Post on 25-Aug-2018
1
Chem 325
UltraViolet-Visible
Spectroscopy
Electromagnetic Spectrum
λνλνλνλν = c
E = hνννν = hc/λλλλ
Organic Spectroscopy
Absorption of different EM radiation produces different
molecular energy changes
1. Radiowaves Nuclear Spin Transitions (NMR)
2. Microwaves Electron Spin Transitions (ESR)
3. Infrared Vibrational Transitions
4. Visible-NIR Raman Scattering (Vibrational)
5. Ultraviolet-Visible Electronic Transitions
UV-vis Absorption Spectroscopy
Light of wavelength 190 to ~800 nm is passed through a
sample. Amount of light that makes it through the sample is
compared to the amount when the sample is not present.
2
Transmittance and Absorbance
The transmittance is the ratio of the light that is detected
when the sample is present to the ratio when the sample is not
present.
Transmittance is measured in the following way:
0I
IT =
Beer-Lambert Law
Transmittance is related to concentration in a non-linear way
(exponential), so it is usually converted to the much more
useful quantity ‘absorbance’, where:
Absorbance is related to concentration in a linear way
according to the Beer-Lambert Law.
Where l is the pathlength in centimeters
And c is the concentration of the absorbing species in Molarity
(mol/L)
And εεεε is the molar absorptivity or molar extinction coefficient
I
IAAbsorbance
0log==
lcAAbsorbance ε==
Molar Absorptivities
Molar absorptivities may be very large for strongly absorbing compounds (ε >10,000) and very small if absorption is weak (ε = 10 to 100).
No absorption gives εεεε = 0!
Often given as logarithmic values
e.g. εεεε = 23,500 equivalent to log εεεε = 4.37
Units of εεεε
cl
A =ε L mol-1 cm-1 or M-1 cm-1
- very rarely stated explicitly!
Absorbance Spectra
The typical UV-vis spectrometer scans the
wavelength range of 190 nm to 800 nm, and the
absorption at each wavelength is plotted vs the
wavelength in nm.
0
0.5
1
1.5
2
255 280 305 330 355 380
Wavelength (nm)
Ab
so
rba
nc
e λλλλmaxλλλλmax λλλλmax λλλλmax
3
Electronic Transitions
UV and Visible light can be absorbed by organic
molecules causing an electronic transition. What
physically happens is an electron from the Highest
Occupied Molecular Orbital (HOMO) is promoted
to a higher energy orbital.
Chromophores
• Normal hydrocarbons: no UV-vis absorptions
• They do have absorption but in the FAR UV
(vacuum UV)
• Requires presence of a chromophore – a group with
easily promoted electrons
• Typically: ππππ-bond systems
Solvents
acetonitrile 190 nm n-hexane 201 nm
chloroform 240 methanol 205
cyclohexane 195 iso-octane 195
1,4-dioxane 215 water 190
95% ethanol 205 trimethylphosphate 210
Solvents generally required due to large absorptivities
Must be ‘transparent’, i.e. not absorb
Must be very pure
Solvent cutoffs (where they start to absorb!):
Solute-Solvent Interactions
OH
4
Electronic Energy Levels Electronic Transitions
Electronic Transitions
Organics that absorb in the UV and Visible region
(200 – 800 nm) generally contain one or more ππππ-
bonds.
Ethylene Orbital and Transitions
5
Molecular Orbital Diagrams
Conjugated molecules such as 1,3-butadiene absorb
at longer wavelengths.
Excited States
When a molecule absorbs light, this energy promotes an
electron from an occupied MO to an unoccupied MO. This
produces a ‘Singlet Excited State. A number of singlet
excited states are possible, and they are labelled S1, S2, …, Sn.
The ground state is always S0.
α
ground state (S0) S1 S2 S3
Electron Spin Selection Rule
S0 →→→→ S1 is spin-allowed
both states are ‘singlet’
spin multiplicity = 2S+1, S = ½ for each unpaired e-
Spin multiplicity = 3
A TRIPLET state
S0 →→→→ T1 is spin-forbidden
T1
6
State Diagrams
The possible excited states are often drawn as a
‘state diagram’ or ‘Jablonski diagram’. The
vibrational energy levels are shown for each
electronic level.
Intersystem
crossing
Electronic and Vibrational Transitions
Rigid Compounds
In rigid molecules, like polyaromatic hydrocarbons,
excitation to each vibrational level is resolved. These
narrow peaks appear in sets, called ‘bands’.
0
0.2
0.4
0.6
0.8
275 325 375 425
Wavelength (nm)
Ab
so
rba
nc
e
S0 �S1
ν = 0
ν = 1
ν = 2
ν = 3
Vibronic Structure
Non-Rigid Compounds
If the molecule is flexible, several conformations are
possible at any given time. The vibrational ‘fingers’
for each conformation are averaged to give a
rounded band.
0
0.4
0.8
1.2
1.6
2
200 250 300 350
Wavelength (nm)
Ab
so
rba
nc
e
S0 �S1
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Types of Excitation
Only two types of excitation commonly encountered, depending
on the orbitals involved. These are:
A) ππππ* n (or n ����ππππ*)
Electron from an n orbital (non-
bonding or lone pair) is excited into a
ππππ* (antibonding) orbital. Commonly
the longest wavelength absorption for
ketones and aldehydes. ππππ* n
transitions are “forbidden”, so these
usually give weak bands (small εεεε)
B) ππππ* ππππ (or ππππ ����ππππ*)
Electron from a ππππ-orbital is excited into a ππππ*-orbital. This is
commonly the longest wavelength absorption for unsaturated
hydrocarbons.
What Factors Affect Absorption?
The major factor that affects absorption is the degree of
conjugation. The longer the conjugation, the lower the energy
required to excite (and therefore longer wavelength band).
Lengthening the conjugation
also increases the molar
absorptivity (more intense
absorption bands).
Adding substituents and
functional groups will have the
same effect but to a much
smaller degree.
Substituent Effects
Substituents may have any of four effects on a chromophore
1. Bathochromic shift (red shift) – a shift to longer λλλλ: lower E
2. Hypsochromic shift (blue shift) – shift to shorter λλλλ: higher E
3. Hyperchromic effect – an increase in intensity: higher εεεε
4. Hypochromic effect – a decrease in intensity: lower εεεε
200 nm 700 nm
εεεε
Hy
po
ch
rom
ic
Hypsochromic
Hy
perc
hro
mic
Bathochromic
Chromophores and Substituents
Empirical rules: help predict the λλλλmax of a specific compound
from the base chromophore and what substituents are
attached to it.
H2CCH2
ββββ-carotene
O
O
λλλλmax nm εεεε
175 15,000
217 21,000
258 35,000
n ���� ππππ* 280 27ππππ ���� ππππ* 213 7,100
465 125,000
n ���� ππππ* 280 12ππππ ���� ππππ* 189 900
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Woodward-Fieser Rules for Dienes
For a compound to absorb above 200 nm, generally it will
contain some degree of conjugation. The simplest conjugated
molecule is a diene.
Woodward and Fieser developed a set of empirical rules to
help predict what the λλλλmax will be for a diene-based
compound.
There are separate rules for cyclic and acyclic dienes.
Butadiene is the simplest acyclic diene and has an absorption
maximum of 217 nm.
acyclic butadiene, λλλλmax = 217 nm
Woodward-Fieser Rules for Dienes
Next we add the contribution from any attached
groups (substitutents).Group Increment
Extended
conjugation
+30
Each exo-cyclic C=C +5
Alkyl +5
-OCOCH3 +0
-OR +6
-SR +30
-Cl, -Br +5
-NR2
-Ph
+60
+60
Examples
Isoprene
acyclic butadiene = 217 nm
one alkyl subs. + 5 nm
222 nm
Experimental value 220 nm
Allylidenecyclohexane
acyclic butadiene = 217 nm
one exocyclic C=C + 5 nm
2 alkyl subs. +10 nm
232 nm
Experimental value 237 nm
Cyclic Dienes
There are two major types of cyclic dienes, with two different
base values.
Heteroannular (transoid): Homoannular (cisoid):
εεεε = 5,000 – 15,000 εεεε = 12,000-28,000
base λλλλmax = 214 base λλλλmax = 253
Increment table is the same as for acyclic butadienes with one
addition:
Additional Homoannular: +39
If two dienes are present in a molecule, the base with longer λλλλmax
is used.
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Example
heteroannular diene = 214 nm
3 alkyl subs. (3 x 5) +15 nm
1 exo C=C + 5 nm
234 nm
Experimental value 235 nm
Structure Determination?
In the pre-NMR era of organic spectral determination, the
power of the method for discerning isomers is readily
apparent:
Consider abietic vs. levopimaric acid:
C
O
OHC
O
OH
levopimaric acidabietic acid
Types of Excitation
Only two types of excitation commonly encountered, depending
on the orbitals involved. These are:
A) ππππ* n (or n ����ππππ*)
Electron from an n orbital (non-
bonding or lone pair) is excited into a
ππππ* (antibonding) orbital. Commonly
the longest wavelength absorption for
ketones and aldehydes. ππππ* n
transitions are “forbidden”, so these
usually give weak bands (small εεεε)
B) ππππ* ππππ (or ππππ ����ππππ*)
Electron from a ππππ-orbital is excited into a ππππ*-orbital. This is
commonly the longest wavelength absorption for unsaturated
hydrocarbons.
Structure Determination!
C
O
OH
heteroannular diene = 214 nm
4 alkyl subs. (4 x 5) +20 nm1 exo C=C + 5 nm
239 nm
homoannular diene = 253 nm
4 alkyl subs. (4 x 5) +20 nm1 exo C=C + 5 nm
278 nmC
O
OH
abietic acid
levopimaric acid
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WARNING !!
Three common errors:
R
This compound has three exocyclic double
bonds; the indicated bond is exocyclic to two
rings
This is not a heteroannular diene; you would use
the base value for an acyclic diene
Likewise, this is not a homooannular diene;
you would use the base value for an acyclic
diene
Woodward-Fieser Rules
See Pavia, Chapter 7, Section 10
for the Rules and worked examples!!
Product Analysis
Two possible enamines from this ketone. Can UV-vis tell
them apart?
O
CH3
CH3
NH2
CH3
NH2
Product Analysis
CH3
NH2
CH3
NH2
Base value 214
Homoannular 39
Alkyl substituents 3 ×××× 5 15
Exocyclic C=C 5
NH2 group 60
Predicted λλλλmax 333 nm
Base value 214
Alkyl substituents 3 ×××× 5 15
Exocyclic C=C 5
NH2 group 60
Predicted λλλλmax 294 nm
∗∗∗∗ ∗∗∗∗
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The Carbonyl Group Absorptions
•Two possible absorptions
– Longer wavelength (lowest E) is n →→→→ ππππ*
– Symmetry ‘forbidden’, thus low εεεε
– Shorter wavelength (highest E) is ππππ →→→→ ππππ*
– Symmetry ‘allowed’, thus high εεεε
– Can conjugate with alkenes ππππ-systems
Woodward-Fieser Rules for Enones
Enones have a strong ππππ* ππππ band and
a longer wavelength ππππ* n that is
usually around 100 times less intense.
If the enones are conjugated enough,
the ππππ* ππππ band completely swamps out
the ππππ* n band.
Also, the positions of ππππ* ππππ bands are
much easier to predict with empirical
rules than ππππ* n bands.
For these reasons, the Woodward-
Fieser rules for enones ONLY APPLY
TO THE ππππ* ππππ TRANSITION!!
Ketones
Group Increment
6-membered ring or acyclic enone Base 215 nm
5-membered ring parent enone Base 202 nm
Acyclic dienone Base 245 nm
Double bond extending conjugation 30
Alkyl group or ring residue α, β, γ α, β, γ α, β, γ α, β, γ and higher 10, 12, 18
-OH α, β, γ α, β, γ α, β, γ α, β, γ and higher 35, 30, 18
-OR α, β, γ, δα, β, γ, δα, β, γ, δα, β, γ, δ 35, 30, 17, 31
-O(C=O)R α, β, δα, β, δα, β, δα, β, δ 6
-Cl α, βα, βα, βα, β 15, 12
-Br α, βα, βα, βα, β 25, 30
-NR2 ββββ 95
Exocyclic double bond 5
Homocyclic diene component 39
C C CC
β α
C
γδ
δO
Aldehydes and Acids/Esters
Unsaturated system Base Value
Aldehyde 208
With αααα or ββββ alkyl groups 220
With α,βα,βα,βα,β or β,ββ,ββ,ββ,β alkyl groups 230
With α,β,βα,β,βα,β,βα,β,β alkyl groups 242
Acid or ester
With αααα or ββββ alkyl groups 208
With α,βα,βα,βα,β or β,ββ,ββ,ββ,β alkyl groups 217
Group value – exocyclic α,βα,βα,βα,β double bond +5
Group value – endocyclic α,βα,βα,βα,β bond in 5
or 7 membered ring
+5
12
Solvent Effects on Enones
For enones, the solvent will also affect the position of λλλλmax.
Solvent correction Increment
Water +8
Ethanol, methanol 0
Chloroform -1
Dioxane -5
Ether -7
Hydrocarbon -11
Examples
cyclic enone = 215 nm
2 x ββββ- alkyl subs.(2 x 12) +24 nm
239 nm
Experimental value 238 nm
cyclic enone = 215 nm
extended conj. +30 nm
ββββ-ring residue +12 nm
δδδδ-ring residue +18 nm
exocyclic double bond + 5 nm
280 nm
Experimental value 280 nm
O
R
O
αααα
ββββ
Product Analysis
Bromination a steroid can produce two possible products.
Dehydrobromination gives two enones. Can we tell them apart?
CH3
OH
CH3
OH
Br CH3
OH
Br
CH3
O
CH3
OH
Br2
-HBr -HBr
Product Analysis
CH3
OH
CH3
O
Base value 215
ββββ-alkyl 12
227 nm
Base value 215
ββββ-alkyl ××××2 24
Exocyclic C=C 5
244 nm
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UV-Vis of Aromatics
Benzene has 6 ππππ-MOs which leads to a number of
transitions.
ππππ4444∗∗∗∗ ππππ5555∗∗∗∗
ππππ6666∗∗∗∗
ππππ2222
ππππ1111
ππππ3333
Aromatics
Benzene has three main bands, the E, K, and B bands.
The E band is also called the primary
band. It is strongly allowed (εεεε = 47,000)
but shows up below 200 nm.
The K band is called the second
primary band, can be observed above
200 nm if substituents cause a red shift.
Its molar absorptivity is 7400.
The longest wavelength B band (260
nm) is called the secondary band and is
forbidden and therefore weak (εεεε = 230)
Aromatics in General
Substitution with auxochromes lead to the same general
effects as observed for dienes and enones, but in a less
predictable way.
The formation of rules for predicting the position of the
bands is not very useful since there tend to be more
exceptions than there are rules. However, we can
certianly highlight qualitative trends.
1. Substituents with lone pairs will red-shift the primary and
secondary bands.
GGG G
Substituent effects
2. Protonating or deprotonating functional groups changes
how they affect the primary and secondary bands.
3. Functional groups that extend the conjugation red shift
the primary and secondary bands.
5602688,700224-C(O)O-
97027311,600230-C(O)OH
1692547,500203-NH3+
1,4302808,600230-NH2
2,6002879,400235-O-
1,4502706,200211-OH
2042547,400203.5-H
ελλλλmaxελλλλmaxSubstituent
SecondaryPrimary
5602688,700224-C(O)O-
97027311,600230-C(O)OH
1692547,500203-NH3+
1,4302808,600230-NH2
2,6002879,400235-O-
1,4502706,200211-OH
2042547,400203.5-H
ελλλλmaxελλλλmaxSubstituent
SecondaryPrimary
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Electronic Effects
Electron withdrawing groups (EWG) red-shift the primary
band and electron donating groups (EDG) red-shift both
bands.
7,800269-NO2
1922617,900210-Br
1902647,400210-Cl
1,4302808,600230-NH2
1,4802696,400217-OCH3
1,4502706,200211-OH
9,800224-C(O)CH3
11,400250-C(O)H
97027311,600230C(O)OH
1,00027113,000224-CN
2252617,000207-CH3
2042547,400203.5-H
ελλλλmaxελλλλmaxSubstituent
SecondaryPrimary
7,800269-NO2
1922617,900210-Br
1902647,400210-Cl
1,4302808,600230-NH2
1,4802696,400217-OCH3
1,4502706,200211-OH
9,800224-C(O)CH3
11,400250-C(O)H
97027311,600230C(O)OH
1,00027113,000224-CN
2252617,000207-CH3
2042547,400203.5-H
ελλλλmaxελλλλmaxSubstituent
SecondaryPrimary
Electron donating
Electron w
ithdrawing
Di-Substituted Aromatics
1) If both are EWG or both are EDG, the effect is equal to the
stronger of the two.
2) If one is an EWG and the other is an EDG, the overall effect
is additive if they are ortho or meta to one another.
3) If one is an EWG and the other is an EDG, the overall effect
is greater than additive if they are para to one another.
NOO
OH OH
NOO+
+
__ _
Woodward and Fieser Again…
Woodward and Fieser were able to come up with some rules
for predicting the λλλλmax of a subset of aromatic compounds –
those containing a carbonyl attached to the ring.
The rules are for R = H, R, OH, and OR.
The rules are not as accurate as for dienes and enones, but
are generally within 5 nm. The following are the possible
base structures.RO
G
230R = OH or O-Alkyl
250R = H
246R = alkyl or ring residue
λλλλmaxParent Chromophore
230R = OH or O-Alkyl
250R = H
246R = alkyl or ring residue
λλλλmaxParent Chromophore
Woodward-Fieser Rules for Aromatics
A substituent will have a different effect depending on
whether it is ortho, meta, or para substituted. Note that the
strongest effects are observed for para substituents.
85
73
45
58
15
10
78
25
10
p
-NHCH3
1313-NH2
22-Br
2011-O-
33Alkyl or ring residue
Substituent increment
00-Cl
77-O-Alkyl, -OH, -O-Ring
moG
2020-N(CH3)2
2020-NHC(O)CH3
85
73
45
58
15
10
78
25
10
p
-NHCH3
1313-NH2
22-Br
2011-O-
33Alkyl or ring residue
Substituent increment
00-Cl
77-O-Alkyl, -OH, -O-Ring
moG
2020-N(CH3)2
2020-NHC(O)CH3
15
Visible Spectrum
Molecules absorbing between 400 and 800 nm can be detected
by the human eye. The common colours and their associated
wavelengths are given below.
UV Vision
Many insects have vision that extends into the UV region,
which is useful for them since many flowers have UV
colourings that are invisible to humans.
Appearance of Coloured Compounds
The appearance of coloured compounds can be determined
by looking at the colour wheel below.
Select the λλλλmax of the compound of interest, and the
appearance of that compound will be the colour opposite it.
What we ‘see’ is the opposite of the colour absorbed.
ββββ-carotene
Consider ββββ-carotene
ββββ-carotene, λλλλmax = 455 nm
This molecule absorbs at 455 nm
(in the blue), therefore the
compound will appear orange.
ββββ-carotene is the primary
pigment that colours carrots.
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Lycopene and Indigo
Similarly, lycopene absorbs at 474 nm and appears red. This
compound is the main pigment found in tomatoes.
λλλλmax for indigo is at 602 nm – in the orange region of the
spectrum – this is absorbed, the compliment is now indigo.
lycopene, λλλλmax = 474 nm
NH
HN
O
O
indigo
Azo Dyes
One of the most common classes of organic dyes are the azo
dyes. They have the following general structure:
These dyes have an EDG on one ring and an EWG on the
other, similar to disubstituted benzenes.
N N
EDGsEWGs
Azo Dyes
Azo Dyes are used to colour a variety of materials.
OH
N
N
NO2
Para Red
NN
NH2
H2N
Fast Brown
NNO3S
HO
SO3
Sunset Yellow (Food Yellow 3)
Azo Dyes
Many azo dyes are pH sensitive, which makes them useful pH
indicators.
NNO3S N
CH3
CH3
NHNO3S N
CH3
CH3
Yellow, pH > 4.4 Red, pH < 3.2
Methyl Orange
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Final Notes
UV-Vis not especially useful as a primary tool for determining
the structures of organic molecules. However, it is useful in
certain instances to distinguish between isomers.
Main contribution that UV-Vis makes to structure
determination is that it can readily identify the presence of
conjugated ππππ-systems and unique chromophores.
Final Notes
• UV-vis most widely used instrumental technique in
chemistry and science/medicine!
• Detection in chromatography.
• Monitoring reaction kinetics (chem, biol, medicine)
• Materials science, synthesis, analytical, inorganic, organic,
physical chemistry, biochemistry, biology, medical
applications, food industry.
(R), (S) Nomenclature• Different molecules (enantiomers) must have different
names.
• Configuration around the
chiral carbon is specified
with (R) and (S).
• Cahn – Ingold - Prelog
Chiral Molecules and Optical Activity Polarimetry
• Use monochromatic light, usually sodium D-line, 589 nm
• Movable polarizing filter to measure angle
• Clockwise = dextrorotatory = d or (+)
• Counterclockwise = levorotatory = l or (-)
• Optical activity
• Not related to (R) and (S)
=>
18
Specific Rotation
Observed rotation depends on the length of the cell and
concentration, as well as the strength of optical activity,
temperature, and wavelength of light.
[αααα] = αααα (observed)
c •••• l
c is concentration in g/mL
l is length of path in decimeters.
Racemic Mixtures
• Equal quantities of d- and l-enantiomers (or R and S forms).
• Notation: (d,l), (±±±±), (R,S)
• No optical activity.
• The mixture may have different b.p. and m.p. from the pure
enantiomers!
Normal UV-vis spectrum
Isotropic radiation
(unpolarized)
Same spectrum for both
enantiomers
It is possible to measure the optical rotation (of an
enantiomer solution) at different wavelengths by
using a monochromator.
The variation of optical rotation with wavelength
of the (plane) polarized light is called OPTICAL
ROTATORY DISPERSION, a.k.a. the ORD
spectrum
19
• Difference in molar absorptivity εεεε is called CIRCULAR DICHROISM
• Optically active compounds rotate the plane of linearly
polarized light
• Different εεεε for left and right circularly polarized light,
∆ε∆ε∆ε∆ε = εεεεL - εεεεR ≠≠≠≠ 0
• Related to the difference in refractive index for left and right
circularly polarized light
Normal UV-vis
CD Spectrum
Chiroptical Spectroscopy
• Optical rotatory dispersion (ORD) and circular
dichroism (CD) known as CHIROPTICAL methods
USES
• Deconvolution of overlapping UV-vis bands
• Relative chiralities of isomers
• Absolute configurations of molecules
• Biomolecules: amino acids, proteins, enzymes, DNA
• Macromolecules, polymers with ‘handedness’