Electronic Excitation by UV/Vis Spectroscopy · UV-Vis problems α,β -unsaturated ketone Table...

transcript

X-ray: core electron excitation

UV: valance

electronic

excitation

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.

} ∆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.

σ σ∗

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.

π π∗Example: ethylene absorbs at longer wavelengths:

λmax = 165 nm ε= 10,000

∆Ε= hv

=hc/λ

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

σ −> σ∗ 135 nm

π −> π∗ 165 nm

n −> σ∗ 183 nm weak

π −> π∗ 150 nmn −> σ∗ 188 nmn −> π∗ 279 nm weak

180 nm

279 nm

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

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

Homoannular heteroannular acyclic exocyclic

For more than 4 conjugated double bonds: λmax = 114 + 5(# of alkyl groups) + n(48.0-1.7n)

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)

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

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

15-23 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:

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*

l aumenta

l diminui

deslocamento batocrômico deslocamento hipsocrômico

polaridade do solventepolaridade do solvente

UV-Vis problems

α,β-unsaturatedketone

Table 7.12

Base value: 202 nm

UV-Vis problems

α,β-unsaturated ketone

Table 7.12

Base value: 202 nm

α-hydroxy group: 35 nm

UV-Vis problems

Table 7.12

Base value: 202 nm

α-hydroxy group: 35 nmβ-alkyl group: 12 nm

UV-Vis problems

Table 7.12

Base value: 202 nm

UV-Vis problems

Table 7.12

249 nm

Base value: 202 nm

UV-Vis problems

Table 7.12

249 nm

Base value: 215 nm

Base value: 202 nm

UV-Vis problems

Table 7.12

249 nm

Base value: 215 nm

α-alkyl group: 10 nm

Base value: 202 nm

UV-Vis problems

Table 7.12

249 nm

Base value: 215 nm

β-alkyl group: 12

Base value: 202 nm

UV-Vis problems

Table 7.12

249 nm

Base value: 215 nm

β-alkyl group: 12

237 nm

UV-Vis problems

Meα,β-unsaturated ketoneTable 7.12

UV-Vis problems

Base value: 215 nm

UV-Vis problems

Base value: 215 nm

UV-Vis problems

Base value: 215 nm

Two β-alkyl groups 24 nm

UV-Vis problems

Base value: 215 nm

Two β-alkyl groups 24 nmTwo exocyclic

double bonds 10 nm

UV-Vis problems

Base value: 215 nm

double bonds 10 nm

259 nm

UV-Vis problems

Base value: 215 nm

double bonds 10 nm

259 nm

UV-Vis problems

Base value: 215 nm

double bonds 10 nm

259 nm

Base value: 215 nm

UV-Vis problems

Base value: 215 nm

double bonds 10 nm

259 nm

Base value: 215 nm

β-hydroxy group: 30 nm

UV-Vis problems

Base value: 215 nm

double bonds 10 nm

259 nm

Base value: 215 nm

β-alkyl group: 12

UV-Vis problems

Base value: 215 nm

double bonds 10 nm

259 nm

Base value: 215 nm

β-alkyl group: 12

257 nm

UV-Vis problems

α,β-unsaturated ketoneTable 7.12

Base value: 215 nm

UV-Vis problems

Base value: 215 nmDouble bond

Extending conjug. 30 nm

UV-Vis problems

β-alkyl group: 12 nm

UV-Vis problems

β-alkyl group: 12 nmγ-alkyl group: 18 nm

UV-Vis problems

>γ-alkyl group: 18 nm

UV-Vis problems

>γ-alkyl group: 18 nmTwo exocyclicdouble bonds 10 nm

UV-Vis problems

303 nm

UV-Vis problems

303 nm

UV-Vis problems

303 nm

Base value: 215 nm

UV-Vis problems

303 nm

Base value: 215 nm

α-alkyl group: 10 n

UV-Vis problems

303 nm

Base value: 215 nm

α-alkyl group: 10 n2 β-alkyl groups: 24 nm

UV-Vis problems

303 nm

Base value: 215 nm

α-alkyl group: 10 n2 β-alkyl groups: 24 nm

249 nm

UV-Vis problems

Base value: 202 nm

UV-Vis problems

Base value: 202 nm

α-alkyl group 10 nm

UV-Vis problems

Base value: 202 nm

Two β-alkyl groups 24 nm

UV-Vis problems

Base value: 202 nm

Two β-alkyl groups 24 nmexocyclic

double bond 5 nm O

UV-Vis problems

Base value: 202 nm

Two β-alkyl groups 24 nmexocyclic

double bond 5 nm

241 nm

UV-Vis problems

Table 7.12

Base value: 215 nm

UV-Vis problems

Table 7.12

Base value: 215 nm

α-alkoxy (OMe) 35 nm

UV-Vis problems

Table 7.12

Base value: 215 nm

β-alkoxy (OMe) 30 nm

UV-Vis problems

Table 7.12

Base value: 215 nm

β-alkoxy (OMe) 30 nmDouble bond

Extending conj 30 nm

UV-Vis problems

Table 7.12

Base value: 215 nm

Extending conj 30 nm

>γ alkyl group 18 nm

UV-Vis problems

Table 7.12

Base value: 215 nm

extending conj 30 nm

>γ alkyl group 18 nmHomodiene 39 nm

UV-Vis problems

Table 7.12

Base value: 215 nm

extending conj 30 nm

>γ alkyl group 18 nmHomodiene 39 nm

367 nm

UV-Vis problems

BrCOOCH3 triene

Table 7.5

UV-Vis problems

trieneTable 7.5

BrCOOCH3

Base value (heteroannular diene): 214 nm

UV-Vis problems

trieneTable 7.5

BrCOOCH3

Base value (heteroannular diene): 214 nmDouble bond extending conjugation 30 nm

UV-Vis problems

trieneTable 7.5

BrCOOCH3

Oalk group 6 nm

UV-Vis problems

trieneTable 7.5

BrCOOCH3

Oalk group 6 nm

N(Alk)2 60 nm

UV-Vis problems

trieneTable 7.5

Oalk group 6 nm

N(Alk)2 60 nm5 alkyl residues 25 nm

BrCOOCH3

UV-Vis problems

trieneTable 7.5

Oalk group 6 nm

4 exocyclic double bonds 20 nm

BrCOOCH3

UV-Vis problems

trieneTable 7.5

Oalk group 6 nm

4 exocyclic double bonds 20 nm

BrCOOCH3

355 nm

UV-Vis problems

Table 7.12

Base value: 202 nm

UV-Vis problems

Table 7.12

Base value: 202 nm

Extended conjugation 30 nm

UV-Vis problems

Table 7.12

Base value: 202 nm

β-NR2 95 nm

UV-Vis problems

Table 7.12

Base value: 202 nm

β-NR2 95 nmδ-OAc 6 nm

UV-Vis problems

Table 7.12

Base value: 202 nm

2 γ and higher alkyls 36 nm

UV-Vis problems

Table 7.12

Base value: 202 nm

2 γ and higher alkyls 36 nmTwo exocyclic olefins 10 nm

UV-Vis problems

Table 7.12

Base value: 202 nm

2 γ and higher alkyls 36 nmTwo exocyclic olefins 10 nm

379 nm

Electronic Excitation by UV/Vis Spectroscopy · UV-Vis problems α,β -unsaturated ketone Table...

Documents