UV Spectroscopy

Utility

• First organic spectral method; rarely used as a primary method for structure determination • Main contribution is that can readily identify the presence of conjugates π-systems or unique chromophores • Can sometimes be used to differentiate double bond isomers • In combination with NMR and IR data can use to elucidate unique electronic features not readily apparent from those methods • Widely used in other applications • Most common detector for HPLC • Can be used to moniter reaction kinetics (chemistry, biology, medicine), etc.

UV Spectroscopy

The Electromagnetic Spectrum

x-rays ultraviolet(UV) visible Infrared

(IR) microwaves radiowaves

190 - 400 nm 400 - 800 nm

absorbed color

observed

color

violet

blue

green

yellow

orange

red

yellow red violet blue

short wavelengthhigh frequency

high energy

long wavelengthshort frequency

low energy

UV Spectroscopy

Colors of Different Wavelength Regions

Absorbance & Transmittance

< 380380 - 435435 - 480480 - 490490 - 500500 - 560560 - 580580 - 590590 - 625625 - 780

> 780

ultravioletvioletblue

greenish bluebluish green

greenyellowish green

yelloworange

rednear IR

---yellowish green

yelloworange

redpurplevioletblue

greenish bluebluish green

red

wavelengthabsorbed (nm)

absorbedcolor

transmitted color(compliment)

UV Spectroscopy

Origin of the Absorption

• The absorption of UV or visible radiation corresponds to the excitation of valence electrons

• Valence electrons are typically found in: - σ bonding orbitals (single bonds) - π bonding prbitals (double or triple bonds) - non-bonding orbitals (lone pair electrons)

ΔE = [Eexcited – Eground] = hν

HOMO

LUMO

UV Spectroscopy

Electronic Transitions

ΔE = [Eexcited – Eground] = hν

σ

σ

π

n

n

σ*

π*

π*

σ*

π*

alkanes

carbonyls

alkenes, carbonyls, alkynes, etc.

heteroatoms (O, N, S, X, etc.)

carbonyls

σ*

π*

n

π

σ

E E

antibonding

bonding

atomic

orbitals possible electronic transitions

UV Spectroscopy

Electronic Transitions

Relative Transition Energies

most useful

• not all transformations that are possible will be observed • some electronic transitions “forbidden” by certain selection rules • even forbidden transitions can be observed, but usually not very intense

for example, n π*

σ σ* n σ* π π* n π* σ π*> > > >

UV Spectroscopy

The Spectrometer

UV Spectroscopy

Beer-Lambert Law

A = log ( I0 / I ) = ε ℓ c

Where: A is absorbance (no units) I0 = intensity of incident light I = intensity transmitted light ε = molar absorbtivity or extinction coefficient ℓ = path length; length of the sample cell (cm) c = sample concentration (mol/L)

describes relationship between absorbance and concentration

ℓ

UV Spectroscopy

Spectrum Features

isoprene

ε = A / ℓ c • values of 104-106 are termed high intensity absorptions • values of 103-104 are termed low intensity absorptions • values of 0 to 103 are the absorptions of forbidden transitions

UV Spectroscopy

Spectrum Features

reporting data

λmax = 206 nm 252

317 376

O

NH2

O

UV Spectroscopy

Spectrum Features

peak broadening

UV Spectroscopy

Choice of Solvent

Solvent should not absorb UV radiation in the same region as the sample (measuring UV spectra below 200 nm is impractical)

solvent cutoff (nm) solvent cutoff (nm) acetonitrile 190 ethanol 205 chloroform 240 hexane 201 cyclohexane 195 methanol 205 diethyl ether 210 isooctane 195 dioxane 215 water 190

sample cell: quartz glass cutoff 210 nm

UV Spectroscopy

Solvent Effect on Spectra

OH

UV Spectroscopy

Chromophores

• All molecules capable of absorbing ultraviolet radiation, though most do so at very high energy (wavelengths < 200 nm).

• Unsaturated groups which give rise to absorptions involving π or π* orbitals in the in the near-UV/visible region are called chromophores.

• Saturated groups with non-bonding electrons, which can give rise to transitions involving non-bonding orbitals are called auxochromes.

• Most useful transitions for analysis are the intense π → π * transitions and the weaker, but lower energy, n → π* transitions.

Basic Information

UV Spectroscopy

Chromophores

Absorptions of Organic Molecules

Alkanes: • Saturated molecules that lack lone pairs • Only transitions possible are σ σ* • high energy; absorb UV radiation at very short wavelengths • not accessible using UV spectroscopy Alcohols, Ethers, Amines, & Sulfur Compounds: • Satuated molecules with lone pairs of electrons • Important transitions are n σ* • high energy, most often at wavelengths shorter than 200 nm • alcohols and amines: 175 – 200 nm • thiols and sulfides: 200 – 220 nm

UV Spectroscopy

Chromophores

Absorptions of Organic Molecules

Alkenes & Alkynes: • Important transitions are π π* • high energy, but impacted by substitution • simple alkenes: 175 nm • simple alkynes: 170 nm Carbonyls: • Important transitions are π π* (188 nm) • n π* also possible (280 – 290 nm) • sensitive to substitution

UV Spectroscopy

Chromophores

UV Spectroscopy

Chromophores

Bathochromic shift: shift of absorption to a longer wavelength Hypsochromic shift: shift of absorption to a shorter wavelength Hyperchromic effect: an increase in absorption intensity Hypochromic effect: a decrease in absorption intensity

Terminology for Absorptive Shifts

200 nm 700 nm

ε

Hypochrom

ic

Hypsochromic

Hyp

erch

rom

ic

Bathochromic

UV Spectroscopy

Conjugation Effects

H2C CH2

O

O

λmax (nm)

175

217

258

465

15,000

21,000

35,000

125,000

280189

280213

12900

277,100

εcompound

n π*

n π*

π π*

π π*

UV Spectroscopy

Conjugation Effects

• Greater the conjugation, the lower the energy required to induce electronic transitions (e.g. the (longer the wavelength) • lengthening conjugation also increases band intensity (greater molar absorbitivity) • adding substitutents may have same effect, but to a much smaller degree

UV Spectroscopy

Conjugation Effects

H2C CH2

Energy

ethylene

butadiene

hexatriene

octatetraene

π

π*

Extended π Systems

UV Spectroscopy

Auxochromes & Conjugation

• atoms with lone pairs can extend conjugation by resonance

RX

RX

X = OH, OR, NH2, halogen, etc.

• alkyl substituents can influence wavelength by overlap of C-H bonding orbital with the π system (e.g. by hyperconjugation)

UV Spectroscopy

Woodward Fieser Rules for Dienes

• Woodward & Fieser derived a set of empirical rules for the estimation of wavelength for the low energy π π*electronic transition • Based on empirical observation of known conjugated structures • Can be used to reliably predict absorption wavelength in dienes, enone, and to a lesser extent aromatic systems

s-trans s-cis homoannular(cisoid)

acyclic dienes cyclic dienes

heteroannular(transoid)

UV Spectroscopy

Woodward Fieser Rules for Dienes

not affected by solvent

Where both types of cyclic dienes are present, the base with the longer λmax is used.

s-trans homoannular(cisoid)

heteroannular(transoid)

base values: 217 nm 253 nm 214 nm

Increments:

For each additional conjugated double bondFor each exocyclic double bondFor each alkyl groupFor each of the following groups:

- OR- O(C=O)R- Cl - Br- SR- NR2- Ph

+ 30 nm+ 5 nm+ 5 nm

+ 6 nm+ 0 nm+ 5 nm+ 5 nm+ 30 nm+ 60 nm+ 60 nm

UV Spectroscopy

Woodward Fieser Rules for Dienes

CAUTION!

RThis compound had three exocyclicdouble bonds; the indicated bond isexocyclic to two rings.λmax calc = 284

this is not a heteroannular diene; must use base value for acyclic diene.λmax calc = 232

this is not a homoannular diene; must use base value for acyclic dieneλmax calc = 237

UV Spectroscopy

Woodward Fieser Rules for Dienes

examples acyclic diene3 alkyl substcalculated valueobserved

217 nm15 nm

232 nm234 nm

acyclic diene2 alkyl subst1 exocyclic dbcalculated valueobserved

217 nm10 nm5 nm

232 nm236 nm

cisoid diene4 alkyl subst1 exocyclic dbcalculated valueobserved

253 nm20 nm5 nm

278 nm275 nm

UV Spectroscopy

Woodward Fieser Rules for Dienes

examples

O

O

cisoid diene2 conj db5 alkyl subst1 acyl subst3 exocyclic dbcalculated valueobserved

253 nm60 nm25 nm0 nm

15 nm353 nm355 nm

EtO

transoid diene3 alkyl subst1 OR subst1 exocyclic dbcalculated valueobserved

214 nm15 nm6 nm5 nm

240 nm241 nm

UV Spectroscopy

Woodward Fieser Rules for Dienes

double bond regioisomers

OOH

OOH

abietic acid levopimaric acid

λmax calc: 239λmax obs: 238

λmax calc: 278λmax obs: 275

cisoid diene4 alkyl subst1 exocyclic dbcalculated value

253 nm20 nm5 nm

278 nm

transoid diene4 alkyl subst1 exocyclic dbcalculated value

214 nm20 nm5 nm

239 nm

UV Spectroscopy

Fieser-Kuhn Rules for Extended Polyenes

where n = number of conjugated double bonds

• Woodward-Fieser Rules work well up to four conjugated double bonds • For more extended conjugation, use the Fieser-Kuhn Rules

λmax = 114 + 5(# alkyl substituents) = n(48 - 1.7n) - 16.5(# endo) - 10(# exo)

λmax = 114 + 5(8) = 11(48 - 1.7•11) - 16.5(0) - 10(0) = 476 nmobserved: 474 nm

λmax = 114 + 5(10) = 11(48 - 1.7•11) - 16.5(2) - 10(0) = 453 nmobserved: 452 nm

β-carotene

lycopene

R

acyclic enone 6-membered ringenone

5-membered ringenone

base values: 215 nm 215 nm 202 nm

Increments:

For each additional conjugated double bondFor each exocyclic double bondFor each homodiene component

For each alkyl groupFor each of the following groups: - OH

- OR- O(C=O)R- Cl - Br- SR- NR2

+ 30 nm+ 5 nm+ 39 nm

+ 10 nm+ 35 nm+ 35 nm+ 6 nm+ 15 nm+ 25 nm+ 30 nm+ 60 nm

R

O

α

β

γ

δ

R

O

α

β

β

OO O

R

O

acyclic dienone

245 nm

β γ δ and higherα

+ 12 nm+ 30 nm+ 30 nm+ 6 nm+ 12 nm+ 30 nm+ 85 nm+ 95 nm

+ 18 nm

+ 17 nm+ 6 nm

+ 18 nm+ 50 nm+ 31 nm+ 6 nm

UV Spectroscopy

Woodward Fieser Rules for Enones

Solvent Correction nmH2OEtOHCHCl3DioxaneEt2OHydrocarbon

+ 80

- 1- 5- 7- 11

UV Spectroscopy

Woodward Fieser Rules for Enones

examples

acyclic enone1 α alkyl2 β alkylcalculated valueobserved

215 nm10

24 nm249 nm249 nm

5-membere enone1 α Br2 β alkyl1 exocyclic dbcalculated valueobserved

202 nm25 nm24 nm5 nm

256 nm251 nm

O

O

Br

UV Spectroscopy

Woodward Fieser Rules for Enones

practice

Me

O H

Me

O H

Me

O H

Br

Br

orBr2

base base

Me

O H

Me

O

or

Can you distinguish the two by UV?

UV Spectroscopy

Woodward Fieser Rules for Enones

practice

Me

O H

Me

O

vs

6-membered enoneβ alkylexocyclic db

215 nm12 nm

227 nm

215 nm24 nm5 nm

244 nm

UV Spectroscopy

Woodward Fieser Rules for Enones

practice

O

absorbance in EtOH?

UV Spectroscopy

Woodward Fieser Rules for Other Conjugated Carbonyls

aldehydes carboxylic acid or ester

Base Values

unsubstituted aldehyde or esterwith α or β alkyl groupswith α,β or β,β alkyl groupswith α,β,β alkyl groups

for an exocyclic α,β double bondfor an endocyclic α,β double bond in a 5- or 7-membered ring

208 nm220 nm230 nm242 nm

OR

O

α

β

H

O

α

β

β

193 nm (not observed)208 nm217 nm225 nm

+ 5 nm+ 5 nm

β

(R = H or R')

as for enones, solvent correction is also relevant

UV Spectroscopy

Woodward Fieser Rules for Aromatic Compounds

• Substitution, auxochromic groups, conjugation and solvent effects can cause shifts in wavelength and intensity of bands for aromatic systems similar to dienes and enones • Show multiple bands, often fine structure • However, shifts are difficult to predict – the formulation of empirical rules is often dificult (there are more exceptions than rules) • Can make some useful predictions for benzoyl derivatives

We will not worry about predicting UV bands in aromatic compounds

UV Spectroscopy

Woodward Fieser Rules for Benzoyl Derivatives

R

aryl ketones(R = alkyl)

benzaldehydes benzoic acidsand esters

base values: 246 nm 250 nm 230 nm

Increments:

For each alkyl groupFor each OH or OR (R = alkyl)For each OFor each of the following groups:

- Cl - Br- NH2- NH(C=O)CH3- NHCH3- N(CH3)2

+ 3 nm+ 7 nm+ 11 nm

+ 0 nm+ 2 nm+ 13 nm+ 20 nm

+ 20 nm

O

+ 3 nm+ 7 nm+ 20 nm

+ 0 nm+ 2 nm+ 13 nm+ 20 nm

+ 20 nm

+ 10 nm+ 25 nm+ 78 nm

+ 10 nm+ 15 nm+ 58 nm+ 45 nm+ 73 nm+ 85 nm

H

O

OR

O

ortho meta para