THEORY and INTERPRETATION of ORGANIC SPECTRA H. D. Roth...
Transcript of THEORY and INTERPRETATION of ORGANIC SPECTRA H. D. Roth...
Organic Spectra Electronic Spectroscopy H. D. Roth
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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
ΔE = hν
n-σ*σ-σ*
π-π*n-π*
E
}
}
anti-bonding
non-bonding
bonding
σ*
σ
π
n
π*
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
c concentration (mol l–1
)
b pathlength of the cell (usually1 cm; sometimes1 mm)
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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)
bathochromichypsochromic shift Spectra of systems with more than one chromophore are additive,
unless the chromophores interact (charge transfer spectra, vide infra).
250 300 300250200 350 250 300200 350
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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
σ
C OOC
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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4. π-π* transitions are typical for alkenes and conjugated systems
ΔE = 2 β
ethene
λmax 165nm
ε 10,000α + β
α – βπ*
π
α – β
α + β
butadiene
λmax 217nm
ε 4,600
ΔE = 1.24 β
π
π*
Ψ1
Ψ2
Ψ3
Ψ4
Ψ1
Ψ2
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.
16,0008,0004,000214259239λmax
ε
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The structures of some naturally occurring anti-oxidants containing
extensive conjugated systems are shown below.
formed by addition of a hydroxyl ion. The equilibriumbetween the pseudobase and the base is independent of pHbut depends on water activity. The heterocycle of the skeletonmay open for both the base and the pseudobase. Theequilibria are further complicated by molecular stackingphenomena, which further affects color. In the kitchen pHis easily controlled, and colors of fruit-based desserts ordrinks may accordingly be adjusted.199
4.3. Textures in Food and How To Make ThemThe overall appeal of any food is determined not only by
its flavor but also, to a large extent, by its texture. Forexample, some foods need to be crisp and crunchy to beproperly enjoyed. No matter how good the flavor a “soggy”potato chip (crisp in the United Kingdom) will not taste right;
ice creams that are not properly smooth due to large icecrystals have an unappealing gritty texture, while a limp saladwill put off even the least discriminating diner. The controland modification of texture is therefore an important aspectof the kitchen repertoire and well worth discussing in somedepth.
Chefs know how to modify the texture of meats to producecrisp yet moist pastries and to prepare the lightest souffles.In many cases they follow long-winded and complex (butwell-tried and -tested) procedures to achieve their desiredresults. However, often, with a little understanding of theunderlying stability criteria, they can achieve the same resultwith far less trouble. A glance at any cookbook on how toproduce a simple mayonnaise shows that many cooks do nothave even a basic grasp of thermodynamics of emulsification.
Figure 15. Lycopene from tomatoes and carotene from carrots is red, while lutein and zeaxanthin, classified as xanthophylls (oxygencontaining), are yellow. Astaxanthin is the pink colorant in salmon.
Figure 16. Equilibria between different forms of anthocyanins affecting color. AH+ is a flavylium cation, A is the quinoidal base, A- isthe ‘anhydro’ base, B is the pseudobase, while C is the chalcone.
Molecular Gastronomy Chemical Reviews, 2010, Vol. 110, No. 4 2337
Role of Alkyl Groups butadiene λmax 217 ε 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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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.
α
β
β
O
R CC
O
H3C
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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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).
purpleyellowcolorless
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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:
n-π*π-π*
π-π*
601,600
12,600244280317
λmax ε transitionO
H3C
Another method of classification uses the type of chromophore, i. e.,
B enzenoid E thylenic
R adical like K onjugated
245 nm π-π* K
O
O
λmax transition type
435 nm n-π* R
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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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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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*
A* + D → A– + D+
D → 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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[Aδ–…..Dδ+] → [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). [A– solvent D+] vs [Aδ–…..Dδ+]
“solvent separated” “contact” radical ion pairs radical ion pairs
SSRIP 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 [Aδ–…..Dδ+] = Keq [A] [D] [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., 1,3,5-
trimethylbenzene (c)] should result in a red-shifted spectrum, as observed.
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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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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.