27: More about Spectroscopy
- Page ID
- 21991
In Chapter 9, we gave an exposition of the most generally useful and practical spectroscopic methods currently employed in modern organic laboratories. However, in our discussions of NMR spectra, we passed rather quickly over the basis of understanding why some lines are broad and others sharp, why rate effects can cause chemical shifts to be averaged, and how to correlate spin- spin splitting with the energies of NMR transitions. These topics will be discussed in this chapter along with a brief explanation of the remarkable effects on NMR spectra associated with some kinds of chemical reactions, namely, chemically induced dynamic nuclear polarization (CIDNP). In addition to the spectroscopic methods covered in Chapter 9, there are a number of other spectroscopic techniques that are less generally used, but can provide, and have provided, critical information with regard to specialized problems. Because some of these are relatively new and may become more widely used in the next few years, it is important that you be aware of them and their potentialities. However, because they may be peripheral to your present course of study, we have reserved consideration of them to this chapter.
- 27.1: Prelude to More Spectroscopy
- In addition to the spectroscopic methods covered in Chapter 9, there are a number of other spectroscopic techniques that are less generally used, but can provide, and have provided, critical information with regard to specialized problems.
- 27.2: Line-Width Differences in NMR
- Some resonances in NMR spectra are sharp and others are broad. We can understand these differences by consideration of the lifetimes of the magnetic states between which the NMR transitions occur. The lifetimes of the states can be related to the width of the lines by the Heisenberg uncertainty principle.
- 27.3: Use of the Uncertainty Principle to Measure the Rates of Chemical Transformations
- We have seen how the uncertainty principle relates the attainable line widths in different kinds of spectroscopy to the lifetimes of the states - the shorter the lifetime, the greater the spread in energy of the states and the greater the spectroscopic line width. So far we have associated short lifetimes with excited states, but this need not necessarily be so. Short lifetimes also may be associated with chemical or conformational changes.
- 27.4: Why Spin-Spin Splitting?
- The underlying basis for spin-spin splitting in NMR involves perturbation of the nuclear magnetic energy levels by magnetic interaction between the nuclei. This Module addresses the origin of spin-spin splittings (and is not needed for the qualitative use of spin-spin splitting in structural analysis) with an emphasis on understanding the origins of the line spacings and line multiplicities. We will confine our attention to protons, but the same considerations apply to other nuclei.
- 27.5: Chemically Induced Dynamic Nuclear Polarization (CIDNP)
- One of the most startling developments in NMR spectroscopy since its inception has been the discovery of chemically induced dynamic nuclear polarization or CIDNP. An especially dramatic example is provided by irradiation of 3,3-dimethyl-2-butanone with ultraviolet light. The CIDNP effect is a complicated one and is observed exclusively for radical reactions. However, it is not expected for chain-propagation steps, but only for termination steps.
- 27.6: Photoelectron Spectroscopy
- The excitation of electrons to higher energy states through absorption of visible and ultraviolet light (usually covering the range of wavelengths from 200 nm to 780 nm) is discussed elsewhere. We now will consider what happens on absorption of much shorter wavelength, more energetic, photons.
- 27.7: Mössbauer Spectroscopy
- Mössbauer spectroscopy is a nuclear spectroscopy that is which is capable of giving chemical information. The technique would be used widely if there were more nuclei with the proper nuclear properties. For organic chemistry, probably the most important available nucleus is the iron nuclide Fe-57 ( 2.2% of the natural mixture of iron isotopes). Iron occurs in many biologically important substances, such as hemoglobin, myoglobin, cytochromes, the iron storage substance ferritin, etc.
- 27.8: Field- and Chemical-Ionization Mass Spectroscopy
- There are two ways of achieving ion formation without imparting as much energy as by electron impact - in other words, "soft" rather than "hard" ionizations. Field ionization is one such method, in which ionization results from passing the molecules through a high electric field. An alternative method is chemical ionization that, as might be expected from its name, more chemically interesting and is closely allied to ion cyclotron resonance, which will be discussed in the next section.
- 27.9: Ion-Cyclotron Resonance
- Ion-cyclotron resonance combines features of mass spectroscopy in that the charge/mass ratio is involved, and of NMR spectroscopy in that detection depends on absorption of energy from a RF oscillator. The applications depend on reactions between the ions during the time they remain in the cyclotron, which may be many seconds. It is possible to measure the concentrations of the ions as a function of time and determine the rates of reaction of ions with neutral molecules in the gas phase.
- 27.10: Electron-Spin Resonance (ESR) Spectroscopy of Organic Radicals
- An important method of studying radicals is electron-spin resonance (ESR) spectroscopy. The principles of this form of spectroscopy are much the same as of NMR spectroscopy, but the language used by the practitioners of these two forms of magnetic resonance spectroscopy is different. The important point is that an unpaired electron, like a proton, has a spin and a magnetic moment such that it has two possible orientations in a magnetic field.
- 27.E: More about Spectroscopy (Exercises)
- These are the homework exercises to accompany Chapter 27 of the Textmap for Basic Principles of Organic Chemistry (Roberts and Caserio).
Contributors and Attributions
John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."