Organic Spectra
Electronic Spectroscopy
H. D. Roth
THEORY and INTERPRETATION of ORGANIC SPECTRA
H. D. Roth
UV/Vis (Electronic) Spectroscopy
Electrons are raised from σ, π, n levels
to
n, π∗, σ∗ levels.
All transitions are strictly quantized
Δ E = hν
σ*
π*
E
}
anti-bonding
n-π*
n
π
non-bonding
}
π-π*
n-σ*
σ-σ*
bonding
σ
Spectral Range
800 - 400 nm
Visible (conjugated π-systems)
400-190 nm
UV (near)
190-100 nm
Vacuum UV
This technique can be used quantitatively; in a typical application the
eluent of an HPLC chromatograph is detected by UV
Lambert–Beer Law
A = ε x c x b = log I0/I
I0/I
intensity of the incident/ transmitted light
ε
molar absorptivity or extinction coeffient (a characteristic
property of substances) may be solvent dependent
(hydrogen bonding solvents) general range of ε: 10 - 10
5
–1
c
concentration (mol l )
b
pathlength of the cell (usually1 cm; sometimes1 mm)
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Organic Spectra
Electronic Spectroscopy
H. D. Roth
For quantitative analysis:
Case 1: measure A, know ε and b calculate c;
Case 2: measure A, know c and b calculate ε.
Some solvents (cut off, nm)
cyclohexane
190
ethanol (95%)
198
hexane
187
methanol
198
CCl4
245
water
197
CHCl3
223
dioxane
215
CH2Cl2
215
isooctane
195
Beware of impurities (and sexist phrases):
("one man's signal is another's impurity")
Chromophore a functional group that absorbs UV
Bathochromic shift, a shift to longer wavelength (lower energies)
Hypsochromic shift, a shift to shorter wavelength (higher energies)
Auxochrome a group that causes a bathochronic shift (it shifts
absorption to a more accessible region)
hypsochromic
shift
bathochromic
Spectra of systems with more than one chromophore are additive,
unless the chromophores interact (charge transfer spectra, vide infra).
200
250
300
250
300
350
200
250
300
350
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Organic Spectra
Electronic Spectroscopy
H. D. Roth
Electronic Transitions
1.
σ → σ* transitions are typical for alkanes;
they require high energies, λmax <150 nm
2.
n-σ* transitions are typical for compounds containing one hetero atom
(O, N, halogen), thus have occupied n-orbitals:
(CH3)3N
λmax 199nm
ε
3950
Absorption Spectra of Haloalkanes
CH3Cl
λmax 173 nm
ε
200
C3H7Br
208 nm
300
CH3I
259 nm
400
CH3OH
177 nm
200
C5H11SH
224 nm
126
Comparing the spectra of three alkyl halides and two alkyl
chalcogenides, we note that there is a shift to lower energies (by 40-50 nm)
when going to the higher halogen or the higher chalcogen.
Rule (of thumb): the λmax of the next higher halogen or chalcogen is
shifted to lower energies (bathochromic) by 40-50 nm.
3.
n-π* transitions are typical for ketones
π∗
π
C
O
C
O
σ
The non-bonding (n-) orbital is orthogonal to the π*orbital
∴ transition is "forbidden"; it has low probability and low extinction
coefficient, ε.
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Organic Spectra
4.
Electronic Spectroscopy
H. D. Roth
π-π* transitions are typical for alkenes and conjugated systems
π*
Ψ2
α – β
ethene
λmax 165nm
ΔE = 2 β
Ψ1
π
α + β
π*
α – β
ε
10,000
Ψ4
Ψ3
ΔE = 1.24 β
Ψ2
π
α + β
butadiene
λmax 217nm
ε
4,600
Ψ1
Conjugated Double Bonds
ethene
λmax 165
ε
10,000
butadiene
217
21,000
hexatriene
263
36,000
octatetraene
304
3,300
Rule (of thumb): each double bond shifts the λmax to lower energies
(bathochromic) by approximately 50 nm.
However, you should be consider also that the configuration and
structure affects the absorption band of a diene in a major way.
λmax
ε
239
4,000
259
8,000
214
16,000
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r Gastronomy
Organic Spectra
Electronic Spectroscopy
H. D. Roth
The structures of some naturally occurring anti-oxidants containing
Chemical Reviews, 2010, Vol. 110, No.
extensive conjugated systems are shown below.
15. Lycopene from tomatoes and carotene from carrots is red, while lutein and zeaxanthin, classified as xanthophylls (
Role of Alkyl Groups
ng), are yellow. Astaxanthin is the pink colorant in salmon.
ε
butadiene
λmax 217
21,000
isoprene
222
11,000
2,5-dimethylhexadiene
241
13,000
Rule (of thumb): each alkyl group shifts the λmax to lower energies
(bathochromic) by 5 nm. The substituent effects on λmax are additive.
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Organic Spectra
Electronic Spectroscopy
H. D. Roth
The Woodward Rules, e.g. for polyenes, cyclic unsaturated systems,
unsaturated ketones, allow one to predict the λmax of an unknown
compound based on characteristic increments for substituents in each
position.
β
O
O
C
R
β
C
H3C
α
Calc. λmax :
215
base
10
α subst.
12
β subst.
237 nm (vs. 232 nm observed)
Benzene and Annulated Aromatic Systems
Increasing the conjugation (the number of annulated benzene rings
in an aromatic compound) shifts the λmax by ≥50 nm to lower energies.
This is demonstrated in the figure for benzene (blue), naphthalene
(yellow), anthracene (green), and tetracene (red).
Rule (of thumb): each annulated benzene ring shifts the λmax to
lower energies (bathochromic) by ≥50 nm.
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Organic Spectra
Electronic Spectroscopy
H. D. Roth
Just as is the case for polyenes, the structure also affects the absorption
band of aromatic π systems. For example, naphthalene (colorless) absorbs at
lower wavelength, than 1,6-methano-cyclodecapentaene (yellow), or azulene
(purple).
colorless
yellow
purple
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Organic Spectra
Electronic Spectroscopy
H. D. Roth
So far, we have classified electronic transitions by the type of orbitals
involved, e.g., n-π* or π-π*.
For example acetophenone has three bands:
λmax ε
O
H3C
transition
244
12,600
π-π*
280
1,600
π-π*
317
60
n-π*
Another method of classification uses the type of chromophore, i. e.,
B enzenoid
E thylenic
R adical like
K onjugated
O
λmax transition
type
245 nm
π-π*
K
435 nm
n-π*
R
O
Banded Spectra
Many typical electronic spectra show only broad, “featureless”
bands. In special cases, however, the spectra have some fine structure:
they are “banded”. The spectra of the aromatic molecules (vide supra) are
excellent examples. This feature is due to vibrationally excited levels in
the “product” of the spectroscopic transition. In our case, the π-π *
transition of an electron from HOMO to LUMO generates an excited state
with two singly occupied orbitals. Although the ground state has only one
vibrational level populated (vibrational relaxation is fast), the excited state
may be populated into several vibrationally excited states with decreasing
probabilities. The vibrational spacing can be determined from the
separation of the lines in the spectrum.
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Organic Spectra
Electronic Spectroscopy
H. D. Roth
Some excited states decay by emitting the energy difference in form of a
photon. Depending on the nature of the excited state this process is called
fluorescence or phosphorescence; the general term is luminescence.
Emission occurs from the vibrational ground level of the excited state
(vibrational relaxation is fast) to several vibrationally excited levels of the
ground state. These considerations explain why luminescence always occurs
at wavelengths slightly longer than the excitation wavelength.
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Organic Spectra
Electronic Spectroscopy
H. D. Roth
Charge Transfer Spectra
We consider the electronic spectra of an electron donor and an
electron acceptor in a polar solvent. Upon irradiation either donor
or acceptor gives rise to an excited state (D* or A*) which can be
quenched by electron transfer (ET) generating a pair of radical ions
(A–
D+). Because ET can occur before the two reacting molecules
are in contact the two ions are separated by a few solvent molecules
(solvent separated radical ion pair, SSRIP).
A
A* +
D
D
D* +
A*
→
→
A–
→
D+
D*
→
A
+
A–
+
D+
Some donors and acceptors may interact without light energy to form
charge transfer complexes with characteristic “charge transfer” (CT)
spectra. A mixture of a donor, D (red band) and an acceptor, A (blue band),
show a new band in the visible region (green band).
D+A
→
Aδ–…..Dδ+
Irradiation of such a mixture at wavelengths where only D or A
absorb generates SSRIPs; irradiating the “donor-acceptor” (CT) complexes
at the characteristic “charge transfer” band also gives rise to radical ion
pairs;
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Organic Spectra
Electronic Spectroscopy
[Aδ–…..Dδ+]
→
H. D. Roth
[Aδ–…..Dδ+]
however, these ion pairs are different from the SSRIPs obtained upon
irradiation of either A or D, because no solvent molecules separate the two
ions (“intimate” or “contact” radical ion pairs, CRIP).
vs
[A– solvent D+]
[Aδ–…..Dδ+]
“solvent separated”
radical ion pairs
SSRIP
“contact”
radical ion pairs
CRIP
The concentration of the charge transfer complex (green spectrum) is
determined by [A], [D] and Keq (an intrinsic constant for each pair)
Keq = [Aδ–…..Dδ+] or
[A] [D]
[Aδ–…..Dδ+] = Keq [A] [D]
The UV spectra of solutions of tetrafluorobenzoquinone (3.5x10–3 M)
in acetonitrile (a) and benzene (b) are an interesting example. The benzene
solution shows a tenfold increase in ε, an effect much too large for a simple
solvent effect. If this were a CT spectrum, a better donor [e.g., 1,3,5trimethylbenzene (c)] should result in a red-shifted spectrum, as observed.
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Organic Spectra
Electronic Spectroscopy
H. D. Roth
Summary
Irradiation of acceptor (top), donor (bottom) or a CT complex (left)
generates either SSRIPs (top, bottom) or CRIPs (left). Some CRIPSs may
diffuse apart to form SSRIPs, and these, in turn, may diffuse apart to form
free radical ions (not coordinated with a counter ion).
Time-Resolved Spectroscopy
The application of time- resolved spectroscopy has become an
important tool for the study of short-lived reactive intermediates. Modern
laser spectroscopy allows the study reactions on timescales of ms, µs, ns, ps
and even fs. The example below covers the range of nano- to microseconds;
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Organic Spectra
Electronic Spectroscopy
H. D. Roth
open circles: irradiation of p-methoxy styrene (D) forms the radical cation.
filled circles: irradiation of chloranil (A) in the presence of D forms both
ions.
If the reactive intermediate (diphenylamine radical cation, 695 nm) is
stabilized by incorporation into a zeolite, the conversion to diphenylaminyl
radical (460 nm) by deprotonation can be studied over a period of several
hours.
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