THEORY and INTERPRETATION of ORGANIC SPECTRA H. D. Roth...

13
Organic Spectra Electronic Spectroscopy H. D. Roth 1 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 I 0 /I I 0 /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)

Transcript of THEORY and INTERPRETATION of ORGANIC SPECTRA H. D. Roth...

Page 1: THEORY and INTERPRETATION of ORGANIC SPECTRA H. D. Roth …ccb.rutgers.edu/sites/default/files/coursefiles/... · n-σ* transitions are typical for compounds containing one hetero

Organic Spectra Electronic Spectroscopy H. D. Roth

1

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)

Page 2: THEORY and INTERPRETATION of ORGANIC SPECTRA H. D. Roth …ccb.rutgers.edu/sites/default/files/coursefiles/... · n-σ* transitions are typical for compounds containing one hetero

Organic Spectra Electronic Spectroscopy H. D. Roth

2

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

Page 3: THEORY and INTERPRETATION of ORGANIC SPECTRA H. D. Roth …ccb.rutgers.edu/sites/default/files/coursefiles/... · n-σ* transitions are typical for compounds containing one hetero

Organic Spectra Electronic Spectroscopy H. D. Roth

3

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.

Page 4: THEORY and INTERPRETATION of ORGANIC SPECTRA H. D. Roth …ccb.rutgers.edu/sites/default/files/coursefiles/... · n-σ* transitions are typical for compounds containing one hetero

Organic Spectra Electronic Spectroscopy H. D. Roth

4

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

Page 5: THEORY and INTERPRETATION of ORGANIC SPECTRA H. D. Roth …ccb.rutgers.edu/sites/default/files/coursefiles/... · n-σ* transitions are typical for compounds containing one hetero

Organic Spectra Electronic Spectroscopy H. D. Roth

5

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.

Page 6: THEORY and INTERPRETATION of ORGANIC SPECTRA H. D. Roth …ccb.rutgers.edu/sites/default/files/coursefiles/... · n-σ* transitions are typical for compounds containing one hetero

Organic Spectra Electronic Spectroscopy H. D. Roth

6

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.

Page 7: THEORY and INTERPRETATION of ORGANIC SPECTRA H. D. Roth …ccb.rutgers.edu/sites/default/files/coursefiles/... · n-σ* transitions are typical for compounds containing one hetero

Organic Spectra Electronic Spectroscopy H. D. Roth

7

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

Page 8: THEORY and INTERPRETATION of ORGANIC SPECTRA H. D. Roth …ccb.rutgers.edu/sites/default/files/coursefiles/... · n-σ* transitions are typical for compounds containing one hetero

Organic Spectra Electronic Spectroscopy H. D. Roth

8

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.

Page 9: THEORY and INTERPRETATION of ORGANIC SPECTRA H. D. Roth …ccb.rutgers.edu/sites/default/files/coursefiles/... · n-σ* transitions are typical for compounds containing one hetero

Organic Spectra Electronic Spectroscopy H. D. Roth

9

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.

Page 10: THEORY and INTERPRETATION of ORGANIC SPECTRA H. D. Roth …ccb.rutgers.edu/sites/default/files/coursefiles/... · n-σ* transitions are typical for compounds containing one hetero

Organic Spectra Electronic Spectroscopy H. D. Roth

10

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;

Page 11: THEORY and INTERPRETATION of ORGANIC SPECTRA H. D. Roth …ccb.rutgers.edu/sites/default/files/coursefiles/... · n-σ* transitions are typical for compounds containing one hetero

Organic Spectra Electronic Spectroscopy H. D. Roth

11

[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.

Page 12: THEORY and INTERPRETATION of ORGANIC SPECTRA H. D. Roth …ccb.rutgers.edu/sites/default/files/coursefiles/... · n-σ* transitions are typical for compounds containing one hetero

Organic Spectra Electronic Spectroscopy H. D. Roth

12

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;

Page 13: THEORY and INTERPRETATION of ORGANIC SPECTRA H. D. Roth …ccb.rutgers.edu/sites/default/files/coursefiles/... · n-σ* transitions are typical for compounds containing one hetero

Organic Spectra Electronic Spectroscopy H. D. Roth

13

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.