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=εεεεbcWhere 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.
energ
yhv
energ
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 CH
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
π
π∗
σ
σ∗
π
π∗
π π∗Example: ethylene absorbs at longer wavelengths:
λmax = 165 nm ε= 10,000
∆Ε= hv
=hc/λ
σ
σ∗
hv
π
π∗
n
σ
σ∗
π
π∗
n
C O
π∗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−π∗ λmax = 279 nm ; ε= 15
C C
C C
C O
C O
H
σ −> σ∗ 135 nm
π −> π∗ 165 nm
n −> σ∗ 183 nm weak
π −> π∗ 150 nmn −> σ∗ 188 nmn −> π∗ 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 HeteroannularParent λ=253 nm λ=214 nm
=217 (acyclic)Increments for:Double bond extending conjugation
+30Alkyl substituent or ring residue +5Exocyclic double bond +5
Polar groupings:-OC(O)CH3 +0-OR +6-Cl, -Br +5-NR2 +60-SR +30
C
C CC
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.
=>
π → π* for ethylene
and
butadiene
Ultraviolet Spectroscopy
• 200-400 nm photons excite electrons
from a π bonding orbital to a π* 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 15-3315-37
EFEITOS DE SOLVENTE
Aumento da polaridade do solvente:deslocamento do máximo da banda deabsorção para o vermelho (isto é paracomprimemntos de onda maiores): energiasmenores.
Modelo:
π* é mais estabilizado do que π pelo aumento
da polaridade do solvente.Conseqüência:
DE
DE
p
p
p
*
p*
Aumento da polaridadedo solvente:
deslocamento do máximo
da banda de absorção π,
π* para o vermelho (istoé para comprimentos deonda maiores): energiasmenores.
deslocamento do máximoda banda de absorção n,
π* para o azul (isto épara comprimentos deonda menores): energiasmaiores.
Modelo: como
π* é mais estabilizado do que π
n é mais estabilizado do que π*
transição π → π* transição n → π*
DE
DE
p
p
p
*
p*
menor
DE
DE
p*p*
maior
n
n
l aumenta
l diminui
deslocamento batocrômico deslocamento hipsocrômico
polaridade do solventepolaridade do solvente
O
OH
UV-Vis problems
α,β-unsaturatedketone
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 nmβ-alkyl group: 12 nm
O
OH
UV-Vis problems
α,β-unsaturated ketone
Table 7.12
Base value: 202 nm
α-hydroxy group: 35 nmβ-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 nmβ-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 nmβ-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 nmβ-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 nmβ-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
Meα,β-unsaturated ketoneTable 7.12
UV-Vis problems
Me
O
Meα,β-unsaturated ketoneTable 7.12
Base value: 215 nm
UV-Vis problems
Me
O
Meα,β-unsaturated ketoneTable 7.12
Base value: 215 nm
α-alkyl group: 10 nm
UV-Vis problems
Me
O
Meα,β-unsaturated ketoneTable 7.12
Base value: 215 nm
α-alkyl group: 10 nm
Two β-alkyl groups 24 nm
UV-Vis problems
Me
O
Meα,β-unsaturated ketoneTable 7.12
Base value: 215 nm
α-alkyl group: 10 nm
Two β-alkyl groups 24 nmTwo exocyclic
double bonds 10 nm
Me
O
Me
Me
O
Me
UV-Vis problems
Me
O
Meα,β-unsaturated ketoneTable 7.12
Base value: 215 nm
α-alkyl group: 10 nm
Two β-alkyl groups 24 nmTwo exocyclic
double bonds 10 nm
259 nm
UV-Vis problems
Me
O
Meα,β-unsaturated ketoneTable 7.12
Base value: 215 nm
α-alkyl group: 10 nm
Two β-alkyl groups 24 nmTwo exocyclic
double bonds 10 nm
259 nm
OH
O
UV-Vis problems
Me
O
Meα,β-unsaturated ketoneTable 7.12
Base value: 215 nm
α-alkyl group: 10 nm
Two β-alkyl groups 24 nmTwo exocyclic
double bonds 10 nm
259 nm
OH
O
Base value: 215 nm
UV-Vis problems
Me
O
Meα,β-unsaturated ketoneTable 7.12
Base value: 215 nm
α-alkyl group: 10 nm
Two β-alkyl groups 24 nmTwo exocyclic
double bonds 10 nm
259 nm
OH
O
Base value: 215 nm
β-hydroxy group: 30 nm
UV-Vis problems
Me
O
Meα,β-unsaturated ketoneTable 7.12
Base value: 215 nm
α-alkyl group: 10 nm
Two β-alkyl groups 24 nmTwo exocyclic
double bonds 10 nm
259 nm
OH
O
Base value: 215 nm
β-hydroxy group: 30 nm
β-alkyl group: 12
UV-Vis problems
Me
O
Meα,β-unsaturated ketoneTable 7.12
Base value: 215 nm
α-alkyl group: 10 nm
Two β-alkyl groups 24 nmTwo 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 ketoneTable 7.12
Base value: 215 nm
O
UV-Vis problems
α,β-unsaturated ketoneTable 7.12
Base value: 215 nmDouble bond
Extending conjug. 30 nm
O
UV-Vis problems
α,β-unsaturated ketoneTable 7.12
Base value: 215 nmDouble bond
Extending conjug. 30 nm
β-alkyl group: 12 nm
O
UV-Vis problems
α,β-unsaturated ketoneTable 7.12
Base value: 215 nmDouble bond
Extending conjug. 30 nm
β-alkyl group: 12 nmγ-alkyl group: 18 nm
O
UV-Vis problems
α,β-unsaturated ketoneTable 7.12
Base value: 215 nmDouble bond
Extending conjug. 30 nm
β-alkyl group: 12 nmγ-alkyl group: 18 nm
>γ-alkyl group: 18 nm
O
UV-Vis problems
α,β-unsaturated ketoneTable 7.12
Base value: 215 nmDouble bond
Extending conjug. 30 nm
β-alkyl group: 12 nmγ-alkyl group: 18 nm
>γ-alkyl group: 18 nmTwo exocyclicdouble bonds 10 nm
O O
O
UV-Vis problems
α,β-unsaturated ketoneTable 7.12
Base value: 215 nmDouble bond
Extending conjug. 30 nm
β-alkyl group: 12 nmγ-alkyl group: 18 nm
>γ-alkyl group: 18 nmTwo exocyclicdouble bonds 10 nm
303 nm
O
UV-Vis problems
α,β-unsaturated ketoneTable 7.12
Base value: 215 nmDouble bond
Extending conjug. 30 nm
β-alkyl group: 12 nmγ-alkyl group: 18 nm
>γ-alkyl group: 18 nmTwo exocyclicdouble bonds 10 nm
303 nm
O
O
UV-Vis problems
α,β-unsaturated ketoneTable 7.12
Base value: 215 nmDouble bond
Extending conjug. 30 nm
β-alkyl group: 12 nmγ-alkyl group: 18 nm
>γ-alkyl group: 18 nmTwo exocyclicdouble bonds 10 nm
303 nm
O
Base value: 215 nm
O
UV-Vis problems
α,β-unsaturated ketoneTable 7.12
Base value: 215 nmDouble bond
Extending conjug. 30 nm
β-alkyl group: 12 nmγ-alkyl group: 18 nm
>γ-alkyl group: 18 nmTwo exocyclicdouble bonds 10 nm
303 nm
O
Base value: 215 nm
α-alkyl group: 10 n
O
UV-Vis problems
α,β-unsaturated ketoneTable 7.12
Base value: 215 nmDouble bond
Extending conjug. 30 nm
β-alkyl group: 12 nmγ-alkyl group: 18 nm
>γ-alkyl group: 18 nmTwo exocyclicdouble bonds 10 nm
303 nm
O
Base value: 215 nm
α-alkyl group: 10 n2 β-alkyl groups: 24 nm
O
UV-Vis problems
α,β-unsaturated ketoneTable 7.12
Base value: 215 nmDouble bond
Extending conjug. 30 nm
β-alkyl group: 12 nmγ-alkyl group: 18 nm
>γ-alkyl group: 18 nmTwo exocyclicdouble bonds 10 nm
303 nm
O
Base value: 215 nm
α-alkyl group: 10 n2 β-alkyl groups: 24 nm
249 nm
O
UV-Vis problems
α,β-unsaturated ketoneTable 7.12
O
UV-Vis problems
α,β-unsaturated ketoneTable 7.12
Base value: 202 nm
O
UV-Vis problems
α,β-unsaturated ketoneTable 7.12
Base value: 202 nm
α-alkyl group 10 nm
O
UV-Vis problems
α,β-unsaturated ketoneTable 7.12
Base value: 202 nm
α-alkyl group 10 nm
Two β-alkyl groups 24 nm
O
UV-Vis problems
α,β-unsaturated ketoneTable 7.12
Base value: 202 nm
α-alkyl group 10 nm
Two β-alkyl groups 24 nmexocyclic
double bond 5 nm O
O
UV-Vis problems
α,β-unsaturated ketoneTable 7.12
Base value: 202 nm
α-alkyl group 10 nm
Two β-alkyl groups 24 nmexocyclic
double bond 5 nm
241 nm
UV-Vis problems
O
O
O
CHOBr
O
OAc
UV-Vis problems
O
O
O
CHOBr
O
OAc
O
O
O
CHOBr
O
OAc
UV-Vis problems
O
O
O
CHOBr
O
OAc
O
O
O
CHOBr
O
OAc
α,β-unsaturated ketone
Table 7.12
Base value: 215 nm
UV-Vis problems
O
O
O
CHOBr
O
OAc
O
O
O
CHOBr
O
OAc
α,β-unsaturated ketone
Table 7.12
Base value: 215 nm
α-alkoxy (OMe) 35 nm
UV-Vis problems
O
O
O
CHOBr
O
OAc
O
O
O
CHOBr
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
CHOBr
O
OAc
O
O
O
CHOBr
O
OAc
α,β-unsaturated ketone
Table 7.12
Base value: 215 nm
α-alkoxy (OMe) 35 nm
β-alkoxy (OMe) 30 nmDouble bond
Extending conj 30 nm
UV-Vis problems
O
O
O
CHOBr
O
OAc
O
O
O
CHOBr
O
OAc
α,β-unsaturated ketone
Table 7.12
Base value: 215 nm
α-alkoxy (OMe) 35 nm
β-alkoxy (OMe) 30 nmDouble bond
Extending conj 30 nm
>γ alkyl group 18 nm
O
O
O
CHOBr
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 nmDouble bond
extending conj 30 nm
>γ alkyl group 18 nmHomodiene 39 nm
O
O
O
CHOBr
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 nmDouble bond
extending conj 30 nm
>γ alkyl group 18 nmHomodiene 39 nm
367 nm
UV-Vis problems
O
NH
Me
BrCOOCH3 triene
Table 7.5
UV-Vis problems
trieneTable 7.5
O
NH
Me
BrCOOCH3
Base value (heteroannular diene): 214 nm
UV-Vis problems
trieneTable 7.5
O
NH
Me
BrCOOCH3
Base value (heteroannular diene): 214 nmDouble bond extending conjugation 30 nm
UV-Vis problems
trieneTable 7.5
O
NH
Me
BrCOOCH3
Base value (heteroannular diene): 214 nmDouble bond extending conjugation 30 nm
Oalk group 6 nm
UV-Vis problems
trieneTable 7.5
O
NH
Me
BrCOOCH3
Base value (heteroannular diene): 214 nmDouble bond extending conjugation 30 nm
Oalk group 6 nm
N(Alk)2 60 nm
UV-Vis problems
trieneTable 7.5
Base value (heteroannular diene): 214 nmDouble bond extending conjugation 30 nm
Oalk group 6 nm
N(Alk)2 60 nm5 alkyl residues 25 nm
O
NH
Me
BrCOOCH3
O
NH
Me
BrCOOCH3
O
NH
Me
BrCOOCH3
O
NH
Me
BrCOOCH3
O
NH
Me
BrCOOCH3
UV-Vis problems
trieneTable 7.5
Base value (heteroannular diene): 214 nmDouble bond extending conjugation 30 nm
Oalk group 6 nm
N(Alk)2 60 nm5 alkyl residues 25 nm
4 exocyclic double bonds 20 nm
O
NH
Me
BrCOOCH3
UV-Vis problems
trieneTable 7.5
Base value (heteroannular diene): 214 nmDouble bond extending conjugation 30 nm
Oalk group 6 nm
N(Alk)2 60 nm5 alkyl residues 25 nm
4 exocyclic double bonds 20 nm
O
NH
Me
BrCOOCH3
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 nmδ-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 nmδ-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 nmδ-OAc 6 nm
2 γ and higher alkyls 36 nmTwo 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 nmδ-OAc 6 nm
2 γ and higher alkyls 36 nmTwo exocyclic olefins 10 nm
N
O
O
AcO
O
379 nm
