This chapter outlines the principles that underlie the absorption of energy in the UV-visible region by electrons in molecules with double bonds and the associated experimental arrangements. It cites examples of organic molecules that have characteristic absorption spectra and are associated with key molecular features. It also explains how λmax is determined and related to structure and shows how the extent of absorption at a given wavelength depends upon the concentration of the sample. The chapter illustrates the effects of conjugation and the consequent use of UV-visible spectroscopy in structural and quantitative analysis. It includes examples that cover inorganic systems and those of medical and biological relevance.
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Chapter
Electronic (ultraviolet–visible) absorption spectroscopy
Chapter
Electronic spectroscopy
This chapter covers spectroscopy involving the excitation of an electron from one state to another. It talks about how electronic transitions occur at longer wavelengths and are often seen in the visible state as the separation of electronic energy levels of open-shell molecules is typically small. It also mentions the characteristic potential energy curve of the electronic state that supports its own set of vibrational and rotational levels, emphasizing that electronic transitions involve a change of associated vibrational and rotational quantum numbers. The chapter defines the quantum numbers of the electronic wavefunction. It describes an electron as a fundamental particle with a very small mass, which requires a full quantum mechanical treatment.
Book
Simon Duckett, Bruce Gilbert, and Martin Cockett
Foundations of Molecular Structure Determination begins with an overview of the topic and an examination of energy levels and the electromagnetic spectrum. The next chapter covers rotational and vibrational spectroscopy. There follows a chapter looking at electronic (ultraviolet-visible) absorption spectroscopy. The text also discusses nuclear magnetic resonance spectroscopy. The final two chapters cover mass spectrometry and X-ray diffraction and related methods.
Chapter
Infrared spectroscopy
This chapter highlights the principles behind infrared spectroscopy, focusing on the energy range of the infrared rays. These cause vibrational excitation of bonds within a molecule. It discusses stretching for higher energy and bending lower energy as the types of bond excitation. It also refers to the absorption of certain wavelengths of infrared radiation which can be correlated with the bending or stretching of specific types of bonds within a molecule. The chapter analyses the infrared spectra of organic molecules. These are complicated by bond oscillations in the whole molecule and affect the absorption of the incident radiation—such absorption gives rise to overtones and harmonics. It mentions spectra recorded in solution or as a neat substance, which show complexity due to hydrogen bonding with solvents or the presence of dimeric or polymeric associated species..
Book
Laurence M. Harwood and Timothy D.W. Claridge
Introduction to Organic Spectroscopy aims to provide an understanding of spectroscopic techniques in the analysis of chemical structures. The book starts with an introduction to the theory. It then looks at ultra violet-visible spectroscopy. Next it considers infrared spectroscopy. Then it considers nuclear magnetic resonance spectroscopy in basic terms and then in more detail. The last chapter is about mass spectrometry.
Chapter
Introductory theory
The chapter provides some background on the development of non-destructive spectroscopic methods of analysis which can be carried out on small amounts of materials. This has provided the fundamental thrust behind the progress of organic chemistry. It looks at the identification of unknown molecules of high complexity which carry out samples that range from several nanograms to a milligram of material. It also refers to techniques that are used routinely by organic chemists for structural analysis, such as ultraviolet (UV) spectroscopy, which came into general use during the 1930s. The chapter presents analytical techniques that involve the absorption of specific energies of electromagnetic radiation. These correspond exactly in energy to specific excitations within the molecule. It explores what we know of the transitions which may be induced by absorption of a certain wavelength of electromagnetic radiation. These can be used to infer structural features.
Chapter
Mass spectrometry
This chapter highlights the fundamental principles of mass spectrometry. It explains the use of this technique in the determination of molecular masses and formulae and the interpretation of fragmentation patterns as a means to the determination of structure. It also provides some examples of structural analysis that show how problems can be solved using the technique and approach of mass spectrometry. The chapter highlights spectra from unknown compounds, emphasizes the use of mass spectrometry, and provides complementary information about the molecules under investigation. It surveys the very wide range of structural and analytical applications of mass spectrometry across the chemical and biochemical sciences, focusing on its extreme sensitivity.
Chapter
Mass spectrometry
This chapter talks about a type of analysis which enables the mass of the molecule to be determined, including how it is constructed. It analyses the basic mass spectrometric experiment, first demonstrated by Wilhelm Wien in 1989, wherein individual molecules of a sample are bombarded with high energy electrons, causing the ejection of electrons from the substrate on impact. It also refers to electron impact ionization. This indicates positively charged ions which are accelerated along an electrical potential and pass through a magnetic field. The chapter explains the production of intact radical cation and the impact of an electron which cause sufficient energy to be transferred to the vibrational modes of the molecule. It mentions fragments that retain the positive charge, which are deflected in the magnetic field.
Book
John M. Brown
Molecular Spectroscopy provides an introduction to the spectroscopy of diatomic molecules. Following a general introduction to the subject, the second chapter lays out the essential quantum mechanical tools required to understand spectroscopy. The following chapter uses this quantum mechanical framework to establish the selection rules which govern spectroscopic transitions. Chapters 4–7 describe the various branches of spectroscopy covered by the book; rotational, rotational—vibrational, Raman, and electronic spectroscopy. Quantum mechanics is used to derive formulae for the various energy levels involved and for the relative intensities of different types of transition. From these, the appearances of the different types of spectra are derived. The molecular parameters on which these spectra depend are defined and the structural information which can be derived from these is discussed.
Chapter
Nuclear magnetic resonance spectroscopy
This chapter talks about the concept of nuclear spin, which focuses on nuclear magnetic moments and their study through nuclear magnetic resonance (NMR) spectroscopy. It uses the proton,1H, as the simplest example that illustrates how the phenomenon arises and describes the operation of a basic NMR spectrometer. It also describes a range of organic molecules that show how electronic effects of different substituent groups enable these to be recognized by the chemical shifts of the protons concerned, leading to the use of the technique as a diagnostic tool. The chapter analyses the occurrence of spin–spin splittings that demonstrate how neighbouring protons interact with each other. It discusses the development of Fourier transform instrumentation and its application to NMR spectroscopy and outlines the principles and uses of magnetic resonance imaging (MRI) in medically-related studies.
Chapter
Nuclear magnetic resonance spectroscopy: further topics
This chapter analyses the power of modern pulse-Fourier to transform nuclear magnetic resonance (NMR). The chapter then shows a variety of techniques that assist in the interpretation of spectra and allows for the routine observation of nuclei other than protons. It reviews the original motivation for establishing pulse-Fourier transform methodologies which may be accumulated with a concomitant increase in signal-to-noise ratio for a given period of data collection. It also looks at the possibility of using more than one pulse prior to data acquisition. This controls the behaviour of nuclear spins. The chapter considers how an ensemble of nuclei behave and the processes that occur following pulse excitation of a sample. It uses a pictorial approach to show the vector model of NMR.
Chapter
Nuclear magnetic resonance spectroscopy: the basics
This chapter describes nuclear magnetic resonance (NMR) spectroscopy as the most powerful and versatile of all analytical techniques used by organic chemists. It highlights the spectroscopic features of NMR spectroscopy, which correlate with individual atoms within a molecule rather than groups of atoms. It also emphasizes the possibility of determining the structures of small-to-medium-sized organic molecules in solution and focuses on solution spectroscopy as it is in this form of NMR that is the most useful in organic chemistry. The chapter explains that NMR occurs as a result of the fact that the nuclei of certain atoms possess spin, which is characterized by a quantum number and takes integer and half-integer values. It points out that nuclei with zero spin are not amenable to NMR observation, although most elements have at least one isotope that possess nuclear spin.
Chapter
Overview, energy levels, and the electromagnetic spectrum
This chapter reviews different experimental strategies that determine the geometrical arrangement of atoms in space which make up a particular molecule. It explains the choice of techniques to use when dealing with crystalline, liquid, or gas phase samples. It also outlines the structural relationship between functional groups in an organic molecule and the arrangements of ligands around a metal centre in a transition metal complex, including the way in which a cardiovascular drug molecule might bind within the protein cavity in haemoglobin. The chapter looks at the methods that rely on the interaction of photons or electrons with the molecule of interest. It illustrates the use photons that exploit whichever regions of the electromagnetic spectrum are appropriate to.
Chapter
Photoelectron spectroscopy
This chapter discusses photoionization, which is a process carried out in a gas phase that gives a large amount of energy to a molecule to make it ionize. It refers to the photoelectric effect in which radiation of a variable frequency is shone on the surface of an easily ionizable metal, such as an alkali. It also examines the energy required to ionize a hydrogen atom in a particular orbital and remove the electron from the influence of the Coulombic attraction of the nucleus. The chapter then clarifies how the ionization energy can be equated with the energy required to remove an electron from a particular molecular orbital of the molecule. It cites an experiment wherein the monochromatic radiation in the vacuum is focused onto a gaseous sample of molecules, which causes them to be ionized.
Chapter
Quantization and molecular energy levels
This chapter discusses the physical properties of atoms and molecules, which are quantized. This means that they can only take certain discrete values rather than a full range of continuously varying values. The chapter examines the quantized energy levels of a molecule. These are called stationary states, which describe an energy that is independent of time. The chapter also outlines the procedure for constructing a quantum mechanical Hamiltonian, which includes writing down the complete and classical expression for the energy of the system. The chapter reviews the general approach to the determination of energy levels of a physical system. This involves two stages: First, the appropriate Hamiltonian is defined and, in a second step, the eigenfunctions and eigenvalues of the Hamiltonian are determined. The chapter also discusses the Schrödinger equation, which can only be solved for one-body systems.
Chapter
Radiation and matter
This chapter begins by defining spectroscopy, which is the study of the way in which electromagnetic radiation interacts with matter as a function of frequency or wavelength. It describes electromagnetic radiation as a transverse waveform that consists of oscillating electric and magnetic fields that point transversely to the propagation of the wave. The chapter emphasizes that the most important point of view of spectroscopy is that energy can be transferred from the source to the detector in the form of electromagnetic radiation. It analyses the exchange between radiation and matter, which involves the interaction between the oscillating electric field in the radiation with the appropriate dipole moment in the molecule. It reviews the practice of spectroscopy that consists of the measurements of the discrete amounts of energy which are passed between molecules and radiation.
Chapter
Raman spectroscopy
This chapter discusses Raman spectroscopy, which is based on a light scattering effect and named after the Indian physicist C.V. Raman, who was the first to observe it after A. Smekal had theoretically predicted it in 1928. Raman spectroscopy provides a way for obtaining information on the rotational and vibrational levels of a homonuclear diatomic molecule, which is not possible using rotational and rotational–vibrational spectra. The chapter also mentions Rayleigh scattering wherein the great majority of photons are scattered in all directions without a change in energy. The chapter illustrates how a modern Raman spectrometer probes light from a monochromatic source directed at the molecular sample. The scattered light is then viewed at right angles to the incoming beam. Its component wavelength is resolved with either a monochromator or an interferometer.
Chapter
Rotational and vibrational spectroscopy
This chapter presents a brief introduction to rotational and vibrational spectroscopy. It explains how pure rotational spectroscopy can be used in the precise determination of molecular geometry in small molecules and provide a direct means to deduce the shape of larger molecules. It also looks at how vibrational spectroscopy can be used in the measurement of geometrical parameters of gas phase molecules and in providing information about the structure and shape of molecules in the condensed phase. The chapter discusses the important role vibrational spectroscopy plays in characterization and analysis in organic chemistry where it is used typically in tandem with mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy. It analyses concepts on a molecular scale by exploring how light interacts with the energy levels associated with rotational and vibrational degrees of freedom.
Chapter
Rotational spectroscopy
This chapter cites a gas phase sample that demonstrates molecules in continual motion, appropriate to its thermal energy. It describes the continual motion of the molecules as rotational energy, which is the kinetic energy associated with the tumbling motion of the molecules relative to an observer in the laboratory. The chapter explains that the transitions between individual rotational levels give rise to the rotational spectrum of the molecule and that the quantum states of a single molecule remain well-defined as long as the molecule is in isolation. However, the collision of a molecule with another species can change the rotational state. The definition of the spectrum is then in danger of being destroyed. It explains that the orientation of individuals in a solid state is usually fixed in space, and energy cannot be stored in the form of rotational motion, which is why rotational spectroscopy is carried out on gaseous samples at low pressure.
Chapter
Transition probabilities and selection rules
This chapter explains how a spectroscopic experiment registers the change of a molecule from one quantum state to another, which is a process known as spectroscopic transition. It points out that the heart of the spectroscopic experiment is the exchange of energy between radiation and matter as the energy required to drive spectroscopic transition is provided by electromagnetic radiation. It also discusses the change of state brought about in a spectroscopic transition. This requires solving the time-dependent Schrödinger equation. The chapter looks at the transitions studied in molecular spectroscopy that involve the interaction between the electric dipole moment of the molecule and the electric field of the radiation. It refers to the conversion of energy expression to quantum mechanical form, which treats the field classically and the dipole moment quantum mechanically.
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