Ultraviolet-Visible Spectroscopy and Mass Spectrometry

In addition to IR and NMR spectroscopy there are many other instrumental techniques that are useful to the organic chemist. Two of these, ultraviolet-visible spectroscopy and mass spectrometry, are discussed in this chapter. Ultraviolet-visible spectroscopy is presented first. The use of this technique to obtain information about the conjugated part of a molecule is described. Then mass spectrometry is discussed. This technique provides the molecular mass and formula for a compound. In addition, the use of the mass spectrum to provide structural information about the compound under investigation is presented. 

Look for this logo in the chapter and go to OrganicChemistryNow at http //now.brookscole.com/hornback2 for tutorials, simulations, problems, and molecular models. 

CHAPTER 15 ULTRAVIOLET-VISIBLE SPECTROSCOPY AND MASS SPECTROMETRY 

The amount of light absorbed by the sample is plotted along the y-axis as the absorbance (A), which is defined as 

Although the Amax and can be used like a melting point to aid in the identification of a compound, they also provide information about the energy separation between the MOs of the compound. Let s see how to use this to obtain information about the structure of the compound. 

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To bring you up to speed on spectroscopy, we cover the basics in Chapter 5. We give you the executive summary on infrared (IR), ultraviolet-visible (UV-vis), mass spectrometry (mass spec), and nuclear me netic resonance (NMR). In addition, many of the chapters in this book have a spectroscopy section at the end where we simply cover the essentials concerning the specific compounds that you study in that chapter. 

We will concentrate upon the most commonly used techniques in organic structure determination nuclear magnetic resonance (NMR), infrared (IR) and ultraviolet-visible (UV-Vis) spectroscopy, and mass spectrometry (MS). The amount of space devoted to each technique in this text is meant to be representative of their current usage for structure determination. 

The techniques considered in this chapter are infrared spectroscopy (or vibrational spectroscopy), nuclear magnetic resonance spectroscopy, ultraviolet-visible spectroscopy (or electronic spectroscopy) and mass spectrometry. Absorption of infrared radiation is associated with the energy differences between vibrational states of molecules nuclear magnetic resonance absorption is associated with changes in the orientation of atomic nuclei in an applied magnetic field absorption of ultraviolet and visible radiation is associated with changes in the energy states of the valence electrons of molecules and mass spectrometry is concerned... 

Coupled spectroscopic methods such as TLC-UV (ultraviolet) and visible spectroscopy, TLC-mass spectrometry, and TLC-FTIR (Fourier transform infrared) have been developed to overcome this difficulty [7]. Their future application in the TLC analysis of natural pigments will markedly increase the information content of this simple and interesting separation technique. The automation of the various steps of TLC analysis (sample application, automated developing chambers, TLC scanners, etc.) greatly increased the reliability of the method, making it suitable for official control and legislative purposes [8]. 

Chromatography, infrared spectroscopy, mass spectrometry, nuclear magnetic resonance spectroscopy, ultraviolet-visible spectroscopy and others have also been benefited from the properties of wavelet processing for data compression, noise removal, base-line correction, zero crossing and regression (Leung et al. 1998). 

D6t6rminQtion of UlolBCUlsr Structuro is one of the central themes of organic chemistry. For this purpose, chemists today rely almost exclusively on instrumental methods, four of which we discuss in this text. We begin in this chapter with infrared (IR) spectroscopy. Then in Chapters 13 and 14, we introduce nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS), respectively. A brief introduction to ultraviolet-visible spectroscopy is contained in Chapter 20 as part of our discussion of conjugated systems. 

Xu, H., Que, G., Yu, D., Lu, J.R., (2005),Characterization of petroporphyrins using ultraviolet-visible spectroscopy and laser desorption ionization time of flight mass spectrometry. Energy and Fuel, 19,517-524. 

Previous authors have taught the principles of solving organic structures from spectra by using a combination of methods NMR, infrared spectroscopy (IR), ultraviolet spectroscopy (UV) and mass spectrometry (MS). However, the information available from UV and MS is limited in its predictive capability, and IR is useful mainly for determining the presence of functional groups, many of which are also visible in carbon-13 NMR spectra. Additional information such as elemental analysis values or molecular weights is also often presented. 

Traditional instrumental techniques, such as nuclear magnetic NMR, mass spectrometry infrared (IR) spectroscopy, ultraviolet-visible spectrophotometry, and gas and liquid chromatography and size-exclusion chromatography, are used extensively for purity assessment and molecular structure and molecular weight measurements of monomers and polymers [61]. 

Numerous, wide-ranging spectroscopic techniques will be presented in this volume, with the exception of nuclear magnetic resonance (NMR), which was the subject of Volumes 176, 177, and 239 of Methods in Enzy-mology, and mass spectrometry, which was the subject of Volume 193. Examples of techniques from each of three major areas, ultraviolet/visible spectroscopy, vibrational spectroscopy, and electron or electron/nuclear magnetic resonance, are presented in this volume. Also included are special topics like rapid-scan diode-array spectroscopy, terbium labeling of chromopeptides, and deconvolution of complex spectra that are covered in chapters in Section IV of this volume. 

Most physical properties of oxazoles have now been extensively explored. This chapter serves as an overview of the most important areas and updates the previous edition, in which the spectroscopic chapter remains relevant in aU details. NMR (surely now the single most important technique to the practicing organic chemist) is covered first and in the most detail, followed by a review of mass spectrometry, infrared and ultraviolet/visible spectroscopy, microwave spectroscopy, and other techniques. This order parallels that used in the previous edition, with some changes the proton and carbon NMR tables have been expanded, oxygen and fluorine NMR are now covered, as are microwave spectroscopy and other methods, such as photoelectron spectra. 

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