Electromagnetic Spectrum
Chem 325
UltraViolet-Visible
Spectroscopy
λν = c
E = hν
ν = hc/λ
λ
Organic Spectroscopy
Absorption of different EM radiation produces different
molecular energy changes
1.
2.
3.
4.
5.
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.
Radiowaves
Nuclear Spin Transitions (NMR)
Microwaves
Electron Spin Transitions (ESR)
Infrared
Vibrational Transitions
Visible-NIR
Raman Scattering (Vibrational)
Ultraviolet-Visible Electronic Transitions
1
Transmittance and Absorbance
Beer-Lambert Law
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 related to concentration in a non-linear way
(exponential), so it is usually converted to the much more
useful quantity ‘absorbance’, where:
T=
I
I0
Transmittance is measured in the following way:
Absorbance = A = log
I0
I
Absorbance is related to concentration in a linear way
according to the Beer-Lambert Law.
Absorbance = A = εlc
Where l is the pathlength in centimeters
And c is the concentration of the absorbing species in Molarity
(mol/L)
And ε is the molar absorptivity or molar extinction coefficient
Molar Absorptivities
No absorption gives ε = 0!
Often given as logarithmic values
e.g. ε = 23,500 equivalent to log ε = 4.37
Units of ε
ε=
A
cl
L mol-1 cm-1 or M-1 cm-1
- very rarely stated explicitly!
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.
2
Absorbance
Molar absorptivities may be very large for strongly absorbing
compounds (ε >10,000) and very small if absorption is weak
(ε = 10 to 100).
Absorbance Spectra
λmax λmax λmax λmax
1.5
1
0.5
0
255
280
305
330
355
380
Wavelength (nm)
2
Electronic Transitions
Chromophores
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.
Solvents
• 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: π-bond systems
Solute-Solvent Interactions
Solvents generally required due to large absorptivities
Must be ‘transparent’, i.e. not absorb
OH
Must be very pure
Solvent cutoffs (where they start to absorb!):
acetonitrile
chloroform
cyclohexane
1,4-dioxane
95% ethanol
190 nm
240
195
215
205
n-hexane
methanol
iso-octane
water
trimethylphosphate
201 nm
205
195
190
210
3
Electronic Energy Levels
Electronic Transitions
Electronic Transitions
Ethylene Orbital and Transitions
Organics that absorb in the UV and Visible region
(200 – 800 nm) generally contain one or more πbonds.
4
Molecular Orbital Diagrams
Conjugated molecules such as 1,3-butadiene absorb
at longer wavelengths.
Excited States
Electron Spin Selection Rule
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.
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
α
S0 → T1 is spin-forbidden
T1
ground state (S0)
S1
S2
S3
5
State Diagrams
Electronic and Vibrational Transitions
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
Rigid Compounds
Non-Rigid Compounds
In rigid molecules, like polyaromatic hydrocarbons,
excitation to each vibrational level is resolved. These
narrow peaks appear in sets, called ‘bands’.
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.
0.8
ν=1
ν=2
0.4
1.6
S0 S1
ν=3
0.2
Absorbance
Absorbance
2
ν=0
0.6
1.2
0.8
S0 S1
0.4
0
275
325
375
Wavelength (nm)
425
0
Vibronic Structure
200
250
300
350
Wavelength (nm)
6
Types of Excitation
What Factors Affect Absorption?
Only two types of excitation commonly encountered, depending
on the orbitals involved. These are:
A) π*
 n (or n π*)
Electron from an n orbital (nonbonding or lone pair) is excited into a
π* (antibonding) orbital. Commonly
the longest wavelength absorption for
ketones and aldehydes. π*
n
transitions are “forbidden”, so these
usually give weak bands (small ε)
B) π*
 π (or π π*)
Electron from a π-orbital is excited into a π*-orbital. This is
commonly the longest wavelength absorption for unsaturated
hydrocarbons.
Substituent Effects
Empirical rules: help predict the λmax of a specific compound
from the base chromophore and what substituents are
attached to it.
H2C
λmax nm
CH2
Hyperchromic
Hypsochromic
ε
175
15,000
217
21,000
258
35,000
465
125,000
β-carotene
Bathochromic
Hypochromic
200 nm
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.
Chromophores and Substituents
Substituents may have any of four effects on a chromophore
1. Bathochromic shift (red shift) – a shift to longer λ: lower E
2. Hypsochromic shift (blue shift) – shift to shorter λ: higher E
3. Hyperchromic effect – an increase in intensity: higher ε
4. Hypochromic effect – a decrease in intensity: lower ε
ε
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).
700 nm
O
n π* 280
π π* 189
12
900
O
n π* 280
π π* 213
27
7,100
7
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-Fieser Rules for Dienes
Next we add the contribution from any attached
groups (substitutents).
Group
Extended
conjugation
Each exo-cyclic C=C
Alkyl
-OCOCH3
-OR
-SR
-Cl, -Br
-NR2
-Ph
Woodward and Fieser developed a set of empirical rules to
help predict what the λmax 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, λmax = 217 nm
Examples
Experimental value
Allylidenecyclohexane
acyclic butadiene =
one exocyclic C=C
2 alkyl subs.
Experimental value
+5
+5
+0
+6
+30
+5
+60
+60
Cyclic Dienes
Isoprene
acyclic butadiene =
one alkyl subs.
Increment
+30
217 nm
+ 5 nm
222 nm
220 nm
217 nm
+ 5 nm
+10 nm
232 nm
237 nm
There are two major types of cyclic dienes, with two different
base values.
Heteroannular (transoid): Homoannular (cisoid):
ε = 5,000 – 15,000
base λmax = 214
ε = 12,000-28,000
base λmax = 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 λmax
is used.
8
Example
heteroannular diene = 214 nm
3 alkyl subs. (3 x 5) +15 nm
1 exo C=C
+ 5 nm
234 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:
Experimental value 235 nm
C OH
O
abietic acid
Types of Excitation
Only two types of excitation commonly encountered, depending
on the orbitals involved. These are:
A) π*
 n (or n π*)
Electron from an n orbital (nonbonding or lone pair) is excited into a
π* (antibonding) orbital. Commonly
the longest wavelength absorption for
ketones and aldehydes. π*
n
transitions are “forbidden”, so these
usually give weak bands (small ε)
B) π*
 π (or π π*)
Electron from a π-orbital is excited into a π*-orbital. This is
commonly the longest wavelength absorption for unsaturated
hydrocarbons.
C OH
O
levopimaric acid
Structure Determination!
abietic acid
heteroannular diene = 214 nm
4 alkyl subs. (4 x 5)
1 exo C=C
C OH
O
+20 nm
+ 5 nm
239 nm
levopimaric acid
homoannular diene = 253 nm
4 alkyl subs. (4 x 5)
1 exo C=C
C OH
O
+20 nm
+ 5 nm
278 nm
9
WARNING !!
Woodward-Fieser Rules
Three common errors:
R
This compound has three exocyclic double
bonds; the indicated bond is exocyclic to two
rings
See Pavia, Chapter 7, Section 10
for the Rules and worked examples!!
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
Product Analysis
Product Analysis
∗
CH3
Base value
CH3
H2N
214
Homoannular
39
Alkyl substituents 3 × 5
15
Exocyclic C=C
H2N
∗
H2N
H2N
O
CH3
CH3
CH3
Two possible enamines from this ketone. Can UV-vis tell
them apart?
NH2 group
Predicted λmax
5
60
333 nm
Base value
Alkyl substituents 3 × 5
Exocyclic C=C
NH2 group
214
15
5
60
Predicted λmax 294 nm
10
The Carbonyl Group Absorptions
Woodward-Fieser Rules for Enones
Enones have a strong π*
 π band and
a longer wavelength π*
 n that is
usually around 100 times less intense.
•Two possible absorptions
– Longer wavelength (lowest E) is n → π*
– Symmetry ‘forbidden’, thus low ε
If the enones are conjugated enough,
the π*
 π band completely swamps out
the π*
 n band.
Also, the positions of π*
 π bands are
much easier to predict with empirical
rules than π*
 n bands.
– Shorter wavelength (highest E) is π → π*
– Symmetry ‘allowed’, thus high ε
– Can conjugate with alkenes π-systems
For these reasons, the WoodwardFieser rules for enones ONLY APPLY
TO THE π*
 π 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
δ γ β α
δ C C C C C
O
Double bond extending conjugation
30
Alkyl group or ring residue
α, β, γ and higher
-OH
α, β, γ and higher
35, 30, 18
-OR
α, β, γ, δ
35, 30, 17, 31
10, 12, 18
α, β, δ
6
-Cl
α, β
15, 12
-Br
α, β
25, 30
β
95
-O(C=O)R
-NR2
Exocyclic double bond
5
Homocyclic diene component
39
Aldehydes and Acids/Esters
Unsaturated system
Aldehyde
With α or β alkyl groups
With α,β or β,β alkyl groups
With α,β,β alkyl groups
Acid or ester
With α or β alkyl groups
With α,β or β,β alkyl groups
Group value – exocyclic α,β double bond
Group value – endocyclic α,β bond in 5
or 7 membered ring
Base Value
208
220
230
242
208
217
+5
+5
11
Solvent Effects on Enones
Examples
α
For enones, the solvent will also affect the position of λmax.
Solvent correction
cyclic enone =
2 x β - alkyl subs.(2 x 12)
O
β
Experimental value
Increment
Water
+8
Ethanol, methanol
0
Chloroform
-1
Dioxane
-5
Ether
-7
Hydrocarbon
-11
R
cyclic enone =
extended conj.
β -ring residue
δ-ring residue
exocyclic double bond
O
Experimental value
Product Analysis
Br
CH3
CH3
Br2
CH3
O
H
O
215
Base value
215
12
β -alkyl ×2
24
H
H
H
CH3
O
O
O
215 nm
+30 nm
+12 nm
+18 nm
+ 5 nm
280 nm
280 nm
Product Analysis
Bromination a steroid can produce two possible products.
Dehydrobromination gives two enones. Can we tell them apart?
CH3
215 nm
+24 nm
239 nm
238 nm
Br
-HBr
Base value
-HBr
β -alkyl
227 nm
CH3
CH3
Exocyclic C=C
5
244 nm
O
O
H
12
UV-Vis of Aromatics
Aromatics
Benzene has 6 π-MOs which leads to a number of
transitions.
π6 ∗
π4 ∗
π5 ∗
π2
π3
π1
Benzene has three main bands, the E, K, and B bands.
The E band is also called the primary
band. It is strongly allowed (εε = 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 (εε = 230)
Aromatics in General
Substituent effects
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.
G
G
G
G
1. Substituents with lone pairs will red-shift the primary and
secondary bands.
2. Protonating or deprotonating functional groups changes
how they affect the primary and secondary bands.
Primary
Secondary
Substituent
λmax
ε
λmax
ε
-H
203.5
7,400
254
204
-OH
211
6,200
270
1,450
-O-
235
9,400
287
2,600
-NH2
230
8,600
280
1,430
-NH3+
203
7,500
254
169
-C(O)OH
230
11,600
273
970
-C(O)O-
224
8,700
268
560
3. Functional groups that extend the conjugation red shift
the primary and secondary bands.
13
Electronic Effects
Di-Substituted Aromatics
Electron withdrawing groups (EWG) red-shift the primary
band and electron donating groups (EDG) red-shift both
bands.
Primary
Substituent
Electron withdrawing
Electron donating
-H
1) If both are EWG or both are EDG, the effect is equal to the
stronger of the two.
Secondary
λmax
ε
λmax
ε
203.5
7,400
254
204
-CH3
207
7,000
261
225
-Cl
210
7,400
264
190
-Br
210
7,900
261
192
-OH
211
6,200
270
1,450
-OCH3
217
6,400
269
1,480
-NH2
230
8,600
280
1,430
-CN
224
13,000
271
1,000
C(O)OH
230
11,600
273
970
-C(O)H
250
11,400
-C(O)CH3
224
9,800
-NO2
269
7,800
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.
_
O + O
N
OH
Woodward and Fieser Again…
_
O
_
N
O
OH
+
Woodward-Fieser Rules for Aromatics
Woodward and Fieser were able to come up with some rules
for predicting the λmax of a subset of aromatic compounds –
those containing a carbonyl attached to the ring.
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.
Substituent increment
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.
O
R
G
o
m
p
Alkyl or ring residue
3
3
10
-O-Alkyl, -OH, -O-Ring
7
7
25
-O-
11
20
78
-Cl
0
0
10
Parent Chromophore
λmax
-Br
2
2
15
R = alkyl or ring residue
246
-NH2
13
13
58
R=H
250
-NHC(O)CH3
20
20
45
R = OH or O-Alkyl
230
-NHCH3
20
20
G
-N(CH3)2
73
85
14
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.
β -carotene
Appearance of Coloured Compounds
The appearance of coloured compounds can be determined
by looking at the colour wheel below.
Consider β-carotene
β-carotene, λmax = 455 nm
This molecule absorbs at 455 nm
(in the blue), therefore the
compound will appear orange.
Select the λmax 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.
β-carotene is the primary
pigment that colours carrots.
15
Lycopene and Indigo
Azo Dyes
Similarly, lycopene absorbs at 474 nm and appears red. This
compound is the main pigment found in tomatoes.
One of the most common classes of organic dyes are the azo
dyes. They have the following general structure:
N N
lycopene, λmax = 474 nm
O
H
N
EWGs
N
H
EDGs
O
indigo
These dyes have an EDG on one ring and an EWG on the
other, similar to disubstituted benzenes.
λmax for indigo is at 602 nm – in the orange region of the
spectrum – this is absorbed, the compliment is now indigo.
Azo Dyes
Azo Dyes
Many azo dyes are pH sensitive, which makes them useful pH
indicators.
Azo Dyes are used to colour a variety of materials.
NO2
Methyl Orange
HO
N
N
H2N
OH
N N
O 3S
N N
O3S
NH2
Yellow, pH > 4.4
SO3
Para Red
Fast Brown
N N
CH3
N
CH3
O3S
H
N N
CH3
N
CH3
Red, pH < 3.2
Sunset Yellow (Food Yellow 3)
16
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 π-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.
Polarimetry
Chiral Molecules and Optical Activity
(R), (S) Nomenclature
• Different molecules (enantiomers) must have different
names.
• Configuration around the
chiral carbon is specified
with (R) and (S).
• Cahn – Ingold - Prelog
•
•
•
•
•
•
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)
=>
17
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.
[α
α] = α (observed)
c•l
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!
c is concentration in g/mL
l is length of path in decimeters.
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
18
• Optically active compounds rotate the plane of linearly
polarized light
• Different ε for left and right circularly polarized light,
∆ε = εL - εR ≠ 0
• Related to the difference in refractive index for left and right
circularly polarized light
Normal UV-vis
CD Spectrum
• Difference in molar absorptivity ε is called CIRCULAR DICHROISM
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’
19