 5 nm 241 nm UV-Vis problems O O O CHO Br O OAc UV-Vis problems O O O CHO Br O OAc O O O CHO Br O OAc UV-Vis problems O O O CHO Br O OAc O O O CHO Br O OAc α,β-unsaturated ketone Table 7.12 Base value: 215 nm UV-Vis problems O O O CHO Br O OAc O O O CHO Br O OAc α,β-unsaturated ketone Table 7.12 Base value: 215 nm α-alkoxy (OMe) 35 nm UV-Vis problems O O O CHO Br O OAc O O O CHO Br O OAc α,β-unsaturated ketone Table 7.12 Base value: 215 nm α-alkoxy (OMe) 35 nm β-alkoxy (OMe) 30 nm UV-Vis problems O O O CHO Br O OAc O O O CHO Br O OAc α,β-unsaturated ketone Table 7.12 Base value: 215 nm α-alkoxy (OMe) 35 nm β-alkoxy (OMe) 30 nm Double bond Extending conj 30 nm UV-Vis problems O O O CHO Br O OAc O O O CHO Br O OAc α,β-unsaturated ketone Table 7.12 Base value: 215 nm α-alkoxy (OMe) 35 nm β-alkoxy (OMe) 30 nm Double bond Extending conj 30 nm >γ alkyl group 18 nm O O O CHO Br O OAc UV-Vis problems α,β-unsaturated ketone Table 7.12 Base value: 215 nm α-alkoxy (OMe) 35 nm β-alkoxy (OMe) 30 nm Double bond extending conj 30 nm >γ alkyl group 18 nm Homodiene 39 nm O O O CHO Br O OAc UV-Vis problems α,β-unsaturated ketone Table 7.12 Base value: 215 nm α-alkoxy (OMe) 35 nm β-alkoxy (OMe) 30 nm Double bond extending conj 30 nm >γ alkyl group 18 nm Homodiene 39 nm 367 nm UV-Vis problems O N H Me Br COOCH3 triene Table 7.5 UV-Vis problems triene Table 7.5 O N H Me Br COOCH3 Base value (heteroannular diene): 214 nm UV-Vis problems triene Table 7.5 O N H Me Br COOCH3 Base value (heteroannular diene): 214 nm Double bond extending conjugation 30 nm UV-Vis problems triene Table 7.5 O N H Me Br COOCH3 Base value (heteroannular diene): 214 nm Double bond extending conjugation 30 nm Oalk group 6 nm UV-Vis problems triene Table 7.5 O N H Me Br COOCH3 Base value (heteroannular diene): 214 nm Double bond extending conjugation 30 nm Oalk group 6 nm N(Alk)2 60 nm UV-Vis problems triene Table 7.5 Base value (heteroannular diene): 214 nm Double bond extending conjugation 30 nm Oalk group 6 nm N(Alk)2 60 nm 5 alkyl residues 25 nm O N H Me Br COOCH3 O N H Me Br COOCH3 O N H Me Br COOCH3 O N H Me Br COOCH3 O N H Me Br COOCH3 UV-Vis problems triene Table 7.5 Base value (heteroannular diene): 214 nm Double bond extending conjugation 30 nm Oalk group 6 nm N(Alk)2 60 nm 5 alkyl residues 25 nm 4 exocyclic double bonds 20 nm O N H Me Br COOCH3 UV-Vis problems triene Table 7.5 Base value (heteroannular diene): 214 nm Double bond extending conjugation 30 nm Oalk group 6 nm N(Alk)2 60 nm 5 alkyl residues 25 nm 4 exocyclic double bonds 20 nm O N H Me Br COOCH3 355 nm UV-Vis problems N O O AcO O UV-Vis problems α,β-unsaturated ketone Table 7.12 Base value: 202 nm N O O AcO O UV-Vis problems N O O AcO O α,β-unsaturated ketone Table 7.12 Base value: 202 nm Extended conjugation 30 nm N O O AcO O UV-Vis problems N O O AcO O α,β-unsaturated ketone Table 7.12 Base value: 202 nm Extended conjugation 30 nm β-NR2 95 nm N O O AcO O UV-Vis problems N O O AcO O α,β-unsaturated ketone Table 7.12 Base value: 202 nm Extended conjugation 30 nm β-NR2 95 nm δ-OAc 6 nm N O O AcO O UV-Vis problems N O O AcO O α,β-unsaturated ketone Table 7.12 Base value: 202 nm Extended conjugation 30 nm β-NR2 95 nm δ-OAc 6 nm 2 γ and higher alkyls 36 nm N O O AcO O UV-Vis problems N O O AcO O α,β-unsaturated ketone Table 7.12 Base value: 202 nm Extended conjugation 30 nm β-NR2 95 nm δ-OAc 6 nm 2 γ and higher alkyls 36 nm Two exocyclic olefins 10 nm N O O AcO O N O O AcO O N O O AcO O UV-Vis problems N O O AcO O α,β-unsaturated ketone Table 7.12 Base value: 202 nm Extended conjugation 30 nm β-NR2 95 nm δ-OAc 6 nm 2 γ and higher alkyls 36 nm Two exocyclic olefins 10 nm N O O AcO O 379 nm
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