Spectroscopic Methods in Organic Chemistry, 2nd Edition 2007 - M. Hesse - E-Book

Spectroscopic Methods in Organic Chemistry, 2nd Edition 2007 E-Book

M. Hesse

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Spectroscopic Methods in Organic Chemistry covers all aspects of modern spectroscopic methodology. It provides the necessary equipment for the application of spectroscopic methods in organic chemistry, as required as part of chemistry courses in all universities. The following methods are explained and examples given: - UV/Vis Spectroscopy - Infrared (IR) and Raman Spectroscopy - Nuclear Magnetic Resonance Spectroscopy (NMR) - Mass spectrometry (MS) The layout and many tables help to introduce the reader to spectroscopy. The extensive and thorough approach makes the text the first choice both as a companion for the professional chemists and as a refresher course in practical spectroscopy.

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Spectroscopic Methods in Organic Chemistry

Preface

Almost 30 years passed since the first German edition of this textbook on spectroscopic methods for the structure determination of organic compounds appeared. Many improvements and refinements of the spectroscopic techniques were achieved in the past three decades - in particular in nuclear magnetic resonance (NMR) and mass spectrometry (MS). The rapid progress in computer technologies was a major prerequisite for the enormous development of instrumental analysis.

Elucidations of molecular structures (constitution, configuration and conformation) can be performed in a short time and with an accuracy which has never been attained before.

Another achievement concerns the study of smaller and smaller probes down to picogram or even femtogram quantities. Moreover, the analysis of multi-component mixtures attached steadily increasing importance.

The detection limit of undesirable or detrimental byproducts has to be lowered more and more. Nature provides many amazing examples for extremely high analytical sensitivity - like the nose of police dogs, which follow the track of criminals, or the sensors of male butterflies which identify the 'one and only' female in a world of fragrances and nasty smells.

Trace analyses and structure identifications play an important role for pharmacologically or biologically active substances and their metabolites. Most people are aware of these problems in the context of doping in sports and body-building. Another widespread area of analytical investigations is caused by environmental pollutions with toxic compounds or harmful, dangerous materials.

Since this book is also directed at students, who start to learn structure determinations of organic compounds, in all chapters strong emphasis is on possible sources of error, adequate troubleshooting and error correction. It is always recommended to apply as many different methods as possible for the solution of difficult analytical problems. Purity, for instance, is a question of benchmark.

Impurities of products may have many and often unexpected sources (starting compounds, solvents, auxiliary materials, decomposition products, softeners of used plastics, impurities present in the device, etc.). Normally the purity of an organic compound is checked by a correct elemental analysis. Many scientific journals nowadays accept high resolution mass spectrometry (HRMS) as an alternative; however, HRMS provides a proof for the empirical formula but not for the purity of the compound.

Chemo-, regio- and stereoselectivities are major challenges in modern organic chemistry. Therefore special attention has to be turned on the presence of isomers with similar spectroscopic properties. Statements about the selectivity of organic reactions should include measurements of the reaction mixtures and not only of the isolated (and possibly enriched) product charges.

We tried to coordinate all these viewpoints in a textbook, which presents the 'state of the art' and discusses difficult applications without going too deep into instrumental and software details.

Compared to the previous English edition, a large number of supplements and methodical improvements was integrated. UV/VIS: Conjugated oligomers and polymers, aggregates and charge-transfer complexes were added; the chapter about chiroptical methods was extended. IR/Raman: More emphasis was put on FT IR and data processing. NMR: Newer, more sophisticated 1D, 2D and 3D techniques were added (TOCSY, HOHAHA, HMQC, HSQC, HMBC); the use of combined methods for a complete assignment of all 1H and 13C signals of a compound to certain nuclei is discussed in detail. MS: FT-ICR mass spectrometry was added; the references were actualized and classified in methods, applications and classes of compounds. Combined examples: The number of examples was doubled from 7 to 14. Additionally, the combination of separation processes and spectroscopic characterization was taken into account in all chapters.

This book is a translation of the 7th German edition which appeared in 2005. We are most grateful to Dr. Richard Dunmun and Dr. Martin Murray for carrying out the translation and to our co-workers Heinz Kolshorn, Dr. Norbert Hanold, Dr. Andrea Schulz (University of Mainz), Dr. Laurent Bigler, Dr. Manuel Tzouros, and Dr. Stefen Bienz (University of Zürich) for their suggestions and measurements, which make the book readable for beginners and on the other side helpful for experts.

Zürich, Mainz, Ludwigshafen

July 2007

Manfred Hesse

Herbert Meier

Bernd Zeeh

Contents

1 UV/Vis Spectroscopy

H. Meier

1 Theoretical Introduction

1.1 Electronic Transitions

1.2 Light Absorption and the Spectrum

2 Sample Preparation and Measurement of Spectra

3 Chromophores

3.1 Individual Chromophoric Groups and their Interactions

3.2 Olefins, Polyenes

3.3 Benzene and Benzenoid Aromatics

3.4 Carbonyl Compounds

3.5 Conjugated Oligomers and Polymers

3.6 Aggregated Molecules, Charge-Transfer Complexes

4 Applications of UV/Vis Spectroscopy

5 Derivative Spectroscopy

6 Chirooptical Methods

Supplementary Literature

2 Infrared and Raman Spectroscopy

B. Zeeh

1 Introduction

2 Basic Principles and Selection Rules

3 IR Spectrometers

3.1 Classical (scanning) IR spectrometers

3.2 Fourier-Transform IR Spectrometers

3.3 Coupled Techniques

4 Sample Preparation

4.1 Measurements in the Gas Phase

4.2 Measurements on Liquids

4.3 Measurements in Solution

4.4 Measurements in the Solid State

5 The IR Spectrum

6 Overview of Characteristic Absorptions

7 IR Absorptions of Single Bonds to Hydrogen

7.1 (C—H) Absorptions

7.2 (O—H) and (N—H) Absorptions

8 IR Absorptions of Triple Bonds and Cumulated Double Bonds

9 IR Absorptions of Double Bonds

10 Typical IR Absorptions of Aromatic Compounds

11 IR Absorptions in the Fingerprint Region

12 Examples of IR-Spectra

13 Information Technology as an Aid to IR Spectroscopy

14 Quantitative IR Spectroscopy

15 Raman Spectroscopy

15.1 The Raman Effect

15.2 Selection Rules

15.3 Raman Spectrometers

15.4 Applications

Literature

3 Nuclear Magnetic Resonance Spectroscopy

H. Meier

1. Physical Principles

1.1 The Resonance Phenomenon

1.2 Chemical Shift

1.3 Spin-Spin Coupling

1.4 Line Widths

1.5 Intensity

2. NMR Spectra and Molecular Structure

2.1 Molecules with “Rigid” Atomic Positions

2.2 Intramolecular Motion

2.3 Chemical Exchange Processes

3. 1H NMR Spectroscopy

3.1 Sample Preparation and Measurement of Spectra (CW and PFT Techniques)

3.2 1H Chemical Shifts

3.3. 1H — 1H Coupling

3.4 Coupling to Other Nuclei

3.5 Correlation of 1H Shifts with Structural Features

3.6 Increment Systems for Estimating 1H Shifts

3.7 1H NMR Data of Representatives of the Commoner Classes of Compounds

3.8 Specialised Techniques

4. 13C NMR spectroscopy

4.1 Sample Preparation and Measurement of Spectra

4.2 13C Chemical Shifts

4.3 13C,1H Couplings

4.4 Coupling of 13C to Other Nuclei (D,F,N,P)

4.5 13C,13C Couplings

4.6 Correlation of 13C Shifts with Structural Features

4.7 Increment Systems for the Estimation of 13C Shifts

4.8 Special Methods

5 Combination of 1H and 13C NMR Spectroscopy

5.1 Complete Assignment of 1H and 13C NMR Signals

5.2 Use of Databases

5.3 1H and 13C NMR Data of representatives of the most important Classes of Compoundsa

6 NMR of other Nuclei

6.1 19F NMR Spectroscopy

6.2 31P NMR Spectroscopy

6.3 15N NMR Spectroscopy

6.4 Other Nuclei

4 Mass Spectra

M. Hesse

1 Introduction

2 Instrumentation and the Recording of Spectra

2.1 The Principle of the Mass Spectrometer

3 Fragmentation of Organic Compounds

4 The Main Fragmentation Reactions of Organic Compounds

4.1 α-Cleavage

4.2 Benzyl- and Allyl-Cleavage

4.3 The Cleavage of “Non-Activated” Bonds

4.4 The Retro Diels-Alder Reaction (RDA Reaction)

4.5 The McLafferty Rearrangement

4.6 The Onium Reaction

4.7 Loss of CO

5 Thermal Reactions in the Mass Spectrometer

5.1 The Most Important Types of Thermal Reactions5

5.2 Recognition of Thermal Reactions

5.3 Prevention of Thermal Reactions in the Mass Spectrometer

6 Mass Spectra of Contaminated Samples and Mixtures

6.1 Solvents

6.2 Foreign Substances in Solvents

6.3 Foreign Substances in Reagents

6.4 Materials from Laboratory Apparatus

6.5 Contaminants from Thin-Layer Chromatography Plates

7. Isotopic Labelling Reactions

7.1 H/D Exchange Reactions

7.2 Transformations of Functional Groups under Deuterating Conditions

7.3 Determination of the Degree of Labelling

8 Additional Ionisation Methods

8.1 Ionisation Methods

8.2 Atmospheric Pressure Chemical Ionisation (APCI)

8.3 Chemical Ionisation (CI)

8.4 Direct Chemical Ionisation (DCI)

8.5 Electrospray Ionisation (ESI)

8.6 Fast Atom Bombardment (FAB)

8.7 Field Desorption (FD)

8.8 Field Ionisation (FI)

8.9 Cation Addition Mass Spectroscopy

8.10 Laser Desorption/Ionisation Mass Spectrometry (LDI)

8.11 MALDI

8.12 Photoionisation (PI)

8.13 Secondary Ion Mass Spectrometry (SIMS)

8.14 Thermal Desorption Mass Spectrometry (TD)

8.15 The Thermospray Ionisation Procedure28 (TSI)

9 Other Aspects of Mass Spectrometry and Terminology

9.1 Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

9.2 Field Ionisation Kinetics (FIK)

9.3 Measurement of High Masses

9.4 Ion Trap Spectrometer

9.5 The Coupling of other Instruments to Mass Spectrometers

9.6 Multiply Charged Ions

9.7 The Memory Effect

9.8 Neighbouring Group Participation Reactions

9.9 Quadrupole Mass Analysers

9.10 Spectral Libraries

9.11 Stereoisomers

9.12 Collision Activation (CA)

9.13 Tandem Mass Spectrometry

9.14 Metastable Signals

10 Tables for Use in Mass Spectrometry

10.1 List of Frequently Occurring Ions and Characteristic Mass Differences Resulting from Mass Spectrometric and Chemical Reactions

10.2 Mass Differences Between the Reactant and Product of Frequently used Chemical Reactions

10.3 Isotopic ratios in compounds containing Cl and Br

10.4 Mass spectra of solvents

10.5 Mass spectra of common impurities

10.6 Mass Numbers and Abundances of the Isotopes of Naturally Occurring Elements

Literature

5 Combined Examples

Introduction

Examples 1–14

Solutions for the Example 1–14

1 UV/Vis Spectroscopy

1. Theoretical Introduction

2. Sample Preparation and Measurement of Spectra

3. Chromophores

4. Applications of UV/Vis Spectroscopy

5. Derivative Spectroscopy

6. Chirooptical Methods

1 Theoretical Introduction

1.1 Electronic Transitions

Electromagnetic radiation is characterised by the wavelengthλ or the frequency ν. These values are connected with each other by the equation

c is the velocity of light (in vacuum ≈2.991010 cm · s−1). A quantum of light with frequency ν has the energy

When visible light of a particular spectral colour is absorbed, the human eye recognises the complementary colour:

absorbed spectral colour

complementary colour

violet

yellow-green

blue

yellow

green-blue

orange

blue-green

red

green

purple

yellow-green

violet

yellow

blue

orange

green-blue

red

blue-green

purple

green

If the energy is based on a quantum or an individual atomic or molecular process, the customary unit is the electron volt (eV). For a mole, i.e., 6.02·1023 quanta of light, the energy is given in kJ. Energy and wavenumber are directly proportional to each other. For conversions the following relationships are recommended:

1000 cm−1 ≅ 12 kJ·mol−1

1 kJ·mol−1 ≅ 84 cm−1

If light with the appropriate frequency ν meets a molecule in the ground stateψ0, it can be absorbed and raise the molecule to an electronic excited stateψ1. By spontaneous emission or by stimulated emission, caused by the light rays, the system can return to the ground state. The word “can” in these senses expresses the transition probability of the two radiative processes, absorption and emission (▶ Fig. 1.2).

The connection with the orbitals involved in the electronic transition is shown in ▶ Fig. 1.3. The difference in energy between the lowest unoccupied orbital (LUMO) and the highest doubly occupied orbital (HOMO) is considerably greater than the excitation energy A for the transition from the singlet ground state S0 to the first excited singlet state S1. The difference arises from the different electronic interactions (Coulomb term J, exchange term 2K). The singlettriplet splitting in this approximation is 2K. Since K > 0 the lowest triplet state T1 is always below S1. Molecules, which have the same HOMO-LUMO gap, can have quite different excitation energies. The colourless anthracene represents a classical example, it has the same HOMO-LUMO energy difference as the blue azulene. As a further result of the configurational interaction the HOMO-LUMO transition is not necessarily the lowest transition S0 → S1.

▶  Fig. 1.2 Electronic transitions and radiative processes.

▶  Fig. 1.3 Energy scheme for the electronic transition between HOMO and LUMO.

For diatomic or linear polyatomic molecules, as with atoms, selection rules for the allowed transitions between two different electronic states can be established based on the rule of the conservation of angular momentum. For other molecules, which constitute the overwhelming majority, these rules result in transition exclusions.

allowed:

g → u

forbidden

g ↛ g

u→g

u ↛ u

Movement of nuclei can reduce the symmetry, so that symmetry-forbidden transitions can, in fact, be seen. (An example of a vibrationally allowed transition is the longwavelength absorption band of benzene; cf. p. 15).

A further possible cause for the disappearance of the electronic transition moment is the so-called overlap exclusion. This takes effect when the two orbitals which are taking part in the electronic transition overlap poorly or not at all. That is quite clearly the case in an intermolecular charge-transfer transition where the electronic transition takes place from the donor to the acceptor molecule. There are also numerous intramolecular examples of the overlap exclusion. (Compare the n → π* transition of carbonyl compounds, p. 17).

If the possibilities of transitions between two orbitals of a molecule are worked out, it becomes apparent that exclusions become the rule and allowed transitions are the exceptions. However, forbidden transitions frequently occur, albeit with low transition probability, i.e., a low f value (10−1 ≥ f ≥ 10−6). The spin exclusion is the most effective. Even spin-forbidden transitions can, however, be observed in cases of effective spin-orbit coupling (e.g., by heavy atoms) or in the presence of paramagnetic species.

The statements in ▶ Section 1.1 apply to single photon transitions. With the use of lasers two-photon spectroscopy has been developed. High photon densities allow the simultaneous absorption of two photons. This leads to altered selection rules; thus, for example, transitions between states of the same parity are allowed (g → g, u → u) and transitions between states of opposite parity are forbidden. The degree of polarisation can also be determined in solution. Two-photon spectroscopy thus provides useful extra information in studies of electronically excited molecules.

At the end of this section, the photophysical processes of electron transitions are summarised in a modified Jablonski term scheme. From the ground state, which in general is a singlet state S0, absorption leads to higher states S1, S2, etc. The return to S0 from S1, and more rarely from higher singlet states Sn, can occur by the emission of radiation, known as fluorescence, or by non-radiative deactivation (internal conversion). Non-radiative spin-inversion processes (intersystem crossing) lead to triplet states T, which can return to S0, disregarding the spin exclusion, either by emission of radiation, known as phosphorescence, or by renewed intersystem crossing (▶ Fig. 1.4).

“True” two-photon absorptions must be differentiated from processes in which two photons are absorbed one after the other. At high light intensities, populations of excited states can be attained which allow further excitation; for example, the process S0 → S1 ⇝ T1 can be followed by a triplet-triplet absorption T1 → T2.

In contrast to atoms, the various electronic states of molecules have rather broad energies because of the added effect of vibrational and rotational levels. Each term in ▶ Fig. 1.4 is therefore split into many energy terms, as shown schematically in ▶ Fig. 1.5. A specific energy level Etot, therefore, corresponds to a particular electronic, vibrational and rotational state of the molecule.

To a first approximation the three energy components can be separated

For an electronic transition it follows that

The electronic part is always much larger than the vibrational part which, in turn, is much larger than the rotational part. The relaxation R (see ▶ Fig. 1.5) is an additional non-radiative deactivation within each electronic state. In addition to the monomolecular processes described here it should also be noted that bimolecular photophysical processes (energy transfer: sensitisation, quenching) and primary photochemical processes can occur.

▶  Fig. 1.4 Jablonski term diagram with a visual representation of the electronic configurations.

Radiative processes: →

A Absorption

F Fluorescence

Ph Phosphorescence

Non-radiative processes: ⇝

IC internal conversion

ISC intersystem crossing

▶  Fig. 1.5 Schematic representation of the superimposition of electronic, vibrational, and rotational states; νi vibrational quantum numbers, Ji rotational quantum numbers.

1.2 Light Absorption and the Spectrum

If a beam of light of intensity I0 falls on a layer of homogeneous, isotropic material of thickness d, then apart from losses through reflection or diffraction it can be weakened by absorption. The intensity I of the emerging beam (transmission) is then given by:

The differential equation for the reduction of the intensity dI by an increment dx of the width of the absorbing layer

dI=−α.Idx

and evaluation of the integral

yields the function

II0·e−αd

Here, α is a characteristic absorption coefficient for the medium. If consideration is restricted to dilute solutions, where only the solute, of concentration c, absorbs, then α can be replaced by 2.303·ε·c to give

Special care is necessary when entering values for the concentrations for compounds which undergo a chemical change when dissolved, e.g. dissociation, dimerisation, etc.

If the absorbance is determined according to the Lambert-Beer law for all λ or ṽ and from that the substancespecific value ε, the absorption plot ε (ṽ) or ε(λ) can be obtained and thus the UV or UV/Vis spectrum. As a consequence of the width (in energy terms) of the electronic states it is a band spectrum. The individual bands are characterised by their properties of position, intensity, shape, and fine structure.

A classification of the electronic transitions (bands) can be made from a knowledge of the molecular orbitals (MOs) involved. From occupied bonding σ- or π-orbitals or from non-bondingn-orbitals (lone pairs of electrons) an electron can be raised to an empty anti-bondingπ*- or σ*-orbital. Correspondingly the electron transitions (bands) are indicated as σ→σ*, π→π*, n→π*, n→σ* etc. (▶ Fig. 1.6).

▶  Fig. 1.6 Molecular orbitals and electron transitions.

Apart from this nomenclature based on a simplified MO description there are several other conventions for the specification of electronic states and the possible transitions between them, of which especially the last one in ▶ Table 1.1 is to be recommended.

From ▶ Fig. 1.6 it follows that the positions of the absorption bands depend on the nature of the electron transitions. For isolated chromophores ▶ Table 1.2 (see p. 9) gives a guide. The position of the absorptions is, however, strongly influenced by steric, inductive, and resonance effects — the latter being particularly strongly affected by inclusion of the chromophore in large conjugated systems (▶ Fig. 1.7).

▶  Table 1.1 Nomenclature for electron transitions (absorptions)

System

Term symbol

State

Examples of electron transitions

enumerative

S

0

singlet ground state

S

0

→ S

1

S

1

, S

2

, S

3

,…

higher singlet states

S

0

→ S

2

according to Mulliken according to Platt

T

1

, T

2

, T

3

,…

triplet states

S

0

→ S

3

N

ground state

V ← N

Q, V, R

excited states

Q ← N

A

ground state

B ← A

B, C, L

excited states

C ← A

L ← A

according to Kasha

σ,

π

,

n

orbital of origin

σ → σ*

π

π

*

σ*,

π

*

orbitals of the excited electrons

n

π

*

n

→ σ*

group theory

symbols of the symmetry classes

*

1

A

2

1

A

1

1

B

1u

1

A

1g

1

B

2u

1

A

1g

1

E

1u

1

A

1g

E: doubly degenerate state

T: triply degenerate state Indices

*

See a textbook on symmetry in chemistry.

▶  Fig. 1.7 Absorption regions of various electron transitions.

For certain chromophores the solvent also has a characteristic influence (see ▶ Fig. 1.26).

A shift to longer wavelengths (red shift) of a transition is called a bathochromic effect, a shift to shorter wavelengths (blue shift) a hypsochromic effect.

The term hyperchromic effect is used to describe an increase in intensity. Hypochromic means the opposite, a decrease in intensity.

As described above, the transition moment |M| or the oscillator strength f is a measure of the intensity of a transition. An alternative measurement for the intensity is the area S

The relationship between f and s for a refractive index of n ≈ 1 is given by:

f≈ 4.32·10−9S

m mass of an electron

e charge of an electron

NA Avogadro’s constant

c speed of light

S can often be determined by graphical integration or estimated very roughly from approximations such as, for example

where b is the width of the band at half height (▶ Fig. 1.8).

▶  Fig. 1.8 True and approximated areas of an absorption band.

The higher the transition probability, the shorter is the radiation lifetime τ0 of an excited state; τ0 can be calculated from f and thus from S

As an approximation τ0 is given in seconds by

Usually the intensity of a band is judged simply by εmax. The following assignments have become customary:

          

ε

≤ 10

transition:

forbidden

    10 <

ε

< 1000

weakly allowed

1000 <

ε

<100000

allowed

          

ε

>100000

Strongly allowed

For two states with an energy differenceΔE the ratio of populations (N) is given by

▶ Fig. 1.9 shows the situation schematically.

Spectra measured in solution do not show rotation lines — the electronic bands are composed of vibrational bands. The degree of structure observed in the absorptions depends on the substance. Vibrational fine structure is most likely to be seen in rigid molecules. In polyatomic molecules the vibrational levels lie very close together. Restricted rotation in solution and line broadening due to local inhomogeneities in the solvation result in unstructured bands. The measurement conditions can also play an important role. ▶ Fig. 1.10 shows the reduction in structure with increasing interaction with the solvent and under the influence of temperature.

In line with the Franck-Condon principle the absorption probability is largest for a vertical transition from the energy hyper-surface of the ground state into that of the electronically excited state, i.e., all molecular parameters (bond lengths and angles, conformation, solvation cage, etc.) remain unchanged during the transition.

▶  Fig. 1.9 Absorption and fluorescence as transitions between electronic, vibrational, and rotational levels.

▶  Fig. 1.10 Vibrational structure of the n → π* absorption of 1,2,4,5-tetrazine (1) (from Mason, S.F. (1959), J. Chem. Soc., 1263).

I Vapour spectrum at room temperature (with vibrational mode)

II Spectrum at 77 K in an isopentane/methylcyclohexane matrix

III Spectrum in cyclohexane at room temperature

IV Spectrum in water at room temperature

The λ scale is referenced to I; II is shifted by 150 cm−1; III by 250 cm−1 to higher wavenumbers; IV by 750 cm−1 to lower wavenumbers

▶  Fig. 1.11 Composition of an absorption band from different vibrational bands in a diatomic molecule; r interatomic distance; E energy

a unsymmetric band with intense 0 ← 0 transition

b symmetric band with intense 2 ← 0 transition

2 Sample Preparation and Measurement of Spectra

▶  Fig. 1.12 Schematic diagram of a double-beam spectrometer

Q radiation source (UV: hydrogen or deuterium lamp, Vis: tungsten-halogen lamp)

M (Double) monochromator using prisms and/or grating for spectral dispersion

Z Beam splitter (rotating mirror)

MK Measurement cell with solution

VK Control cell with pure solvent

D Detector (photoelectron multiplier, array of diodes)

S Computer/display/printer that records the transmission or absorption

Most instruments show the absorption A as a function of the wavelength λ. In contrast to A, the extinction coefficient ε has a specific value for a certain substance. It is therefore better to record a plot of ε against λ or even better against the wave number ṽ. Also, ṽ, unlike λ, is proportional to the energy. In the long wavelength region spectra which have a linear λ scale are expanded, in the short wavelength region compressed. If strong and weak bands occur in the same spectrum, it is better to have log ε on the ordinate. ▶ Fig. 1.13 shows a comparison of the four frequently used ways in which UV/Vis spectra are commonly displayed.

A special form of measurement is the recording of the fluorescence as a function of the wavelength of the excitation. The excitation spectra thus obtained are not always identical with the absorption spectra. Even with a very pure compound the participation of different rotational isomers can result in different spectra. Two-photon spectroscopy is often carried out by this technique.

3 Chromophores

3.1 Individual Chromophoric Groups and their Interactions

As shown in ▶ Section 1.1, the position of an absorption band depends on the nature of the electronic transition involved. ▶ Table 1.2 gives a list of the excitation energies of σ,π, and n-electrons in various isolated chromophoric groups.

If a molecule has several π or n orbitals, which do not interact with each other, the spectrum will usually be the sum of absorptions assigned to the individual isolated chromophores. Steric effects, ring strain, etc., can lead to exceptions. Non-conjugated chromophores can also interact if they are near to each other, causing a shift or splitting of the bands (Davidov splitting). Where there are two identical chromophores there will often be two bands instead of the expected one, one of higher energy, one of lower energy than the energy of the isolated chromophore. This is shown by the examples of 1,4-pentadiene (3) and norbornadiene (4).

Conjugated chromophores are of special importance for UV/Vis spectroscopy. Classical examples are the polymethine dyes. As the conjugated system becomes larger so the lowest energy π → π* transition moves to longer wavelengths and becomes more intense; however, a convergence limit is reached for series of oligomers. A bathochromic effect and hyperchromic effect are in general also observed when atoms or groups with n orbitals ( etc.) are directly bound to a chromophoric group. In this context the term auxochromic group is used.

▶  Table 1.2 Absorptions of isolated chromophoric groups (lowest energy transitions)

Chromophore

Transition

Example

λ

max

a

(nm)

ε

max

a

(cm

2

·mmol

−1

)

C—H

σ → σ*

CH

4

122

strong

C—C

σ → σ*

H

3

c—cH

3

135

strong

n → σ*

H

2

O

167

1500

n → σ*

H

3

C—

OH

183

200

n → σ*

C

2

H

5

—O—C

2

H

5

189

2000

n → σ*

H

3

C—SH

235

180

n → σ*

H

3

C—S—CH

3

228

620

n → σ*

C

2

H

5

—S—S—C

2

H

5

250

380

n → σ*

NH

3

194

5700

n → σ*

C

2

H

5

—NH

2

210

800

n → σ*

C

2

H

5

—NH—C

2

H

5

193

3000

n → σ*

(C

2

H

5

)

3

N

213

6000

n → σ*

H

3

C—Cl

173

200

n → σ*

H

3

C—Br

204

260

n → σ*

H

3

C—I

258

380

n → σ*

CHI

3

349

2170

π

π

*

H

2

C=CH

2

165

16000

π

π

*

C

2

H

5

—CH=CH—C

2

H

5

185

7940

—C≡C—

π

π

*

HC≡CH

173

6000

π

π

*

H—C≡C—C

2

H

5

172

2500

n →

π

*

H

3

C—CH=O

293

12

π

π

*

187

950

n →

π

*

273

14

n →

π

*

460

weak

π

π

*

H

3

C—CH=N—OH

190

8000

n →

π

*

H

3

C—CH=N—OH

279

15

n →

π

*

353

240

343

25

n →

π

*

(H

3

C)

3

C—NO

300

100

(H

3

C)

3

C—NO

665

20

—NO

2

π

π

*

H

3

C—NO

2

210

10000

n →

π

*

278

10

a

The

λ

max

and, to a lesser extent, the

ε

max

values depend on the solvent used (see

 

Section 3.3

,

p. 17

).

▶  Fig. 1.14 Long wavelength electronic transitions S0 → S1 in formaldehyde and s-trans-glyoxal.

Interactions between several chromophores or chromophores and auxochromes will be extensively discussed in the following chapters. As explicit examples formaldehyde (5) and glyoxal (6) will be treated here.

The forbidden n → π* transition of formaldehyde gives a band in the gas phase with extensive fine structure and a maximum at 303 nm.

Glyoxal (6) which, in contrast to the colourless formaldehyde (5), is yellowish-green in the gas phase, shows an absorption at 450 nm, shifted by some 150 nm. In the associated n+ → π*3 transition neither the n nor the π orbital is comparable with the orbitals in formaldehyde. The two conjugated π-bonds in glyoxal are described by the bonding orbitals π1 and π2 and the two antibonding orbitals π*3 and π*4; the latter are empty in the ground state. The two lone pairs of electrons (with p character) also interact and split into n+ and n−, with the symmetric combination n+ having the higher energy (▶ Fig. 1.14).

3.2 Olefins, Polyenes

▶  Fig. 1.15 Schematic energy diagram explaining the bathochromic shift of the π → π* transition of ethylenes with auxochromic groups X.

▶  Fig. 1.16 HOMO-LUMO transitions in ethylene, 1,3-butadiene, and 1,3,5-hexatriene. The absorption coefficients εmax increase in parallel with λmax.

The introduction of alkyl groups also leads to a shift of the π → π* absorption. This effect is frequently explained on the basis of hyperconjugation.

When two or more olefinic double bonds are conjugated the centre of gravity of the π orbitals is indeed reduced by the mesomeric effect, but the energy difference between the HOMO and LUMO gets less with increasing chain length, as shown in ▶ Fig. 1.16 and Table 1.3.

Accurate calculations demonstrate, in agreement with the observations, that λmax approaches a finite limiting value (n → ∞).

Remarkably, the lowest excited state S1 of linear all-trans-polyenes is not the optically allowed 1Bu state, reached by the HOMO-LUMO transition, but a forbidden 1Ag state. A doubly excited configuration makes a considerable contribution to this state. This was first predicted by quantum mechanical calculations taking into account the interactions between configurations, and has been experimentally confirmed by two-photon spectroscopy. For the example of 1,3,5,7-octatetraene (7), ▶ Fig. 1.17 illustrates the electron distributions in S0, S1 and S2.

▶  Table 1.3 Longest wavelength absorptions of conjugated all-trans-polyenes (λmax in nm, εmax in cm2·mmol−1)

R—(CH=CH)

n

—R

n

R=CH

3

R=C

6

H

5

λ

max

(nm)

a

ε

max

λ

max

(nm)

b

ε

max

1

174

24000

306

24000

2

227

24000

334

48000

3

275

30200

358

75000

4

310

76500

384

86000

5

342

122000

403

94000

6

380

146500

420

113000

a

Measured in petroleum ether or ether.

b

Measured in benzene.

The configuration of olefins also affects the position and intensity of absorptions. (Z)-Stilbene [(Z)-8] absorbs at slightly shorter wavelength and less intensively than the (E)-isomer (E)-8 (▶ Fig. 1.18).

▶  Fig. 1.17π-Electron distribution of 1,3,5,7-octatetraene (7) in the singlet states S0, S1 and S2.

▶  Fig. 1.18 UV spectrum of (Z)- and (E)-stilbene 8 at 295 K in methylpentane (from Dyck, R.H. and McClure, D.S. (1962), J. Chem.Phys. 36, 2336).

The (Z)- or (E)-configuration has particular influence on the higher energy transitions of polyolefins. The first overtone in β-carotene lies at 340 nm. In the all-(E)-configuration (9) it is symmetry forbidden (cf. parity rule). On inclusion of a (Z)-double bond the symmetry is changed. The transition is allowed and leads to the so-called (Z)-peak of the carotenes (▶ Fig. 1.19).

▶  Fig. 1.19 Absorption spectra of β-carotenes of different configurations (9, 10).

Empirical rules for the absorption maxima of the longest wavelength π → π* transitions of dienes and trienes were established by Woodward in 1942 and later and independently by Fieser and Scott. These start from specific base values for open-chain, homo- or heteroannular dienes with s-cis or s-trans configurations, increments being added for the various substituents (▶ Table 1.4).

The last two examples in ▶ Table 1.5 show that these rules break down when strong steric effects are present. The influence of steric factors on the 1,3-diene chromophore is well demonstrated by the series of (Z,Z)-1,3-cycloalkadienes (▶ Table 1.6). Because of the many exceptions, these rules have lost part of their significance.

The incremental rules also break down when special electronic effects are present, as in the case of the annulene series (▶ Tab 1.7). Here there may be aromatic (4n + 2)-π-electron systems, antiaromatic 4n-π-electron systems and non-planar molecules with so-called non-aromatic (olefinic) character. The similarity of the UV/Vis spectra of, for example, the aromatic [18] annulene and [6] annulene (≡benzene) serves as an introduction to the next section.

▶  Table 1.4 Incremental system for the calculation of the long wavelength absorption maximum of dienes and trienes

preferred

s

-

trans

s

-

cis

s

-

trans

(e.g., acyclic)

(homoannular)

(heteroannular)

217 nm

253 nm

214 nm

Increments

for each further conjugated double bond

+30 nm

for each exocyclic position of a double bond

+5 nm

for each C-alkyl or C-aryl group

+5 nm

for each auxochromic group

O-alkyl

+6 nm

O-acyl

±0

S-alkyl

+30 nm

N(alkyl)

2

+60 nm

Cl

+5 nm

Br

+5 nm

▶  Table 1.5 Examples of the calculation of λmax values of conjugated dienes and trienes

▶  Table 1.6 Long-wavelength UV absorptions of homoannular 1,3-dienes

Compound

λ

max

(nm)

ε

max

(cm

2

·mmol

−1

)

Cyclopentadiene

238

3400

1,3-Cyclohexadiene

256

8000

1,3-Cycloheptadiene

248

7500

1,3-Cyclooctadiene

228

5600

▶  Table 1.7 Absorptions of annulenes

Compound

λ

max

Ig

ε

Solvent

Colour of the solution

Character

Cyclobutadiene

≈305

≈2.0

antiaromatic

Benzene

262

2.41

Hexane

colourless

aromatic

208

3.90

189

4.74

Cyclooctatetraene

285

2.3

Chloroform

yellow

non-aromatic

[10]Annulene

265

4.30

Methanol

yellow

non-aromatic

257

4.46

[14]Annulene

374

3.76

Iso-octane

red-brown

aromatic

314

4.84

[16]Annulene

440

2.82

Cyclohexane

red

antiaromatic

282

4.91

[18]Annulene

764

2.10

Benzene

yellow-green

aromatic

456

4.45

379

5.5

[24]Annulene

530

3.23

Benzene

violet

(antiaromatic)

375

5.29

360

5.26

3.3 Benzene and Benzenoid Aromatics

In contrast to 1,3,5-hexatrienes (see p. 11) the π2/π3 and π*4/π*5 orbitals of benzene form pairs of degenerate (i.e., equal energy) orbitals. As can be theoretically demonstrated, the four conceivable π2/3 → π*4/5 transitions lead from the 1A1g ground state to the excited singlet states 1B2u, 1B1u, and 1E1u. (The latter state is, as the symbol E implies, a degenerate state.) Because of the electron correlation the three excited states and therefore the three transitions are of different energy (▶ Fig. 1.20a and ▶ b).

In the UV spectrum of benzene (▶ Fig. 1.21) the highly structured α-band and the p-band correspond to symmetry forbidden transitions. The p-band, which appears as a shoulder, “borrows” intensity from the neighbouring allowed transition (β-band). Because of the symmetry prohibition there is no 0 ← 0 transition in the α-band. The ν′A vibration distorts the hexagonal symmetry and leads to the longest wavelength vibrational band. Further vibrational bands follow, separated by the frequency of the symmetric breathing vibration ν′B (▶ Fig. 1.21).

▶  Fig. 1.20a Energy scheme of the π-orbitals of benzene b Electronic excitations in benzene.

λ

max

ε

max

I

1

B

2u

1

A

1g

according to Platt

1

L

b

1

A according to Clar:

α

-band

256 nm

204 cm

2

· mmol

−1

II

1

B

1u

1

A

1g

according to Platt

1

L

b

1

A according to Clar:

p

-band

203 nm

7400 cm

2

· mmol

−1

III

1

E

1u

1

A

1g

according to Platt

1

B ←

1

A according to Clar:

β

-band

184 nm

60000 cm

2

· mmol

−1

vibrational bands follow, separated by the frequency of the symmetric breathing vibration ν′B (▶ Fig. 1.21).

The introduction of a substituent reduces the symmetry of benzene, enlarges the chromophoric system, and changes the orbital energies and thus the absorptions, so that the p-band can overtake the α-band. The α-band, sometimes also called the B-band, gains intensity and often loses its fine structure; because of the reduction in symmetry its 0 ← 0 transition becomes visible.

An overview of monosubstituted benzenes is given in ▶ Table 1.8.

The change in the spectra caused by the introduction of two or more substituents into the benzene ring, compared to the monosubstituted derivatives, is particularly marked in those cases where both an electron-withdrawing group and an electron-donating group are present (▶ Table 1.9).

In these cases the increase in the size of the chromophore is linked to the possibility of an intramolecular charge transfer:

▶  Fig. 1.21 Absorption spectrum of benzene

The effect should be even stronger in the p-nitrophenolate anion than in p-nitrophenol itself (11; ▶ Fig. 1.22a). This is confirmed by ▶ Fig. 1.22b which, however, also shows that a similar effect occurs where there is m-substitution, and hence independently of the participation of quinonoid resonance structures.

The solvent can have a particularly strong effect in such cases, and even change the energetic order of the states. A good example of this is dimethylaminobenzonitrile 12.

▶  Table 1.8 UV Absorptions of monosubstituted benzenes C6H5—R

Substituent R

Long wavelength, stronger transition

Long wavelength, (forbidden) transition

Solvent

λ

max

(nm)

ε

max

(cm

2

· mmol

−1

)

λ

max

(nm)

ε

max

(cm

2

· mmol

−1

)

H

204

7400

254

204

Water

198

8000

255

230

Cyclohexane

CH

3

207

9300

260

300

Ethanol

C

2

H

5

200

31600

259

158

Ethanol

CH(CH

3

)

2

251

250

Hexane

F

259

1290

Ethanol

Cl

210

7400

264

190

Water

Br

210

7900

261

192

Water

I

207

7000

257

700

Water

OH

211

6200

270

1450

Water

O

235

9400

287

2600

Water

OCH

3

217

6400

269

1480

Water

OC

6

H

5

255

11000

272

2000

Water

278

1800

NH

2

230

8600

280

1430

Water

NH

+

3

203

7500

254

160

Water

N(CH

3

)

2

251

12900

293

1590

Ethanol

NO

2

269

7800

Water

CH=CH

2

244

12000

282

450

Ethanol

C≡CH

236

12500

278

650

Hexane

C≡N

224

13000

271

1000

Water

CH=O

242

14000

280

1400

Hexane

330

60

CO—CH

3

243

13000

278

1100

Ethanol

319

50

COOH

230

11600

273

970

Water

COO

224

8700

268

560

Water

SO

3

H

213

7800

263

290

Ethanol

▶  Table 1.9 Long wavelength absorptions λmax (nm) of some para-disubstituted benzenes X1—C6H4—X2 (in water)

▶  Fig. 1.22 UV/Vis spectra of o-, m-, and p-nitrophenol:

a in 10−2 molar hydrochloric acid,

b in 5·10−3 molar sodium hydroxide (from Kortüm, G. (1941) Ber. Dtsch. Chem. Ges. 74, 409),

In the spectra of condensed benzenoid aromatics there are many common features. The two highest occupied orbitals πn−1 and πn and the two lowest π*n+1 and π*n+2 are no longer degenerate as they are in benzene. Four electronic transitions are possible between them (▶ Fig. 1.23).

▶  Fig. 1.23 Electronic transitions in polyacenes

a Orbital scheme

b State diagram (from HMO theory)

c Electronic transitions taking into account the configurational interactions

With increasing anellation the α-, p-, and β-bands shift to longer wavelengths. For polyacenes the p-band overtakes the weak α-band and is superimposed on it (▶ Fig. 1.24). The intensity of the p-band remains more or less constant. (The increase in the number of rings has no effect, because this electronic transition is polarised parallel to the short axis.) The bathochromic shift in the acene series leads to the members from tetracene onwards being coloured:

Benzene, naphthalene, anthracene

colourless

Tetracene (Naphthacene)

orange-yellow

Pentacene

blue-violet

Hexacene

dark green

If the anellation is non-linear, characteristic changes occur in the spectra (cf., for example, anthracene and phenanthrene, ▶ Fig. 1.24). As well as the linear anellated tetracene the condensation of four benzene rings can lead to four angular systems, benz[a]anthracene, benzo[c]phenanthrene, chrysene, and triphenylene, and the peri-condensed system pyrene. Of these only tetracene absorbs in the visible region; the others are colourless, but show coloured fluorescence. Even more pronounced are the differences among the C30H18 isomers in ▶ Table 1.10. When using a circle in the ring to show the existence of a complete π-electron sextet, one can see that their number decreases on going from the top to the bottom of the table. This corresponds to a red shift of the absorption with the longest wavelength which leads from the UV region into the near infrared region.

▶  Fig. 1.24 UV/Vis spectra of condensed aromatic hydrocarbons (in heptane).

The strong band structure of nearly all condensed benzenoid aromatic hydrocarbons is of particular analytical value for the identification of the individual members of the series.

▶  Table 1.10 Long-wavelength absorptions of benzenoid aromatics of the C30H18 series

Compound

λ

max

[nm]/solvent

Colour of crystals

Phenathro[9,10-

b

] triphenylene

382/benzene

Colourless

Tribenzo[

a

,

c

,

j

] tetracene

423/benzene

Yellow

Dibenzo[

a

,

c

] pentacene

539/benzene

Red-violet

Benzo[

a

] hexacene

651/1-methylnaphthalene

Blue-green

Heptacene

840/1-methylnaphthalene

Black-green

3.4 Carbonyl Compounds

▶  Table 1.11 n → π* Transitions in saturated carbonyl compounds

Compound

λ

max

(nm)

ε

max

(cm

2

·mmol

−1

)

Solvent

Acetaldehyde

293

12

Hexane

Acetone

279

15

Hexane

Acetyl chloride

235

53

Hexane

Acetic anhydride

225

50

Iso-octane

Acetamide

205

160

Methanol

Ethyl acetate

207

70

Petroleum ether

Acetic acid

204

41

Ethanol

Auxochromes, such as OH, OR, NH2, NHR, NR2, Hal, etc., directly bonded to the carbonyl group, act as π-donors to increase the energy of the π* orbital and as σ-acceptors to lower the n level. The result is a short wavelength shift of the n → π* transitions in carboxylic acids and their derivatives (▶ Table 1.11).

Conjugation of the carbonyl group with a (C=C) bond leads to a marked shift of the π-level; to a first approximation the n-orbital is unaffected (▶ Fig. 1.25).

With increasing length of the conjugated chain in enones the longest wavelength π → π* transition moves further into the visible region, reaches the position of the n → π* band and hides it because of its considerably greater intensity, which also increases strongly with increasing chain length (▶ Table 1.12).

The λmax values of the n → π* transitions of α,β-unsaturated carbonyl compounds can be estimated from the extended Woodward rules (▶ Table 1.13).

▶  Fig. 1.25 Energy diagram of the electronic transitions in conjugated enones compared to alkenes and saturated carbonyl compounds.

▶  Table 1.12 Absorption maxima of the long wavelength π → π* transition in the vinylogous series C6H5–(CH=CH)n–CO–R (in methanol)

n

0

244

12000

254

20000

1

285

25000

305

25000

2

323

43000

342

39000

3

355

54000

373

46000

The agreement between experimental absorption maxima and the values calculated from the increment system is apparent in ▶ Tab 1.14

▶  Table 1.13 Increment system for calculating absorption maxima of α,β-unsaturated carbonyl compounds

▶  Table 1.14 Observed and calculated π → π* absorptions of some enones (in ethanol)

As already mentioned in ▶ Sec. 3.1 (p. 9), certain absorptions are very solvent dependent. Such effects have been particularly carefully investigated for ketones. ▶ Fig. 1.26 shows the example of benzophenone (13).

The electronic states of benzophenone are lowered by solvation, hydrogen bonding in polar, protic solvents being especially effective. The strongest effect is observed with the state of highest polarity, the π,π* singlet state. Since the doubly occupied n-orbital on the oxygen atom is mostly responsible for the hydrogen bonding, the n,π* singlet state of ketones has much poorer solvation properties (▶ Fig. 1.27).

▶  Fig. 1.26 Absorption spectra of benzophenone (13)

______ in cyclohexane

- - - - - - in ethanol

b bathochromic solvent effect (on increasing the solvent polarity)

h hypsochromic solvent effect (on increasing the solvent polarity)

Similar solvent effects appear in certain heterocycles, azo compounds, nitroso compounds, thioketones, etc. However, the use of solvent dependence for characterising the n → π* and π → π* transitions should be confined to aldehydes and ketones. Extreme solvatochromism such as, for example, that of the zwitterionic pyridinium phenolates, is used to determine the polarity of solvents.

The quinones represent special “enone” chromophores. As shown by a comparison of 1,4-(14) and 1,2-benzoquinone (15), o-quinones absorb at longer wavelengths than the corresponding p-isomers (measurement in benzene):

▶  Fig. 1.27 The bathochromic and hypsochromic shifts of the π → π* and n → π* transitions of ketones on increasing the solvent polarity

The reason for this is that the lowest π*-orbital of the linearly conjugated o-quinone lies lower than that of the cross-conjugated p-quinone. Because of the interaction of the n(p)-orbitals of the two oxygen atoms two n → π* transitions are expected. In general they lie very close together.

3.5 Conjugated Oligomers and Polymers

In general, linear conjugated oligomers show a systematic bathochromic shift of the absorption at longest wavelength with increasing number (n) of repeating units.

In the case of cyanines and the related polymethine dyes with degenerate mesomeric resonance structures (see compound 20 for an example) we observe for the lower members a more or less linear increase of λmax with n (see following table). This rule is valid till for higher members the so-called cyanine limit is reached.

In the case of non-degenerate systems, e.g., when the iminium group, C=N+(CH3)2, in 20 is replaced by a formyl group, CH=O, to give a so-called merocyanine, as well as with completely different repeating units with aromatic or heteroaromatic building blocks, a convergent behaviour of not only the energy but also the wavelength of the absorption bands of lowest energy is established.

E(n)→E∞andλ(n)→λfor n→∞

At first one should check this convergence for the 0 → 0 transitions (λ0,0) in the series of conjugated compounds; however, it is often also valid for the absorption maxima (λmax). The absorption spectra of oligo(2,5-dipropoxyphenylenevinylene)s 21 are shown as an example.

When the energies E(n) of the electronic transitions of 21a - j are plotted against the reciprocal of the number of benzene rings one obtains an apparently useful linear correlation. However, extrapolation to the polymer 21p fails completely. On the other hand, when the E values of 21a -j are based on an e function (dotted curve in ▶ Fig. 1.28)

the limiting value E∞ for n → ∞ corresponds to the measured value for the polymer 21p. The difference E1 − E∞ describes the conjugation effect; it affords the bathochromic shift between the first member and that with the “infinitely long” chain of the respective conjugated series. Furthermore, the effective conjugation length nECL indicates which oligomer reaches within λ∞ ± 1 nm (the error limit of a routine spectrophotometer) the limiting value. In the compound series 21, this is the case for the undecamer 21i according to calculation and measurement.

The synthesis of polymers is always accompanied structural errors; E∞ and nECL are important parameters for evaluating the length of defect-free segments in conjugated chains.

For extended chromophores the lowest energy electronic transitions may lie in the near infrared (NIR) region. For example, when a poly(phenylvinylene) system (PPV) is doped with an oxidant, an electron transfer leading to polymeric radical ions and doubly charged ions (Polarons, bipolarons). The insulator 22 is thus transformed to the electrical semiconductor 23.

On absorption measurements in solution one finds that, due to the doping, new bands occur beyond the absorption edge. The lowest energy transition can be shifted out of the visible wavelength region (λmax ≈ 2000 nm); it can then only be detected by using a special spectrophotometer equipped for the NIR region.

In total, for conjugated oligomers there are the four major possibilities shown in ▶ Fig. 1.30 for changes of the longest wavelength absorption with increasing number of repeating units n (extension of the conjugation). The bathochromic effect (case a) with convergence to λ∞ is by far the most common; hypsochromic effects (case b) can occur for push-pull-substituted oligomers with strong donors and strong acceptors if the repeating unit contains aromatic building blocks.

▶  Fig. 1.30 Changes in the long wavelength absorption on extension of the conjugation: a) bathochromic effect with convergence to mλ∞, b) hypsochromic effect with convergence to m λ∞, c) linear increase in λmax, d) “hyperlinear” increase in λmax.

A linear increase of λmax with n (case c) up to a certain limit is typical for degenerate systems such as the cyanines 20 (p. 20). A “hyperlinear” increase of λmax with n (case d) can be seen when the extension of the conjugation is not linear but proceeds rather in two or more directions (area-like). The phenes are an example of this: from phenanthrene through pentaphene and on to heptaphene λmax