Spectroscopic Methods in Organic Chemistry E-Book

Stefan Bienz

• Herausgeber: Thieme
• Kategorie: Wissenschaft und neue Technologien
• Serie: Foundations series
• Sprache: Englisch
• Veröffentlichungsjahr: 2021

Seitenzahl: 944

Beschreibung

Boost your knowledge of modern spectroscopic methods!

This reference work provides you with essential knowledge for the application of modern Spectroscopic Methods in Organic Chemistry. All methods are explained based on typical practical examples, theoretical aspects, and applications.

The following spectroscopic methods are explained and examples are given:

• UV/Vis Spectroscopy
• Infrared (IR) and Raman Spectroscopy
• Nuclear Magnetic Resonance Spectroscopy (NMR)
• Mass Spectrometry (MS)

The textbook has been a standard reference for decades. As it conveys necessary knowledge for examinations at all universities it is compulsory reading for every organic chemistry student!

Leseprobe

Prefixes used with SI units

1UV/Vis Spectroscopy

2Infrared and Raman Spectra

3Nuclear Magnetic Resonance Spectroscopy

4Mass Spectrometry

5Handling of Spectra and Analytical Data: Practical Examples

Hesse–Meier–Zeeh

Spectroscopic Methods in Organic Chemistry

Prof. Dr. Stefan Bienz

Department of ChemistryUniversity of ZurichWinterthurerstrasse 190CH-8057 ZurichSwitzerland

Prof. Dr. Laurent Bigler

Department of ChemistryUniversity of ZurichWinterthurerstrasse 190CH-8057 ZurichSwitzerland

Dr. Thomas Fox

Department of ChemistryUniversity of ZurichWinterthurerstrasse 190CH-8057 ZurichSwitzerland

Prof. Dr. Herbert Meier

Department of ChemistryJohannes-Gutenberg-UniversityDuesbergweg 18-14D-55099 MainzGermany

3rd fully revised and extended Edition335 Figures, 132 Tables, 59 Schemes, 10 Graphical representations of full analytical datasets

Georg Thieme VerlagStuttgart · New York

Library of Congress Cataloging-in-Publication Data is available from the publisher

This book is an extended authorized translation of the 9th German edition published and copyrighted 1979, 1984, 1987, 1991, 1995, 2002, 2005, 2012, 2016 by Georg Thieme Verlag, Stuttgart, Germany. Title of the German edition:Spektroskopische Methoden in der organischen Chemie.

© 2021. Thieme. All rights reserved,

Georg Thieme Verlag KGRüdigerstraße 14, 70469 Stuttgart, Germanywww.thieme-chemistry.com

Cover: © ThiemeImages: © Stefan Bienz and Laurent Bigler, Zurich, Switzerland.

Printed in Germany

by Firmengruppe Appl, Aprinta Druck GmbH,Senefelderstraße 3–11, 86650Wemding

ISBN (print) 978-3-13-243408-0ISBN (ePDF) 978-3-13-243410-3ISBN (ePUB) 978-3-13-243411-0DOI 10.1055/b000000049 1 2

Some of the product names, patents and registered designs referred to in this book are in fact registered trademarks of proprietary names even though specific reference to this fact is not always made in text. Therefore, the appearance of a name without designation as proprietary is not to be construed as a representation by the publisher that it is in the public domain.

This book, containing all parts thereof, is legally protected by copyright. Any use, exploitation or commercialization outside the narrow limits set by copyright legislation, without the publisher’s consent, is illegal and liable to prosecution. This applies in particular to photostat reproduction, copying, mimeographing or duplication of any kind, translating, preparation of microfilms, and electronic data processing and storage.

Preface

Already more than 10 years have passed since the second edition of this textbook on spectroscopic methods appeared in 2008. Time has not been standing still, however, and organic analytics have again experienced a tremendous development. Not only did the instruments get refined, delivering better spectra and more data with lesser amounts of sample material, but also, hand in hand, computer technology made a big step forward, allowing us to handle and process efficiently the enormous amount of data arising with modern instruments and experiments—in particular with nuclear magnetic resonance (NMR) and mass spectrometry (MS) analyses.

The book Spektroskopische Methoden in der organischen Chemie by Manfred Hesse, Herbert Meier and Bernd Zeeh took account of these developments in the German version by two subsequent editions that appeared in 2012 and 2016. Even though the authorship has partly changed after more than 32 years, the textbook remained devoted to its original objective. It is still meant as a straightforward read and source of reference to complement lecture and laboratory courses devoted to structure elucidation and analytical characterization of organic compounds. It shall still offer enough information also to make it a reliable companion for Bachelor, Master, and PhD students, as well as for professionals in chemical teaching and research outside of universities.

The new English edition appears in a completely new guise. Not only has the layout been changed to a more lucid, modern, and colorful look, but also the content has been updated significantly, reflecting important developments in organic analytics. The major changes as compared with the previous edition are summarized shortly below.

In the Ultraviolet/Visible Spectroscopy (UV/Vis) chapter, attended by Herbert Meier (University of Mainz), the fundamentals of allowed and symmetry-forbidden electronic transitions are newly discussed by means of simple molecules. In addition, acknowledging the increasing importance of optoelectronic materials, the section about compounds with larger conjugated systems (aromatics, heteroaromatics, and open-chained oligomers) has been extended. More room is also given to determination of solvent polarities in the chapter on applications of UV/Vis spectroscopy.

Thomas Fox (University of Zurich) has taken over the authorship of the chapter of IR and Raman spectroscopy from Bernd Zeeh.

He particularly revised and complemented the parts describing the basics of these spectroscopic methods. For instance, the relationships between bond strengths and vibration frequencies or between molecule symmetries and resulting IR and Raman activities are newly presented. With regard to the instrumental part, the construction of IR and Raman spectrometers is discussed in more detail, giving special attention also to the laser technology that became increasingly important. A new section dedicated to the interpretation of spectra was added, considering more deeply vibration couplings, overtones, Fermi resonance, and combination and difference bands. Adaptions were also done to enhance the efficiency of the interpretation of IR spectra by use of already well-approved sample spectra. The important absorption bands are now directly linked to the tables of the characteristic group frequencies, which are completely revised and partly reorganized for even more effective use.

Major changes have been made in the NMR chapter, authored by Herbert Meier. Great emphasis is placed on modern one- and two-dimensional NMR techniques such as DEPT, APT, COSY, DQFCOSY, Ph-COSY, E-COSY, TOCSY, NOESY, ROESY, EXSY, HETCOR, HSQC, HSQC-TOCSY, HSQC-NOESY, HMBC, and INADEQUATE. The description of earlier methods, however, has not been forgone because their knowledge is required to be able to understand less recent publications. Since organic molecules are mainly constructed on the basis of carbon frameworks, 13C NMR signals play a crucial role in the characterization of organic compounds. It is striking to observe, however, that respective signal assignments are omitted or erroneous in many publications. It thus became a special issue to deeply discuss electronic, steric, and anisotropic factors that affect the chemical shifts of 13C signals in open-chain as well as cyclic compounds. Several respective tables have been added, and the graphical table with the compilation of 1H and 13C chemical shifts—displayed with compounds ordered according to substance classes—has been significantly extended and complemented with examples of more rare substance classes. In the NMR spectra shown, two opposing tendencies are taken into account: on the one hand there is the trend towards high field strengths; on the other hand,bench spectrometers with, e.g. 60 MHz became more important.

Stefan Bienz and Laurent Bigler (both University of Zurich) are the new authors of the MS chapter, replacing Manfred Hesse who died in 2011. Performance and user friendliness of mass spectrometers have tremendously advanced over the recent decades. New and sophisticated ion generation, ion separation, and ion detection methods, and many new accessories such as coupled separation and automatization modules, sample preparation kits, and evaluation software led to a fresh affection for the MS method and to a broad spread of MS instruments as routine and open-access equipment throughout many chemical facilities.

To account for these developments, the MS chapter was completely redesigned: the content was fully regrouped, freed of obsolete techniques and methods, and complemented with information to newest instrumental and methodical advances. Because the MS techniques became more and more different and facetted, a new chapter has been introduced, which gives guidance for selecting proper sample preparations and appropriate measuring procedures. The newest methods for the structural elucidation of small molecules up to biopolymers are introduced, based on accurate mass measurements (high-resolution MS, HR-MS) and collision-induced dissociation (CID). Although not in detail, fragmentation mechanisms and fragmentation patterns for CID processes are addressed along broad lines, which is relevant in connection with structural elucidations of unknown analytes, where information can often be gained from conclusions by analogy.

The completely new final chapter of the textbook, Handling of Spectra and Analytical Data: Practical Examples, is also authored by Stefan Bienz and Laurent Bigler. It shows by means of 10 real cases how analytical data are described and what kind of strategies could be followed to come up with these data to reasonable structural proposals. The compounds were measured without any exception with modern instruments, and the examples were chosen to demonstrate the information gain that can be acquired by the most commonly applied analytical methods, including two-dimensional NMR and HR-MS. As a supplement, a set of freely accessible exercises is provided online.

For the preparation of the new English edition, which embraces also the preparation of the two German editions of 2012 and 2016, we owe our gratitude to many colleagues. First of all, the marvelous groundwork of the two former authors, Manfred Hesse and Bernd Zeeh, is warmly acknowledged. Special thanks are also due to Heinz Berke and Ferdinand Wild (both University of Zurich), as well as Klaus Bergander (University of Münster) for their contributions to the IR/Raman chapter; Heinz Kolshorn, Johannes Liermann, and Ingrid Schermann (all University of Mainz) for their supports for the NMR chapter; and Urs Stalder, Armin Guggisberg, Yvonne Forster, Jrène Lehmann, and the students of the advanced chemical laboratory courses (all University of Zurich) for their inputs, analytical measurements, and samples that were used for the MS chapter, the practical examples in Chapter 5, and the electronic supplements.

Stefan Bienzin the name of the authors

Contents

1UV/Vis Spectroscopy

1.1Theoretical Introduction

1.1.1Electromagnetic Waves and Electron Transitions in Molecules

1.1.2Light Absorption and the Spectrum

1.2Sample Preparation and Measurement of Spectra

1.3Chromophores

1.3.1Individual Chromophoric Groups and Their Interactions

1.3.2Olefins and Polyenes

1.3.3Benzene and Benzenoid Aromatics

1.3.4Heteroaromatics (Hetarenes)

1.3.5Carbonyl Compounds

1.3.6Conjugated Oligomers and Polymers

1.3.7Aggregated Molecules, Charge-Transfer Complexes

1.4Applications of UV/Vis Spectroscopy

1.5Derivative Spectroscopy

1.6Chiroptical Methods

Supplementary Literature

UV/Vis spectroscopy

Chiroptical Methods

2Infrared and Raman Spectra

2.1Introduction

2.2Basic Principles

2.3Infrared Spectrometer

2.3.1Classical (Scanning) Infrared Spectrometers

2.3.2Fourier Transform Spectrometer

2.4Sample Preparation

2.4.1Sample Preparation for Measurements in Transmission

2.4.2Reflection Measurements

2.4.3Raman Measurements

2.5Infrared Spectrum

2.5.1Number and Types of Vibrations

2.5.2Spectrum Interpretation

2.6Characteristic Absorptions: An Overview

2.7Infrared Absorptions of Single Bonds with Hydrogen

2.7.1(C–H) Absorption

2.7.2(O–H) and (N–H) Absorptions

2.8Infrared Absorptions of Triple Bonds and Cumulated Double Bonds

2.9Infrared Absorptions of Double Bonds CO, CN, CC, NN, and NO

2.10Infrared Absorption of Aromatic Compounds

2.11Infrared Absorption in the Fingerprint Range

2.12Examples of Infrared Spectra

2.13Information Technology Assisted Spectroscopy

2.14Quantitative Infrared Spectroscopy

2.15Raman Spectroscopy

2.15.1Excitation Mechanisms

2.15.2Selection Rules

2.15.3Raman Spectrometer

2.15.4Applications

2.15.5Comparison of Infrared and Raman

Literature

Raman Spectroscopy

Particular Techniques

Databases and Application

3Nuclear Magnetic Resonance Spectroscopy

3.1Physical Principles

3.1.1The Resonance Phenomenon

3.1.2Chemical Shift

3.1.3Spin–Spin Coupling

3.1.4Linewidths

3.1.5Intensity

3.2NMR Spectra and Molecular Structure

3.2.1Molecules with “Rigid” Atomic Positions

3.2.2Intramolecular Motion

3.2.3Chemical Exchange Processes

3.31H NMR Spectroscopy

3.3.1Sample Preparation and Measurement of 1H NMR Spectra

3.3.21H Chemical Shifts

3.3.31H,1H Coupling

3.3.4Coupling to Other Nuclei

3.3.5Correlation of 1H Shifts with Structural Features

3.3.6Increment Systems for Estimating 1H Chemical Shifts

3.3.71H NMR Data of Representatives of Common Classes of Compounds

3.3.8Specialized Techniques

3.3.9Two-Dimensional 1H NMR Spectroscopy

3.3.10Simulation of 1H NMR Spectra

3.3.11NMR Spectra of Oriented Phases and Solids

3.3.12Combination of Separation Methods with NMR Measurements

3.413C NMR Spectroscopy

3.4.1Sample Preparation and Measurement of Spectra

3.4.213C Chemical Shifts

3.4.3Correlation of 13C Chemical Shifts with Structural Features

3.4.4Increment Systems for the Estimation of 13C Chemical Shifts

3.4.513C,1H Couplings

3.4.6Coupling of 13C to Other Nuclei (D, F, N, and P)

3.4.713C,13C Couplings

3.4.8Special Techniques

3.4.9Multidimensional 13C NMR Spectra

3.4.10Solid-State Spectra

3.5Combination of 1H and 13C NMR Spectroscopy

3.5.1Complete Assignment of 1H and 13C NMR Signals

3.5.2Use of Databases

3.5.31H and 13C NMR Data of Representatives of the Most Important Classes of Compounds

3.6NMR of other Nuclei

3.6.119F NMR Spectroscopy

3.6.231P NMR Spectroscopy

3.6.315N NMR Spectroscopy

3.6.4Complete Assignment of the NMR Signals of a Compound Containing 1H, 13C, 15N, …

3.6.5Other Nuclei

Literature

General Works

Special Methods and Effects

Special Compound Classes, Applications

Catalogues

Serials, Periodicals

4Mass Spectrometry

4.1Introduction

4.2General Aspects of Mass Spectrometry

4.2.1The Principle of Mass Spectrometry

4.2.2The Mass Spectrum

4.3Instrumental Aspects

4.3.1Sample Introduction (Injection) and Ion Types

4.3.2Ionization Methods

4.3.3Mass Analyzers

4.3.4Detectors

4.3.5Tandem Mass Spectrometry

4.3.6Coupling of Mass Spectrometry with Chromatographic Methods

4.3.7Ion-Mobility Mass Spectrometry

4.3.8Selection of the Appropriate Method

4.4Interpretation of Spectra and Structural Elucidation

4.4.1Preparation for the Interpretation

4.4.2Structural Information from HR-MS

4.4.3Fragmentation Reactions in EI-MS

4.4.4Collision-Induced Dissociation

4.4.5Neighboring Group Participation and Stereoisomerism

4.4.6Spectral Libraries

4.4.7Special Techniques

4.4.8Elucidation of Fragmentation Mechanisms

4.5Sample Preparation

4.5.1Purification, Preparation, and Enrichment

4.5.2Sample Submission and Declaration of Sample Properties

4.5.3Specific Preparations for the Measurement

4.6Artifacts

4.6.1Memory Effect

4.6.2Formation of Artifacts in the Ion Source

4.6.3Identification of Artifacts

4.6.4Prevention of Artifact Formation

4.7Tables to the Mass Spectrometry

4.7.1Frequently Detected Ions

4.7.2Frequently Detected Mass Differences

4.7.3Isotope Distributions of Halogenated Compounds

4.7.4Solvents and Frequent Impurities

4.7.5Isotopes of Naturally Occurring Elements

Literature

5Handling of Spectra and Analytical Data: Practical Examples

5.1Introduction

5.2Characterization of Compounds

Example 1

Example 2

5.3Structure Elucidation of Allegedly Known Compounds and of Products Arising from Syntheses

Example 3

Example 4

Example 5

5.4Structure Elucidation of Completely Unknown Compounds

Example 6

Example 7

Example 8

Example 9

Example 10

Literature

Index

1

Herbert Meier

UV/Vis Spectroscopy

UV/Vis Spectroscopy

1.1Theoretical Introduction

1.2Sample Preparation and Measurement of Spectra

1.3Chromophores

1.4Applications of UV/Vis Spectroscopy

1.5Derivative Spectroscopy

1.6Chiroptical Methods

1 UV/Vis Spectroscopy

1.1 Theoretical Introduction

1.1.1 Electromagnetic Waves and Electron Transitions in Molecules

Electromagnetic radiation is characterized by the wavelength λ or the frequencyv. These values are connected with each other by the equation:

where c is the velocity of light (in vacuum ≈ 2.99 ·1010 cm × s–1). A quantum of light with frequency v has the energy

When visible light of a particular spectral color is absorbed, the human eye recognizes the complementary color:

Absorbed spectral color

Complementary color

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 x 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:

1,000 cm–1 ≅ 12 kJ·mol–1

1 kJ·mol–1 ≅ 84 cm–1

If light with the appropriate frequency v meets a molecule in the ground state ψ0, it can be absorbed and will raise the molecule to an electronically excited state ψ1. 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 sentences 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. Taking Koopmans’ theorem for granted, the energy of the highest occupied molecular orbital (HOMO) corresponds to the negative ionization potential (IP) and the energy of the lowest unoccupied molecular orbital (LUMO) to the negative electron affinity (EA). The orbital energies used here are related to one-electron configurations:

According to Fig. 1.3, the following equation applies for the energy difference of the states:

The difference in energy between LUMO and 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 singlet-triplet 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 colorless 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 (cf. Fig. 1.21, p. 16). Modern density functional theory (DFT) calculations avoid the problem of additional terms for the configuration interaction and quote HOMO and LUMO levels, which correspond to those obtained, for example, from redox potentials determined by cyclic voltammetry. The measured spectrum can be directly compared with the energy of the electron transitions, when time-dependent DFT (TD-DFT) calculations are performed. Some explicit examples will be given in sections 1.3.3 and 1.3.4.

Fig. 1.2 Electronic transitions and radiative processes.

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.

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

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 long-wavelength absorption band of benzene; cf. p. 18.)

An important possible cause for the disappearance of the electronic transition moment is the so-called overlap exclusion, a special case of the symmetry 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 (CT) transition where the electronic transition takes place from the donor to the acceptor molecule. There are also numerous intramolecular examples of the overlap exclusion (cf. intramolecular charge transfer [ICT] or the n→π* transition of carbonyl compounds, p. 30 and p. 25, respectively).

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 rule 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.

If the molecule under investigation is considered in a Cartesian coordinate system whose axes are established, for example, with reference to the molecular axes, the vector M01 can be separated into its spatial components Mx, My, and Mz. For M01≠ 0 at least one of the three components must be nonzero. When MxMyMz ≠ 0, the absorbed or emitted radiation is polarized in the z-direction. This optical anisotropy of the molecule cannot normally be observed, since the molecules are present in a random orientation. Polarization measurements are performed on single crystals or stretched plastic films.

Fig. 1.4 Molecular orbitals and electron transitions.

A classification of the electronic transitions (bands) can be made from a knowledge of the molecular orbitals (MOs) involved. From occupied bondingσ- or π-orbitals or from nonbondingn-orbitals (lone pairs of electrons), an electron can be raised to an empty antibondingπ*- or σ*- orbital. Correspondingly, the electron transitions (bands) are indicated as σ→σ*, π →π*, n→π *, n→ σ*, etc. (Fig. 1.4).

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. Especially the last nomenclature (group theory) in Table. 1.1 is to be recommended.

Ethene (1) and formaldehyde (2) shall be used here as simple molecules for the illustration of allowed and forbidden electron transitions.

If the five σ bonds of ethene are considered to be located in the yz plane of a three-dimensional coordinate system, the p lobes of the π and π* orbitals are aligned in the x-direction (Fig. 1.5). Ethene molecules have the following elements of symmetry in this arrangement:

▪The x-, y-, and z-axes are twofold symmetry axes: C2(x), C2 (y), C2 (z).

▪The origin of coordinates is an inversion center i.

▪The xy-, xz-, and yz planes of the coordinate system are symmetry planes: σ(xy), σ(xz), σ(yz).

Ethene (1) belongs to the point group D2h, in short ethene (1) has D2h symmetry. Table. 1.2 shows in the first column the irreducible representations of this point group and in the first row the above-mentioned symmetry elements (symmetry operations). The identity I is added because of group theoretical reasons. Each orbital of 1 must belong to a certain symmetry class (irreducible representation). Only group orbitals can be used and not, for example, single C–H bond orbitals. The characters +1 and –1 in Table. 1.2 express the symmetric or antisymmetric behavior toward the respective symmetry operation.

Fig. 1.5 π → π* electron transition in ethene (1).

The column on the right end of the character table (Table. 1.2) contains the symmetry behavior of the x-, y-, and z-component of the transition vector M. The z-component corresponds to b1u (B1u). That means the π→ π* transition (1B1u ← 1Ag ) is allowed and polarized in z-direction. It is observed in the far-UV at ~165 nm.

Formaldehyde (2), the second example, belongs to the point group C2v with symmetry elements which can be seen in the drawing of the formula.

▪Twofold symmetry-axis C2 (z).

▪Symmetry plane σv (xz).

▪Symmetry plane σv(yz) (plane of the σ-bonds).

Table. 1.3 shows the symmetry classes of the point group C2v.

The group orbitals in the valence shell of formaldehyde (2) are depicted in Fig. 1.6. The symmetry classes of the orbitals can be seen from orbital pictures and formula 2 in the coordinate system. As already pointed out in the discussion of ethene (1), it does not make sense to consider single C–H orbitals, since they do not belong to a symmetry class of the point group C2v.

Fig. 1.6 does not contain the 1s orbitals of C and O, which have the symbols 1a and 2a. The depicted orbitals 3a1, 4a1, 1b2, 5a, 1b1, and 2b2 contain 6 x 2 electrons. The antibonding orbitals 2b1* 6a1* 3b2*, and 7a1* are lying above the occupied orbitals. The n→ π* transition (2b1 ←2b2) is the energy-lowest transition, in which the nonbonding py orbital of the oxygen atom delivers an electron to the π* orbital of the CO double bond. The singlet ground state S0 (an 1A1 state) is thereby transformed to the S1 state 1A2 (b2 × b1 a2, Table. 1.3). The column on the right side of Table. 1.3 reveals that none of the components x, y, and z of the transition vector has the characters of a2. Accordingly, the n→π* transition is forbidden. This statement corresponds to the overlap exclusion: n(pyO) and π* are orthogonal to each other. A transition with a low intensity can be observed at ~300 nm.

In general, n→π* and π → π* transitions are discussed for carbonyl compounds. Ab initio calculations reveal that the π →π* transition of formaldehyde (2) has a high energy and should lie in the far-UV. Thus, a Rydberg transition seems to be the second transition, in which an n(2b2) electron is transferred to the 3s orbital of the next higher shell.

Considerations, which other electron transitions between the orbitals shown in Fig. 1.6 are allowed or forbidden, are certainly a good exercise. However, it should be noted here that mixed transitions occur in many organic molecules. The discussion of allowed and forbidden electron transitions in other point groups exceeds the scope of this book. Textbooks on UV/Vis spectroscopy are compiled in the references on p. 40.

The statements in Section 1.1.1 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 polarization 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 summarized in a modified Jablonski term scheme. From the ground state, which in general is a singlet state S0, absorption leads to higher singlet states S1, S2, etc.

Fig. 1.7 Jablonski term diagram with a visual representation of the electronic configurations.

Radiative processes: →

A Absorption

F Fluorescence

Ph Phosphorescence

Nonradiative processes: ⇝

IC Internal conversion

ISC Intersystem crossing

The return to S0 from S1, and more rarely from higher states Sn, can occur by the emission of radiation, known as fluorescence, or by nonradiative deactivation (internal conversion). Nonradiative spin-inversion processes (intersystem crossing) lead to triplet states T, which can return to S0, either by emission of radiation known as phosphorescence—disregarding the spin exclusion—or by renewed intersystem crossing (Fig. 1.7).

“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. 1.7 is therefore split into many energy terms, as shown schematically in Fig. 1.8. A specific energy level Etot corresponds to a particular electronic, vibrational, and rotational state of the molecule.

To a first approximation the three energy components can be separated

EtotEelectr.+Evibr.+Erot

Born-Oppenheimer approximation

For an electronic transition it follows that

ΔEtotEelectr. + ΔEvibr. + ΔErot

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. 1.8) is an additional nonradiative deactivation within each electronic state. In addition to the monomolecular processes described here it should also be noted that bimolecular photophysical processes (energy transfer: sensitization, quenching) and primary photochemical processes can occur.

Fig. 1.8 Schematic representation of the superimposition of electronic, vibrational, and rotational states; vi vibrational quantum numbers, Ji rotational quantum numbers.

1.1.2 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 d/by an increment dx of the width of the absorbing layer is

and evaluation of the integral

yields the function

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·e·c to give

Lambert-Beer law

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

If the absorbance is determined according to the Lambert-Beer law for all measured λ or ṽ, and from that the substance-specific value ɛ, the absorption plot ɛ(ṽ) or ɛ(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 characterized by their properties of position, intensity, shape, and fine structure.

From Fig. 1.9 it follows that the positions of the absorption bands depend on the nature of the electron transitions. For isolated chromophores, Table. 1.4 (see p. 13) 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. 1.9).

Fig. 1.9 Absorption regions of various electron transitions.

For certain chromophores the solvent also has a characteristic influence (see for example Table. 1.5 and Fig. 1.34).

A shift to longer wavelengths (red-shift) of a transition is called a bathochromic effect, and 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

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

where b is the width of the band at half height (Fig. 1.10).

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 ɛmax. The

following assignments have become customary:

Fig. 1.10 True and approximated areas of an absorption band.

For two states with an energy difference ∆E, the ratio of populations is given by

The transition to S1 leads to vibrational states with v’ 0, 1, 2, 3 … Because of very rapid relaxation to v’ 0 the fluorescence starts entirely from v’ 0 and leads to So with v 0, 1, 2 …

Fig. 1.11 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. The 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. 1.12 shows for 1,2,4,5-tetrazine 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 hypersurface of the ground state into that of the electronically excited state, i.e., all molecular parameters (bond lengths and angles, configuration, conformation, solvation cage, etc.) remain unchanged during the transition.

Fig. 1.11 Absorption and fluorescence as transitions between electronic, vibrational, and rotational levels.

Fig. 1.12 Vibrational structure of the n→ π* absorption of 1,2,4,5-tetrazine (3) (from Mason SF. 1959, J. Chem. Soc, 1263).

IVapor spectrum at room temperature (with vibrational mode).

IISpectrum at 77 K in an isopentane/methylcyclohexane matrix.

IIISpectrum in cyclohexane at room temperature.

IVThe 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. 1.13 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.

1.2 Sample Preparation and Measurement of Spectra

For analytical purposes, UV/Vis spectra are normally measured in solution. Optically pure solvents, available commercially, are used and allow transitions measured at concentrations of ~10–4 mol·L–1. For the weak bands of forbidden transitions, the concentration must be increased appropriately. (As a guide the absorbance A should be ≈ 1. For a layer thickness of 1 cm-length of the wave path through a quartz cell—it follows from the Lambert-Beer law that c·ɛ ≈ 1. If ɛmax =10n M–1 cm–1 the measurement should therefore be made at a concentration of 10–n mol·L–1.)

Solvents with their own absorptions in the measurement region are unsuitable. The best transparency down to the vacuum-UV region is shown by perfluorinated alkanes such as perfluorooctane. Sufficiently transparent down to 195 nm (for d 1 cm) are the saturated hydrocarbons pentane, hexane, heptane, or cyclohexane, and the polar solvents water and acetonitrile.

Methanol, ethanol, and diethyl ether are useable down to ca. 210 nm. In order of increasing the lower measurement limit, then follow dichloromethane (220 nm), chloroform (240 nm), and carbon tetrachloride (250 nm). Benzene, toluene, and tetrahydrofuran are generally only applicable above 280 nm. An increase in the interaction between the compound being measured and the solvent leads to the loss of fine structure. It is therefore recommended to use nonpolar solvents wherever possible. The effect of solvent polarity on the position of absorption bands is discussed in Table. 1.5 and Fig. 1.34 using the case of ketones as an example. In the customary double-beam spectrometers, the cell with the solution to be measured is placed in one beam and a cell with the pure solvent in the other beam. The intensities are then compared over the whole spectral region. Fig. 1.15 shows schematically the construction of a double-beam spectrometer.

Most instruments show the absorption A as a function of the wavelength A. In contrast to A the extinction coefficient ɛ has a specific value for a certain substance. It is therefore better to record a plot of ɛ against λ or even better against the wave number. 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 ɛ on the ordinate. Fig. 1.16 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 performed by this technique.

Some modern spectrometers permit the simultaneous measurement of several probes. Another modern technique concerns the determination of an ATEEM matrix by the simultaneous measurement of absorption, transmission, excitation, and emission.

1.3 Chromophores

1.3.1 Individual Chromophoric Groups and Their Interactions

As shown in Section 1.2, the position of an absorption band depends on the nature of the electronic transition involved. Table. 1.4 gives a list of the excitation energies of σ, π, and n-electrons in various isolated (chromophoric) groups. Although group orbitals have principally to be considered, bond orbitals are often used for the first characterization of an absorption.

Table. 1.5 (p. 14) shows the influence of the solvent on the λmax values of the π → π* transition of acetophenone.

Fig. 1.15 Schematic diagram of a double-beam spectrometer.

S Radiation source (UV: hydrogen or deuterium lamp, Vis: tungsten–halogen lamp).

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

Sp Beam splitter (rotating mirror).

MS Measurement cell with solution.

CC Control cell with pure solvent.

D Detector (photoelectron multiplier, array of diodes).

C/D/P Computer/display/printer that records the transmission or absorption.

The blue color of azulene (5) arises from an absorption in the visible region of the spectrum, not shown in the above spectra.

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 a 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.

Interactions between several chromophores or chromophores and auxochromes will be extensively discussed in the following chapters. As explicit examples formaldehyde (2) and glyoxal (8) 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 (8) which, in contrast to the colorless formaldehyde (2), is yellowish-green in the gas phase, shows an absorption at 450 nm, shifted by some 150 nm. In the associated n+→ π3* 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 π and π2and the two antibonding orbitals π3* and π4*; 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. 1.17, p. 14).

Fig. 1.17 Long-wavelength electronic transitions S0 → S1 in formaldehyde (2) and s-trans-glyoxal (8).

Table 1.5 Absorption maxima (π →π* transition) of acetophenone (H3C-CO-C6H5) in different solvents

λ

max

(nm)

Solvent

237

Cyclohexane

238

Dioxane

241

Dichloromethane

242

Ethanol

245

Water

1.3.2 Olefins and Polyenes

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 average energy level 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 Figs. 1.19 and 1.20, and Table. 1.6 (p. 15).

Fig. 1.18 Schematic energy diagram explaining the bathochromic shift of the π → π* transition of ethylenes with auxochromic groups X.

Accurate calculations demonstrate, in agreement with the observations, that λmax approaches a finite limiting value (n→ ∞). According to perturbation theory, different bond lengths (Peierls distortion) in the chain are the reason of this convergence. The limiting value λmax ≈ 500 nm should be reached for n ≈ 17. The corresponding HOMO-LUMO gap of 2.48 eV is much higher than the band gap (0.56 eV) of solid all-trans-polyacetylene, which is a semiconductor.

Fig. 1.19 HOMO–LUMO transitions in ethylene, 1,3-butadiene, and 1,3,5-hexatriene.

Fig. 1.20 Allowed long-wavelength transitions in all-trans-polyenes (9a–i).

Fig. 1.21 π-Electron distribution of 1,3,5,7-octatetraene (9d) in the singlet states S0, S1, and S2.

Fig. 1.22 UV spectrum of (Z)- and (E)-stilbene 10 at 295 K in methyl-pentane (from Dyck RH, McClure DS. 1962, J. Chem. Phys. 36, 2336).

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 an 1Ag state, reached by a for-bidden transition. Apart from HOMO - 1 → LUMO and HOMO → LUMO + 1 transitions a doubly excited configuration can make 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 (9d), Fig. 1.21 illustrates the electron distributions in S0, S1, and S2.

Fig. 1.23 Absorption spectra of p-carotenes of different configurations (11, 12).

The configuration of olefins also affects the position and intensity of absorptions. (Z)-Stilbene [(Z)-10] absorbs at slightly shorter wavelengths (higher wavenumbers) and less intensively than the (E)-isomer (E)-10 (Fig. 1.22).

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 (11) it is symmetry forbidden (cf. parity rule). On inclusion of a (Z)-double bond, the symmetry is changed as shown for (15-Z)-β-carotenes (12). The electron transition at 340 nm is allowed and leads to the so-called (Z)-peak of the carotenes (Fig. 1.23).

Table 1.8 UV absorption of dendralenes and radialenes

Empirical rules for the absorption maxima of the long-wavelength π → π* transitions of dienes and trienes were established by Woodward in 1942 and later and independently by Fieser and Scott. Because of many exceptions, these increment rules have lost their significance.

The series of (Z,Z)-1,3-cycloalkadienes in Table. 1.7 demonstrates the influence of steric factors on the 1,3-diene chromophore in carbocycles. Moreover, the effect of (Z)- and (E)-configurations is shown for 1,3-cyclododecadiene.

In contrast to the linear conjugated compounds, cross-conjugated systems do not exhibit a bathochromic shift, when the number of double bonds is increased. Typical dendralenes and radialenes are compiled in Table. 1.8.

Table. 1.9 contains the series of annulenes: aromatic (4n + 2)-π-electron systems, antiaromatic 4n-π-electron systems, and nonplanar molecules with the so-called nonaromatic (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.

Fig. 1.24 (a) Energy scheme of the π-orbitals of benzene; (b) electronic excitations in benzene.

1.3.3 Benzene and Benzenoid Aromatics

In contrast to 1,3,5-hexatrienes (see p. 15) the π2/π3 and π4*/π5* 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. 1.24a, b).

In the UV spectrum of benzene (Fig. 1.25), the highly structured a-band and the p-band correspond to symmetry forbidden transitions. The p-band, which appears as a shoulder, “borrows” intensity from the neighboring allowed transition (β-band). Because of the symmetry prohibition, there is no 0←0 transition in the α-band. The v’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 v’B (Fig. 1.25).

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.

Fig. 1.25 Absorption spectrum of benzene.

An overview of monosubstituted benzenes is given in Table. 1.10.

The change in the spectra caused by the introduction of two or more substituents into the benzene ring, compared with the monosubstituted derivatives, is particularly marked in those cases where both an electron-withdrawing group and an electron-donating group are present (see Table. 1.11). In these cases the increase in the size of the chromophore is linked to the possibility of an intramolecular charge transfer (ICT). p-Nitrophenol (13) is an example.

The effect should be even stronger in the p-nitrophenolate anion than in p-nitrophenol itself (Fig. 1.26a). This is confirmed by Fig. 1.26b 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 p-dimethylaminobenzonitrile (14).

Biphenyl (15) has in comparison to benzene a bathochromically shifted p-band (Fig. 1.27). Steric interaction between the ortho-standing H atoms of 15 causes a torsion of ~42° between the benzene ring planes. Increasing planarization in 9,10-dihydrophenanthrene (16) and 4,5,9,10-tetrahydropyrene (17) effects a further bathochromic shift of the absorption (Fig. 1.27).

An analogous argumentation is valid for the series of fluorene 18, 19 and 20 for which the λmax values 261, 247, and 235 nm are found for the allowed electron transition in ethanol.

Fig. 1.26 UV/Vis spectra of o-, m-, and p-nitrophenol:

(a) in 10–2 molar hydrochloric acid,

(b) in 5 x 10–3 molar sodium hydroxide.

The prototype of polycyclic aromatic hydrocarbons (PAHs) consists exclusively of condensed benzene rings. Linear anellation as in anthracene or naphthacene and angular anellation as in phenanthrene, chrysene, etc. are subsumed as kata-condensation. Peri-condensed systems, such as pyrene or perylene, represent the second category of benzenoid aromatic hydrocarbons.

In the spectra of condensed benzenoid aromatics are many common features. The two highest occupied orbitals πn-1 and πn and the two lowest empty orbitals π*n+1 and π*n+2 are no longer degenerate as they are in benzene. Four electron transitions IIV are possible between them (Fig. 1.28). Due to the symmetrical arrangement of π and π* orbitals related to the α-line (Coulomb integral) in the energy level scheme of alternating hydrocarbons, the transitions II and III are isoenergetic.

Fig. 1.27 UV spectra of biphenyl (15 …), 9,10-dihydrophenanthrene (16 - - -), and 4,5,9,10-tetrahydropyrene (17 —) in isooctane.

However, configuration interaction removes this degeneration. The topology of the condensed, benzenoid aromatics differentiates between a large (type a) and a small (type b) splitting of the orbital energies (Fig. 1.28). The α-band (2 ≤ log ɛ ≤ 3) can be easily identified in the long-wavelength region of the absorption of type a systems, whereas it is engulfed in type b by the higher intensity of the p-band. The p-band corresponds to the HOMO → LUMO transition, that is polarized for acenes in the direction of the short axis. The β-band has an even higher intensity (log ɛ ≈ 5). The neighboring transition IV (β’-band) and still higher transitions play a minor role for the characterization of the UV/Vis spectra of benzenoid aromatics.

Fig. 1.29 shows the UV/Vis spectra of benzene, naphthalene, anthracene, naphthacene (tetracene), and phenanthrene. The α-band is visible in benzene, naphthalene, and phenanthrene; in anthracene and naphthacene it is superimposed by the intense p-band.

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 polarized parallel to the short axis. The bathochromic shift in the acene series leads to the members from tetracene onwards being colored:

Fig. 1.28 Schematic representation of the excitation of electrons and the corresponding transitions in condensed benzenoid aromatics having strong (type a) or weak (type b) configuration interactions.

Benzene, naphthalene, anthracene colorless

Tetracene (naphthacene) orange-yellow

Pentacene blue-violet

Hexacene dark green

If the anellation is nonlinear, characteristic changes occur in the spectra (e.g., anthracene and phenanthrene, Fig. 1.29). As well as the linear anellated tetracene the condensation of four benzene rings can lead to four angular systems, benz[a]anthra-cene, benzo[c]phenanthrene, chrysene, and triphenylene, and the peri-condensed system pyrene. Of these only naphthacene absorbs in the visible region; the others are colorless, but show colored fluorescence.

Fig. 1.29 UV/Vis spectra of condensed aromatic hydrocarbons (in heptane).

The diversity of structures in the series of kata-condensed aromatics increases fast with increasing numbers n of benzene rings:

Number of benzene rings

n

General formula C

4

n

+2

H

2

n

+4

Possible structure isomers

1

C

6

H

6

1

2

C

10

H

8

1

3

C

14

H

10

2

4

C

18

H

12

5

5

C

22

H

14

17

6

C

26

H

16

37

7

C

30

H

18

123

8

C

34

H

20

446

9

C

38

H

22

1,689

10

C

42

H

24

6,693

Table 1.12 Long-wavelength absorptions of benzenoid aromatics of the C30H18 series

Compound

λ

max

[nm]/(Solvent)

Color of crystal

Phenanthro[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

]hexacen

651(1-methylnaphthalene)

blue-green

Heptacene

840(1-methylnaphthalene)

black-green

Many of these, only to a small part, known arenes are nonplanar because of steric hindrance and show additionally stereoisomerism.

The number of isomers is still higher, when peri-condensed systems are included. (Peri-anellation has to be exercised with caution, because this anellation can not only lead to arenes with Kekulé structures but also to mono- and biradicals.)

Table. 1.12 contains a selection of five compounds C30H18, which consist of seven kata-condensed benzene rings. 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, which leads from the UV into the NIR region.

Monolayers of graphite are called graphenes. Graphene sheets can be very large and are insoluble in organic solvents. Their UV/Vis/NIR spectra can be only measured in thin films or in suspensions. Fig. 1.30 presents the absorption of C216-nano-graphene (21) (p. 23), obtained in a selective synthesis. It has the area of a regular hexagon, but contains a hollow part in its center. Therefore, it can be regarded as an extended coronoid. The absorbance of 21 was measured in a suspension. Time-dependent density functional theory (TD-DFT) calculations provide the wavelengths of the p, β, and β’ bands of this huge arene in an impressive manner.

Nonalternating PAHs are compounds which contain one or more odd-numbered rings. Whereas biphenylene (22) is an alternating hydrocarbon, fluoranthene (23) represents nonalternating hydrocarbons.

The energy scheme of the 77-orbitals of nonalternating hydrocarbons is nonsymmetrical relative to the α line (Coulomb integral).

The number of energetically close lying electronic transitions of PAHs increases with increasing numbers of w-electrons. At the end of this section, a comparison shall be drawn between the planar PAH C60H24 (24) and fullerene C60 (25), which has a spherical topology of 12 five-membered and 20 six-membered rings. Both compounds absorb in the entire range between 200 and 700 nm. Their maxima at longest wavelengths have a low intensity. Compound 24 is insoluble. It can only be measured in the solid state as a thin film.

Fig. 1.30 UV/Vis/NIR spectrum of the C216-nanogra-phene (21), extended coronoid: measurement in chlor-obenzene suspension (red curve), TD-DFT calculation of the p-band (613 nm), β-band (519 nm), and the cluster of β′-bands (black sticks) and the corresponding simulation of the absorbance (blue curve) (from Müllen K et al. 2016, J Am Chem Soc138, 4322).

1.3.4 Heteroaromatics (Hetarenes)

Furan (26), pyrrole (27), and thiophene (28) show in hexane almost structureless absorptions at the border to the far-UV.

The spectra of five-ring hetarenes containing two or in some cases even three heteroatoms look similar. 1,2,3-Triazole (29) for example has in THF an absorption maximum at 215 nm, whereas 1,3,4-thiadiazole (30) has in cyclohexane a maximum at 305 nm.

The UV spectrum of pyridine (31) resembles the spectrum of benzene. However, the long-wavelength n → π* transition is allowed. The n → π* transition is located at ~270 nm below the foot of the ππ* band. Table. 1.13 presents the long-wavelength absorption maxima of several azines. In particular, neighboring N-atoms lead to an interaction of their electron lone-pairs, which causes an energetical splitting to a higher and a lower lying n orbital. Hence, a bathochromic shift of the n → π* transition occurs. The log e values of the π → π* transitions are between 3 and 4 and those of the n → π* transition between 2 and 3.

The condensation of benzene rings to hetarenes leads to UV/ Vis spectra, which are similar to the spectra of polycyclic are-nes with the same number of rings. Going from pyrrole (27) to indole (32) and carbazole (33), a red-shift can be observed. An analogous effect is found for the series pyridine, quinoline, and acridine. The longest-wavelength maximum of quinoline and isoquinoline lies in cyclohexane at 314 and 317 nm, respectively, and for acridine even at 380 nm.

Indigo (34) has a central CC double bond with captodative-substitution on both sides (Fig. 1.31). It represents a twofold cross-conjugated system whose long-wavelength absorption is predominantly due to a HOMO → LUMO electron transition. These frontier orbitals have π and π* characters, respectively. The excitation S0 → S1 increases the electron density at the O atoms and decreases it at the N atoms. The long-wavelength absorption maximum is observed at 600 nm (acetonitrile). TD-DFT calculations give a similar value.

Table 1.13 Long-wavelength absorption maxima of azines in cyclohexane: λmax (nm)/log ɛ

Pyridazine246/3.11350/2.50

Pyrimidine243/3.31298/2.47

Pyrimidine260/3.75398/3.02

1,3,5-Triazine272/3.00

1,2,4-Triazine*248/3.48374/2.60

1,2,4,5-Tetrazine252/3.33320/1.42542/2.92

aMeasurement in methanol

Fig. 1.31 HOMO → LUMO electron transition in indigo (34).

Heterocycles can act as electron donors or electron acceptors in D-77-A systems. Compound 35 contains a carbazole unit as donor D and a terpyridine moiety as acceptor A. The HOMO of 35 is predominantly localized in the donor region, whereas the LUMO has a high electron density in the acceptor region (Fig. 1.32a).

Fig. 1.32 (a) Electron density plot of the frontier orbitals of 35; (b) UV spectrum of 35 (D–π