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2005 | Buch

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Organic Spectroscopy

verfasst von: L. D. S. Yadav

Verlag: Springer Netherlands

Enthalten in: Springer Professional "Wirtschaft+Technik" , Springer Professional "Technik"

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Über dieses Buch

Organic Spectroscopy presents the derivation of structural information from UV, IR, Raman, 1H NMR, 13C NMR, Mass and ESR spectral data in such a way that stimulates interest of students and researchers alike. The application of spectroscopy for structure determination and analysis has seen phenomenal growth and is now an integral part of Organic Chemistry courses.

This book provides:
-A logical, comprehensive, lucid and accurate presentation, thus making it easy to understand even through self-study;
-Theoretical aspects of spectral techniques necessary for the interpretation of spectra;
-Salient features of instrumentation involved in spectroscopic methods;
-Useful spectral data in the form of tables, charts and figures;
-Examples of spectra to familiarize the reader;
-Many varied problems to help build competence ad confidence;
-A separate chapter on ‘spectroscopic solutions of structural problems’ to emphasize the utility of spectroscopy.

Organic Spectroscopy is an invaluable reference for the interpretation of various spectra. It can be used as a basic text for undergraduate and postgraduate students of spectroscopy as well as a practical resource by research chemists. The book will be of interest to chemists and analysts in academia and industry, especially those engaged in the synthesis and analysis of organic compounds including drugs, drug intermediates, agrochemicals, polymers and dyes.

Inhaltsverzeichnis

Frontmatter

1. Introduction to Spectroscopy (Spectrometry)

Abstract

Organic chemists use spectroscopy as a necessary tool for structure determination. Spectroscopy may be defined as the study of the quantized interaction of electromagnetic radiations with matter. Electromagnetic radiations are produced by the oscillation of electric charge and magnetic field residing on the atom. There are various forms of electromagnetic radiation, e.g. light (visible), ultraviolet, infrared, X-rays, microwaves, radio waves, cosmic rays etc.

L. D. S. Yadav

2. Ultraviolet (UV) and Visible Spectroscopy

Abstract

Ultraviolet and visible spectroscopy deals with the recording of the absorption of radiations in the ultraviolet and visible regions of the electromagnetic spectrum. The ultaviolet region extends from 10 to 400 nm. It is subdivided into the near ultraviolet (quartz) region (200–400 nm) and the far or vacuum ultraviolet region (10–200 nm). The visible region extends from 400 to 800 nm.

L. D. S. Yadav

3. Infrared (IR) Spectroscopy

Abstract

Infrared spectroscopy deals with the recording of the absorption of radiations in the infrared region of the electromagnetic spectrum. The position of a given infrared absorption is expressed in terms of wavelength in micron μ or more commonly in terms of wavenumber (cm⁻¹) since it is directly proportional to energy. Note that wavenumbers are often called frequencies, although strictly it is incorrect. However, it is not a serious error as long as we keep in mind that and ν = c/λ. The ordinary infrared region 2.5–15 μ (4000–667 cm⁻¹) is of greatest practical use to organic chemists. The region 0.8–2.5 μ (12,500–4000 cm⁻¹) is called the near infrared and the region 15–200 μ (667–50 cm⁻¹) the far infrared. The absorption of infrared radiation by a molecule occurs due to quantized vibrational and rotational energy changes when it is subjected to infrared irradiation. Thus, IR spectra are often called vibrational-rotational spectra.

L. D. S. Yadav

4. Raman Spectroscopy

Abstract

Infrared and Raman spectroscopy are closely related as both originate from transitions in vibrational and rotational energy levels of the molecule on absorption of radiations. Since different methods of excitation are used, the spectroscopic selection rules* are different. The intensity of IR absorption depends on the change in dipole moment of the bond, whereas Raman intensity depends on the change in polarizability of the bond accompanying the excitation. Thus, an electrically symmetrical bond (i.e. having no dipole moment) does not absorb in IR region (i.e. the transition is forbidden) but it does absorb in Raman scattering (i.e. the transition is allowed). In other words, an electrically symmetrical bond is Raman active but IR inactive. However, an electrically unsymmetrical bond may be IR active and Raman inactive or both IR and Raman active.

L. D. S. Yadav

5. Proton Nuclear Magnetic Resonance (PMR or 1H NMR) Spectroscopy

Abstract

Similar to the UV and IR spectroscopy, nuclear magnetic resonance (NMR) spectroscopy is also an absorption spectroscopy in which samples absorb electromagnetic radiation in the radio-frequency region (3 MHz to 30,000 MHz) at frequencies governed by the characteristics of the sample. As the name itself implies, NMR spectroscopy involves nuclear magnetic resonances which depend on the magnetic property of atomic nuclei. Thus, NMR spectroscopy deals with nuclear magnetic transitions between magnetic energy levels of the nuclei in molecules. NMR signals were first observed in 1945 independently by Prucell at Harvard and Bloch at Stanford. The first application of NMR to the study of structure was made in 1951 and ethanol was the first compound thus studied. In 1952, Prucell and Bloch won the Nobel Prize in Physics for their discovery.

L. D. S. Yadav

6. 13C NMR Spectroscopy

Abstract

The possibility of carbon NMR appears surprising at a first glance because

(i)

the most abundant isotope of carbon ¹²C (natural abundance 98.9%) has no net nuclear spin (spin number I is zero). Hence, it does not exhibit NMR phenomenon.

(ii)

the far less abundant carbon isotope ¹³C (natural abundance only 1.1%) has (like ¹H) a nuclear spin of 1/2 and is detectable by NMR.

L. D. S. Yadav

7. Electron Spin Resonance (ESR) Spectroscopy

Abstract

Electron spin resonance (ESR) spectroscopy, invented by Zavoiskii in 1944, is similar to NMR spectroscopy. ESR spectroscopy is an absorption spectroscopy which involves the absorption of radiation in the microwave region (10⁴–10⁶ MHz) by substances containing one or more unpaired electrons. This absorption of microwave radiation takes place under the influence of an applied magnetic field. The substances with one or more unpaired electrons are paramagnetic and exhibit ESR. Thus, ESR spectroscopy is also called electron paramagnetic resonance (EPR) spectroscopy or electron magnetic resonance spectroscopy.

L. D. S. Yadav

8. Mass Spectroscopy (MS)

Abstract

The technique of mass spectrometry was first used by J.J. Thompson in 1911 to provide the conclusive proof for the existence of isotopes. However, the extensive application of mass spectroscopy in solving structural problems only began around 1960. Mass spectroscopy is based on a single principle, i.e. it is possible to determine the mass of an ion in the vapour phase. Thus, a mass spectrometer ionizes the sample into a beam of ions in the vapour phase, separates the ions according to their mass to charge ratios (m/e or m/z values) and records the mass spectrum as a plot of m/e of ions against their relative abundances (Fig. 8.1). Actually, m/e is obtained from a mass spectrum, but for all practical purposes it is equal to the mass m of the ion because multicharged ions are very much less abundant than those with a single charge (e or z = 1).

L. D. S. Yadav

9. Spectroscopic Solutions of Structural Problems

Abstract

Various spectroscopic methods described in this book provide sufficient information for the structure determination of organic compounds. A general approach to solving structural problems by a combination of spectroscopic methods is given as follows:

Molecular formula and molecular weight of a compound is known from its elemental analysis and mass spectrum. The molecular formula gives an idea about the number and kinds of possible functional groups.

Index of hydrogen deficiency is determined from the molecular formula that gives the sum of multiple bonds and rings in the compound (Section 8.8(i)). It helps in limiting the possibilities of structures for further consideration.

The UV spectrum gives indication about the presence (or absence) of a conjugated system, an aromatic ring, a carbonyl group (aldehydic or ketonic) etc.

The IR spectrum shows the presence (or absence) of certain functional groups, e.g. carbonyl groups, hydroxyl groups, -NH- etc., and C≡C and C=C bonds.

The PMR spectrum reveals the environment of hydrogen atoms in the molecule. It gives the number and kinds of protons present in the molecule. Spin-spin splitting tells about the neighboring protons. In brief, we should examine the PMR spectrum for the presence of —CH₃, CH₃CH₂—, —CH₂—CH₂, (CH₃)₂CH—, —CH=CH—, —C≡CH etc. groups; aromatic protons, and protons on heteroatoms, i.e. exchangeable protons. Thus, the PMR spectrum leads to some extent to the molecular skeleton.

The CMR spectrum gives the number of kinds of carbons, and the number of methine carbons, methylene carbons, methyl carbons, and carbons having no hydrogen. Thus, the CMR suggests the carbon skeleton of the molecule.

The mass spectrum, in addition to molecular weight, shows the presence of certain structural units, and the fragmentation pattern indicates their points of attachment in the molecule.

L. D. S. Yadav

Backmatter

Titel: Organic Spectroscopy
verfasst von: L. D. S. Yadav
Verlag: Springer Netherlands
Electronic ISBN: 978-1-4020-2575-4
Print ISBN: 978-94-017-2508-8
DOI: https://doi.org/10.1007/978-1-4020-2575-4

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