Principles of Spectroscopy

These methods—which often required weeks, months, even years—are still used in appropriate situations. But since the 1940s, various types of spectroscopy have simplified and speeded up the process of structure determination greatly. Automated instruments have been developed that permit us to determine and record spectroscopic properties often with little more effort than pushing a button. And these spectra, if properly interpreted, yield a great deal of structural information. For example, 2-phenylethanol can easily be identified from its NMR spectrum alone. 

Spectroscopic methods have many advantages. Usually only a very small sample of material is required, and it can often be recovered if necessary. The methods are rapid, sometimes requiring only a few minutes. And usually we obtain more detailed structural information from spectra than from ordinary laboratory methods. 

In this chapter, we will describe some of the more important spectroscopic techniques used today and how they can be applied to structural problems. But first, let us examine some general principles that form the basis of most of these techniques. 

Equation 12.2 describes the relationship between the energy of light (or any other E= hv. Theenei of light form of radiation), E, and its frequency, v (Greek nu, pronounced new ). directly proportional to its 

The equation says that there is a direct relationship between the frequency of light and its energy the higher the frequency, the higher the energy. The proportionality constant between the two is known as Planck s constant, h. Because the frequency of light and its wavelength are inversely proportional, the equation can also be written 

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In the preceding section, we presented principles of spectroscopy over the entire electromagnetic spectrum. The most important spectroscopic methods are those in the visible spectral region where food colorants can be perceived by the human eye. Human perception and the physical analysis of food colorants operate differently. The human perception with which we shall deal in Section 1.5 is difficult to normalize. However, the intention to standardize human color perception based on the abilities of most individuals led to a variety of protocols that regulate in detail how, with physical methods, human color perception can be simulated. In any case, a sophisticated instrumental set up is required. We present certain details related to optical spectroscopy here. For practical purposes, one must discriminate between measurements in the absorbance mode and those in the reflection mode. The latter mode is more important for direct measurement of colorants in food samples. To characterize pure or extracted food colorants the absorption mode should be used. 

General principles of spectroscopy and spectroscopic analysis 5.3.5 Sample handling... 

The fundamental principles of spectroscopy which are applied for visible spectroscopy are also applicable to ultraviolet region. 

Chang, R., Basic Principles of Spectroscopy, McGraw-Hill, New York, 1971. A concise, informative treatment of many branches of spectroscopy. 

The major kinds of spectroscopy used for structural analysis of organic compounds are listed in Table 9-1. The range of frequencies of the absorbed radiation are shown, as well as the effect produced by the radiation and specific kind of information that is utilized in structural analysis. After a brief account of the principles of spectroscopy, we will describe the methods that are of greatest utility to practical laboratory work. Nonetheless, it is very important to be aware of the other, less routine, methods that can be used to solve special problems, and some of these are discussed in this and in Chapters 19 and 27. 

Change R (1971) Basic Principles of Spectroscopy, McGraw-Hill, New York. 

For this reason, the reviewer proposes to introduce the reader to Fourier transform spectroscopy in the hope that he will make use of it. The basic physical principles of spectroscopy and the theory and practice of Fourier transform spectroscopy are described. Its advantages and disadvantages are discussed relative to spectroscopic problems and always with reference to the grating spectrometer as representing conventional spectroscopy. 

It is proposed to recapitulate the basic physical and optical principles of spectroscopy in this review. For the comparison of different methods, we concentrate on the determination of wavelength as an essential part of spectroscopy. We also comment on the power of resolution of the various instruments and the instrument line-shape function. 

Fig. 35. The n molecular orbitals according to Huckel MO calculations (kfi), energy level diagram (center) and term diagram of benzene with electronic transitions indicated (right). Quoted from [6]. Copyright John Wiley and Sons, Ltd. Reproduced with permission. The other parts are taken from Chang R (1971) Basic principles of spectroscopy. McGraw-Hill Kogakushi, Ltd, Tokyo. Reproduced with permission of the McGraw-Hill Companies...

The principle of spectroscopy is very simple a sample is exposed to a varying frequency of radiation. The outgoing radiation is captured by a detector. Until the frequency reaches a quantum value, the radiation flux remains constant. When the frequency corresponds to a transition, described by Eq. 8.12, between two energy levels, a quantum of energy is absorbed. The absorption results in the... 

Spectroscopic techniques (IR, MS, and NMR) are covered in Chapters 12 and 13, so that they can be included in the first semester. This early coverage is needed to allow effective use of spectroscopy in the laboratory. Still, a large amount of organic chemistry has been covered before this digression into structure determination. The principles of spectroscopy are practiced and reinforced in later chapters, where the characteristic spectral features of each functional group are summarized and reinforced by practice problems. 

Principles of Spectroscopy Nuclear Magnetic Resonance Spectroscopy A WORD ABOUT... 

Infrared (IR) and Raman are both well established as methods of vibrational spectroscopy. Both have been used for decades as tools for the identification and characterization of polymeric materials in fact, the requirement for a method of analysis synthetic polymers was the basis for the original development of analytical infrared instrumentation during World War II. It is assumed that the reader has a general understanding of analytical chemistry, and a basic understanding of the principles of spectroscopy. A general overview of vibrational spectroscopy is provided in Sec. 5 for those unfamiliar with the infrared and Raman techniques. 

These are the simplest processes in spectroscopy. The principles of spectroscopy will be a recurring theme as we probe the microscopic structure of many-electron atoms and molecules, because spectroscopy remains the most precise and adaptable tool for controlling and measuring the quantum mechanical characteristics of a chemical system. From spectroscopy comes our most precise molecular geometries and successful theories of chemical bonding, as well as many of our most powerful analytical techniques. 

A detailed discussion of the principles of spectroscopy is beyond the scope of this text, but it is hoped that sufficient information is given to point the reader in the right direction. Most spectroscopic methods are based on empirical collations of vast amounts of data obtained with model compounds of known stmcture, and in the interpretation of the spectra of unknowns, some knowledge of these data is required. It will only be possible to reproduce a few selected examples here. Applications of spectroscopic procedures in lipid analysis have been reviewed in some detail elsewhere [153,483]. The mass spectra of fatty acid derivatives are described in Chapter . 

A review of spectroscopy as a tool for polymerization process monitoring and control is given in this chapter. Initially, fundamental principles of spectroscopy are reviewed, including a brief discussion about the nature and properties of the electromagnetic wave and its interaction with matter, followed by a more detailed discussion about NIR radiation. In the second part of this chapter, cafibration techniques used most frequently for the development and implementation of control applications are briefly discussed. Several applications of spectroscopy to other fields and to other control of polymerization reactors are also included in order to illustrate the versatility of spectroscopic methods for the development of monitoring and control strategies of chemical processes. The final part of the chapter presents relevant applications of the spectroscopic methods for in-situ and in-line monitoring and control of solution, suspension, and emulsion polymerization processes. 

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